Key Laboratory of Aquatic Biodiversity and Conservation of Chinese Academy of Sciences, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China
2.
Institute of Evolution and Marine Biodiversity & Key Laboratory of Mariculture, Ministry of Education, Ocean University of China, Qingdao 266003, China
3.
Department of Biology, Shenzhen MSU-BIT University, Shenzhen 518172, China
4.
Department of Zoology, Faculty of Science, Charles University, Prague 12843, Czech Republic
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In sharp contrast to their pelagic relatives, the oligotrichs, the overwhelming majority of hypotrich ciliates inhabit the benthos. Only a few species, including those of the genus Hypotrichidium Ilowaisky, 1921, have adapted to a planktonic lifestyle. The ontogenetic mode of the highly differentiated ciliate, Hypotrichidium tisiae (Gelei, 1929) Gelei, 1954, is unknown. In this study, the interphase morphology and the ontogenetic process of this species are investigated. Accordingly, the previously unidentified ciliary pattern of Hypotrichidium is redefined. The main morphogenetic features are as follows: (1) The parental adoral zone of membranelles is inherited completely by the proter and the oral primordium of the opisthe arises in a deep pouch. (2) Five frontoventral cirral anlagen (FVA) are formed: FVA Ⅰ contributes to the single frontal cirrus, FVA Ⅱ–Ⅳ generate three frontoventral cirral rows, FVA Ⅴ migrates and forms postoral ventral cirri. (3) All marginal cirral row anlagen develop de novo: each of the two left anlagen forms a single cirral row, while the single right anlage fragments into anterior and posterior parts. (4) Two dorsal kinety anlagen occur de novo, with the right one fragmenting to form kineties 2 and 3. (5) Two long caudal cirral rows are formed at the ends of dorsal kineties 1 and 3. On the basis of the morphogenetic features and phylogenetic analyses, the assignment of Hypotrichidium to the family Spirofilidae Gelei, 1929 within Postoralida is supported. The establishment of separate families for the slender "tubicolous" spirofilids and the highly helical spirofilids is also validated.
Ciliates (Alveolata, Ciliophora) are unicellular eukaryotes having a combination of unique characters, e.g., nuclear dimorphism, the sexual phenomenon of conjugation, the occurrence of cilia during at least some stage of the life cycle, and complex, highly specialized organelles and ontogenetic processes, that have been studied in many fields, such as genetics, cytology, evolutionary biology, and cell differentiation and dedifferentiation (Bai et al. 2020; Gao et al. 2016; Hausmann and Bradbury 1996; Liu et al. 2021; Miao et al. 2020). The hypotrichs are an extremely diverse and highly differentiated group of ciliates, the vast majority of which have a substrate-oriented or benthic lifestyle. They are usually dorsoventrally flattened with prominent cirri on the ventral side and inconspicuous bristles on the dorsal side (Hu et al. 2019; Jin et al. 2022; Jung et al. 2021; Shao et al. 2020; Song et al. 2009). Their considerable morphological and morphogenetic diversity make the hypotrich ciliates ideal model organisms for studying unicellular ontogenesis (Berger 1999, 2006, 2008, 2011; Song and Shao 2017; Wang et al. 2021a). The morphogenesis of hypotrichs has been the subject of extensive studies, especially in recent years. However, there are few studies of these processes in planktonic representatives of the group (Chen et al. 2013; Foissner 2016; Li et al. 2021; Omar et al. 2021, 2022; Zhang et al. 2020, 2022). In addition, because descriptions of most species have been based solely on live observations or on silver-stained preparations of only interphase (morphostatic) specimens, morphogenetic features and molecular information for most planktonic hypotrichs are still lacking (Bourland 2015; Chen et al. 2013; Lynn 2008). Therefore, the binary divisional pattern and systematic relationships of this group remain poorly understood.
Hypotrichidium Ilowaisky, 1921, a planktonic representative of the hypotrichs, is a distinctive ciliate genus with highly specialized features e.g., a pyriform rather than dorsoventrally flattened body and spiral cirral rows distributed around the whole body rather than restricted to the ventral surface. Because of this unique combination of features, Hypotrichidium has been considered a distinctive genus ever since it was first established by Ilowaisky (1921). In the most recent revision of the genus Hypotrichidium, Chen et al. (2013) recognized five valid species, i.e., H. conicum Ilowaisky, 1921, H. tisiae (Gelei, 1929) Gelei, 1954, H. africanum Jankowski, 1979, H. tetranucleatum Deshmukh et al., 2011, and H. paraconicum Chen et al., 2013, and several incertae sedis species, most of which lack sufficiently detailed descriptions. Moreover, since only a few morphogenetic stages of two species, namely H. conicum and H. paraconicum, have been reported, a comprehensive description of the ontogeny of Hypotrichidium has not been possible.
Because of the limited data on morphological and morphogenetic characteristics, as well as the lack of molecular information, the taxonomy and systematics of Hypotrichidium have long been a matter of debate. Originally, the genus was classified within the family Oxytrichidae Ehrenberg, 1838 by Ilowaisky (1921), which was accepted by Kahl (1932), Corliss (1961), and Dingfelder (1962). Fauré-Fremiet (1961) included Hypotrichidium in a newly established stichotrichid family, Strongylidae, emended and accepted by Tuffrau (1972) as Strongylidiidae, and also accepted by Stiller (1975). Jankowski (1975, 1979) assigned Hypotrichidium to the family Hypotrichidiidae Jankowski, 1975. In more recent classification schemes, Hypotrichidium has been placed in the stichotrichid family Spirofilidae Gelei, 1929 (Borror 1972; Corliss 1979; Dragesco and Dragesco-Kernéis 1986; Jankowski 2007; Lynn 2008; Lynn and Small 2002; Tuffrau 1987; Tuffrau and Fleury 1994). Chen et al. (2013) performed a phylogenetic analysis based on the first molecular sequence for Hypotrichidium (H. paraconicum). However, tree topologies were rather unstable and the phylogenetic position of Hypotrichidium was not confidently resolved. Consequently, a description of the binary divisional pattern and further molecular characterization of this highly distinctive ciliate genus are urgently needed to provide insights into its systematic position and evolutionary history.
In the present work, previously unknown morphological characteristics and details of ontogenesis for the planktonic hypotrich Hypotrichidium tisiae are studied on the basis of live observation, protargol preparations, and scanning electron microscopy (SEM). In addition, phylogenetic analyses are carried out using the 18S rRNA gene sequence for this species, which is reported here for the first time. This integrative study not only helps to redefine the ciliary pattern of H. tisiae, but also elucidates the systematics and phylogeny of Hypotrichidium and the family Spirofilidae.
Improved diagnosis. Body 90–145 × 45–85 μm in vivo, body shape pyriform, with posterior end forming a short, acutely pointed tail; two types of cortical granules, both biconcave, both 1.0–1.5 μm in diameter, one type yellowish, distributed in rows or diffusely scattered, the other type colorless, densely packed; adoral zone occupies about half of body length, composed of about 40 membranelles; undulating membranes in Australocirrus pattern; single frontal cirrus, three frontoventral cirral rows, a row of postoral ventral cirri, long right marginal cirral row separated into anterior and posterior parts, two left marginal cirral rows, and two caudal cirral rows; three dorsal kineties; planktonic freshwater lifestyle.
Voucher slides. Two permanent voucher slides (registration no. LXT2017052301/1, 2) with multiple protargol-stained specimens are deposited in the Laboratory of Protozoology, Ocean University of China, Qingdao, China.
Figure
1.A–H Drawings (A–C) and photomicrographs (D–H) of Hypotrichidium tisiae in vivo (A, D–H) and after protargol staining (B, C). A Ventral view of a representative specimen, arrowhead marks the acutely pointed tail, arrow indicates the contractile vacuole. B, C Ventral (B) and dorsal (C) views of a typical specimen, showing infraciliature and nuclear apparatus, arrowhead in (B) marks the single frontal cirrus, arrow in (B) indicates the separation between adoral zone and frontoventral cirral row 2, arrowheads in (C) mark micronuclei, arrows in (C) show macronuclear nodules, double-arrowhead in (C) marks the single cirrus between anterior and posterior parts of right marginal cirral row. D Lateral view of a representative specimen, arrowhead marks the acutely pointed tail. E Ventral view of a compressed individual, showing contractile vacuole (arrow) and the buccal depression (arrowhead). F Cortical granules and contractile vacuole (arrow). G, H The yellowish cortical granules (arrowheads) and the colorless cortical granules (arrows), double-arrowheads in (G) show dorsal bristles. ARMR anterior part of right marginal cirral row, AZM adoral zone of membranelles, C1–6 cirral rows according to Chen et al. (2013), CC1, 2 caudal cirral rows, CG cortical granules, EM endoral membrane, FVR1–3 frontoventral cirral rows, LMR1, 2 left marginal cirral rows, MR1–4 meridional rows according to Chen et al. (2013), PM paroral membrane, PRMR posterior part of right marginal cirral row, PVC postoral ventral cirri, 1–3 dorsal kineties. Scale bars: 50 μm
Figure
2.A–H Scanning electron micrographs of Hypotrichidium tisiae. A, B Ventral views of representative individuals, showing cirral pattern, arrowhead in A indicates the acutely pointed tail, arrowhead in B marks the single cirrus between anterior and posterior parts of the right marginal cirral row. C Ventral view of an individual with four frontoventral cirral rows, arrowhead indicates the cytopyge. D Left-lateral view of a representative individual, showing the strongly developed paroral membrane. E, F Dorsal views of representative individuals, showing three dorsal kineties and the acutely pointed tail (arrowheads). G Dorsal view of an individual with extra dorsal bristles (arrowheads), showing the contractile vacuole on dorsal side, arrow indicates the acutely pointed tail. H Dorsal view of a starved individual. ARMR anterior part of right marginal cirral row, AZM adoral zone of membranelles, CC1, 2 caudal cirral rows, CV contractile vacuole, EXR extra cirral row, FC frontal cirrus, FVR1–3 frontoventral cirral rows, LMR1, 2 left marginal cirral rows, PM paroral membrane, PRMR posterior part of right marginal cirral row, PVC postoral ventral cirri, 1–3 dorsal kineties. Scale bars: 30 μm
Figure
3.A–I Scanning electron micrographs of Hypotrichidium tisiae. A Right-lateral view from anterior end, showing paroral membrane and dorsal kineties. B Posterior view of a representative individual, showing six cirral rows located around the posterior end, arrowhead indicates the acutely pointed tail. C Posterior-dorsal view of an individual. D Apical view, showing the flat buccal area and the strongly developed paroral membrane. E Details of anterior part, arrowhead indicates the buccal depression. F Details of the oral apparatus, showing paroral and endoral membrane, arrowhead indicates the buccal depression. G Cirri and basal bodies. H Detail of a dorsal kinety. I Diatoms (arrowheads) in the cell. AZM adoral zone of membranelles, EM endoral membrane, FC frontal cirrus, FVR1–3 frontoventral cirral rows, PM paroral membrane, 1–3 dorsal kineties. Scale bars: 30 μm (A–D), 20 μm (E), 10 μm (F–H)
Table
1.
Morphometric data for Hypotrichidium tisiae based on the Boise population
Characteristics
Min
Max
Mean
Med
SD
CV
n
Body length
107
142
124.4
126.0
9.2
7.4
25
Body width
77
100
87.4
87.0
5.4
6.2
25
Body width: body length (%)
63
80
70.4
69.8
4.1
5.8
25
Buccal cavity, length
53
77
64.8
64.0
6.0
9.2
25
Buccal cavity length: body length (%)
44
62
52.2
51.6
4.5
8.6
25
Adoral membranelles, number
32
44
39.6
40.0
2.8
7.1
25
Frontal cirrus, number
1
1
1.0
1.0
0.0
0.0
25
Frontoventral cirral rows, number
3
3
3.0
3.0
0.0
0.0
25
Cirri in frontoventral cirral row 1, number
8
17
10.7
10.0
2.8
26.1
23
Cirri in frontoventral cirral row 2, number
10
18
13.1
13.0
2.3
17.6
24
Cirri in frontoventral cirral row 3, number
11
19
15.5
15.0
1.6
10.4
25
Extra cirral row, number
0
1
0.0
0.0
0.2
500.0
25
Cirri in extra cirral row, number
9
9
9.0
9.0
0.0
0.0
1
Postoral ventral cirri, number
19
26
22.9
23.0
1.5
6.5
24
Cirri in anterior part of right marginal cirral row, number
12
18
15.3
15.0
1.4
9.0
25
Cirri in posterior part of right marginal cirral row, number
20
29
24.9
25.0
2.5
9.9
25
Left marginal cirral rows, number
2
2
2.0
2.0
0.0
0.0
19
Cirri in left marginal cirral row 1, number
17
23
19.9
20.0
1.6
8.0
10
Cirri in left marginal cirral row 2, number
23
28
25.7
26.0
1.6
6.2
19
Caudal cirral rows, number
2
2
2.0
2.0
0.0
0.0
25
Cirri in caudal cirral row 1, number
26
34
29.0
29.0
2.3
8.1
21
Cirri in caudal cirral row 2, number
23
32
27.6
28.0
2.4
8.8
25
Dorsal kineties, number
3
3
3.0
3.0
0.0
0.0
25
Dikinetids in dorsal kinety 1, number
10
20
16.4
17.0
2.4
14.5
21
Dikinetids in dorsal kinety 2, number
13
20
15.9
16.0
2.1
13.3
21
Dikinetids in dorsal kinety 3, number
12
18
15.0
15.0
1.7
11.5
21
Macronuclear nodules, number
2
2
2.0
2.0
0.0
0.0
25
Anterior macronuclear nodule, length
23
35
27.4
28.0
3.1
11.2
25
Anterior macronuclear nodule, width
13
20
16.0
15.0
1.7
10.9
25
Posterior macronuclear nodule, length
22
37
29.8
30.0
4.3
14.3
25
Posterior macronuclear nodule, width
14
23
19.2
19.0
2.1
10.7
25
Micronuclei, number
2
3
2.4
2.0
0.5
20.8
20
Micronuclei, length
3.6
5
4.3
4.3
0.3
7.0
25
Micronuclei, width
3
4.9
3.9
4.0
0.5
12.1
25
All data are based on randomly selected protargol-stained specimens. Measurements in μm CV coefficient of variation in %, Max maximum, Mean arithmetic mean, Med Median, Min minimum, n number of specimens observed, SD standard deviation
Body 90–145 × 45–85 μm in vivo, pyriform (pear-shaped) (Figs. 1A, D, 2A–G), with rear end forming an acutely pointed tail (Figs. 1A, D, 2A, E–G; Supplementary Fig. S1A). Starved cells slender, tail usually inconspicuous (Fig. 2H). Buccal apparatus occupies an obliquely flattened area on left anterior ventrolateral aspect of cell (Figs. 2A–D, 3B, D, E). Buccal cavity itself slightly vaulted dorsally (Figs. 2D, 3A). Posterior half of cell almost circular in cross-section (Fig. 3B). Cell more or less flexible, not contractile. Contractile vacuole near left margin of cell with excretory pore on dorsal side, slightly anterior to level of midbody (Figs. 1F, 2G). Two types of cortical granules, both of which are biconcave in shape and 1.0–1.5 μm in diameter, one type yellowish in color, distributed in rows of variable length or diffusely scattered; other type colorless, densely packed on both ventral and dorsal sides (Fig. 1F–H). Cytoplasm colorless, usually packed with small globules and food vacuoles (sometimes containing ingested diatoms) (Fig. 3I) that render cell opaque and dark at low magnification (Fig. 1D). Two macronuclear nodules, ovoid to ellipsoid in outline, 22–35 × 13–23 μm in protargol-stained specimens. Two to four globular micronuclei, about 4 μm across after protargol staining, each attached to or near macronuclear nodules (Fig. 1C). Locomotion by swimming straight ahead with rotation around the main body axis, seldom gliding on substrate.
Buccal cavity large, rather deep, occupies about half of body length. Adoral zone composed of about 40 membranelles (Fig. 1B). A buccal depression present in anterior end of buccal cavity, conspicuous in vivo and in SEM preparations (Figs. 1E, 3E, F). Undulating membranes in a more or less Australocirrus pattern (paroral distinctly curved but not recurved). Paroral and endoral membranes separated by buccal ridge, intersect optically near their posterior ends, paroral membrane prominent and strongly curved anteriorly, endoral membrane strongly curved posteriorly, much shorter than paroral anteriorly (Figs. 1B, 2D, 3A, D–F).
Cirral pattern as shown in Figs. 1B, 2A–H, 3A–E, Supplementary Fig. S1A. Single, slightly enlarged frontal cirrus positioned anterior to undulating membranes. Three frontoventral cirral rows located to right of undulating membranes and left of anterior part of right marginal cirral row. Sometimes an extra cirral row distributed between frontoventral cirral rows and right marginal cirral row (one out of 25 specimens in protargol preparations, Table 1; see also specimen in SEM preparations, Fig. 2C). Six oblique cirral rows posterior to buccal vertex, arranged spirally around long axis of body, including a row of postoral ventral cirri, two left marginal cirral rows, two caudal cirral rows, and posterior part of right marginal cirral row. Anterior and posterior parts of right marginal cirral row usually separated by a small gap, sometimes a single cirrus present in this gap (Figs. 1C, 2B). Cirri usually comprise two files, each with three or four basal bodies (Fig. 3G). Three dorsal kineties distributed anteriorly on dorsal side, sometimes a short extra row of dikinetids present beside one of them (Figs. 1C, 2E–G). Dorsal bristles about 4–5 μm in vivo (Fig. 1G).
Dividers and reorganizers (Figs. 4, 5, Supplementary Fig. S1B–J)
Figure
4.A–I Drawings showing morphogenetic stages of Hypotrichidium tisiae after protargol staining. A Oral primordium of the opisthe between postoral ventral cirri (arrow) and left marginal cirral row 1 (arrowhead). B, C Ventral (B) and dorsal (C) views of an early divider, showing cirral anlagen and dorsal kinety anlagen, double-arrowhead in B marks the V-shaped anlage, double-arrowheads in C show dorsal kinety 2 anlage, arrowheads in C indicate the replication bands, arrows in C show the micronuclei, the purplish section shows the oral primordium located in the deep pouch. D, E Ventral (D) and dorsal (E) views of an early-middle divider, showing the elongated cirral anlagen and dorsal kinety anlagen, arrowhead in D indicates the dedifferentiated endoral membrane, arrows in D show left marginal cirral row 1 anlage, double-arrowheads in D mark left marginal cirral row 2 anlage, double-arrowheads in E show dorsal kinety 2 anlage, arrowheads in E show the micronuclei, arrows in E show the macronuclear nodules. F, G Ventral F and dorsal G views of a middle divider, showing new cirri formed from cirral anlagen and the fused macronucleus and micronucleus, arrowhead in F shows the dedifferentiated endoral membrane, double-arrowhead in F marks the single frontal cirrus formed from the undulating membranes anlage, arrowheads in G indicate the fragmentation of dorsal kineties, double-arrowheads in G mark the connection between dorsal kinety 3 anlage and caudal cirral row 2 anlage. H, I Ventral (H) and dorsal (I) views of a middle-late divider, double-arrowheads in H mark the new undulating membranes, arrows show the gap between anterior and posterior parts of the right marginal cirral row, arrowheads in H show single frontal cirrus formed from undulating membranes anlage, arrowheads in I indicate caudal cirral row 2 fragmented from the end of dorsal kinety 3. ARMR new anterior part of right marginal cirral row, CC1, 2 new caudal cirral rows, CC1, 2A caudal cirral row anlage, DK1–3A dorsal kinety anlagen, FVR1–3 new frontoventral cirral rows, LM2A left marginal cirral row 2 anlage, LMR1, 2 new left marginal cirral rows, Ma macronuclear nodules, Mi micronuclei, OP oral primordium, PRMR new posterior part of right marginal cirral row, PVC new postoral ventral cirri, RMA right marginal cirral row anlage, I–V frontoventral cirral anlagen, 1–3 old dorsal kineties. Scale bars: 50 μm
Figure
5.A–I Scanning electron micrographs of Hypotrichidium tisiae showing morphogenetic stages. A A very early divider, showing the oral primordium of the opisthe between postoral ventral cirri and left marginal cirral row 1. B An early divider, showing oral primordium and the newly formed frontoventral cirral anlagen. C Details of an early-middle divider, showing the oral primordium in a deep pouch and five frontoventral cirral anlagen on cell surface. D Ventral view of a middle divider, showing the new cirri formed from frontoventral cirral anlagen. E Ventral view of a middle divider, showing the frontoventral cirral anlage of the proter. F Details of the disaggregated old paroral membrane (arrows) and the undulating membranes anlagen (arrowheads). G Dorsal view of an early divider, showing dorsal kinety anlagen and right marginal cirral row anlagen. H Dorsal view of a reorganizer, showing dorsal kinety anlagen, arrowheads indicate extra dorsal bristles. I Dorsal view of a late divider, showing the newly formed dorsal kineties and caudal cirral rows. CC1, 2 new caudal cirral rows, CV contractile vacuole, DK1–3 new dorsal kineties, DK1, 2A dorsal kinety anlagen, LM1, 2A left marginal cirral row anlage, LMR1 old left marginal cirral row, OP oral primordium, PVC old postoral ventral cirri, RMA right marginal cirral row anlage, I–V frontoventral cirral anlagen, 1–3 old dorsal kineties. Scale bars: 30 μm
Stomatogenesis. In very early dividers, the oral primordium for the opisthe is a field of closely spaced basal bodies that develops in a deep pouch between the postoral ventral cirri and left marginal cirral row 1 (Figs. 4A, 5A, Supplementary Fig. S1C). At this stage, all parental cirri remain intact and are not involved in the formation of the oral primordium. Subsequently, the oral primordium enlarges by further proliferation of basal bodies; differentiation into new membranelles begins at the anterior portion and progresses posteriorly (Figs. 4B, 5B, C, Supplementary Fig. S1F). The new membranelles, originally formed in the deep pouch, gradually migrate to the cell surface in a posterior direction. At the same time, the undulating membranes anlage (UMA) for the opisthe appears to the right of the oral primordium and is probably generated from the oral primordium (Figs. 4B, 5C). Later, the parental endoral membrane begins to disintegrate to form the UMA for the proter (Figs. 4D, 5F). In the next stages, the differentiation of the oral primordium for the opisthe is gradually completed, and finally all the new adoral membranelles migrate to the cell surface. Meanwhile, the UMA for both proter and opisthe splits longitudinally into paroral and endoral membranes (Figs. 4F, H, 5D). The parental adoral zone is inherited by the proter unchanged (Figs. 4B, D, F, H, 5A, B, D).
Frontoventral cirri. In very early dividers, no other anlagen are yet formed as the oral primordium for the opisthe appears (Fig. 4A). Subsequently, five longitudinal streak-like frontoventral cirral anlagen (FVA), including the UMA, are generated to the right of the oral primordium in the opisthe (Figs. 4B, 5C). Simultaneously in the proter, a V-shaped anlage comprising FVA Ⅳ and Ⅴ develops de novo to the right of the parental frontoventral cirral row 3 (FVR3) (Fig. 4B, Supplementary Fig. S1D). In the next stage, the V-shaped anlage extends along the parental FVR3. At the same time, the proter's FVA Ⅱ and Ⅲ develop intra-kinetally within the parental FVR1 and FVR2, respectively (Figs. 4D, 5E, Supplementary Fig. S1E). In middle dividers, the V-shaped anlage differentiates into two parallel streaks, and all the streaks broaden, lengthen, break apart, and differentiate into new cirri in both daughter cells (Figs. 4F, 5D, Supplementary Fig. S1H). Later, the five FVA, including the UMA, differentiate into new cirri in the following pattern: FVA Ⅰ gives rise to the single frontal cirrus; FVA Ⅱ–Ⅳ contribute to FVR1–3; the rightmost cirral row originated from FVA Ⅴ migrates below the buccal vertex to form the postoral ventral cirri (Fig. 4H). Most of the parental cirri of FVR1–3 have been resorbed by the middle–late stage of division (Fig. 4H).
Formation of marginal cirral rows. In early dividers, the left marginal cirral row 2 anlage (LM2A) for the proter develops to the left of the parental adoral zone of membranelles and LM2A for the opisthe appears to the right of the parental left marginal cirral row 2 (LMR2). At the same time, the right marginal cirral row anlage (RMA) for the proter originates along the parental anterior part of the right marginal cirral row (ARMR), and the RMA for the opisthe is formed to the right of the parental posterior part of the right marginal cirral row (PRMR) (Fig. 4B, C). In the next stage, the left marginal cirral row 1 anlage (LM1A) for the opisthe develops to the right of the parental left marginal cirral row 1 (LMR1), and LM1A for the proter is formed to the right of LM2A (Fig. 4D, E). No parental cirri take part in the formation of any of the anlagen mentioned above. In middle dividers, all the anlagen extend bidirectionally and begin to differentiate into new cirri (Fig. 4F, G). Later, the RMA fragments to form the new ARMR and PRMR in both proter and opisthe (Figs. 4H, I, Supplementary Fig. S1J). The parental cirral rows remain intact up to the middle-late stage of division (Figs. 4H, I, 5I).
Dorsal ciliature. In early dividers, two thread-like dorsal kinety anlagen for the proter develop to the left of parental dorsal kineties 1 and 2. Meanwhile, in the opisthe, dorsal kinety 1 anlage forms to the left of the parental caudal cirral row 1, and dorsal kinety 2 anlage appears to the left of the parental caudal cirral row 2. All the dorsal kinety anlagen develop de novo, that is, no parental structures contribute to the formation of any of the anlagen (Figs. 4C, E, 5D). In middle dividers, the rightmost anlage fragments and forms two anlagen (Fig. 4G). At the same time, a long row of cirri is generated at the end of the dorsal kinety 1 anlage and dorsal kinety 3 anlage, respectively (Figs. 4F, G, Supplementary Fig. S1I). In the next stages, three rows of dorsal kineties and two caudal cirral rows are formed in each daughter cell (Figs. 4I, 5I). The parental dorsal kineties remain intact until the middle-late stage (Fig. 4I). Later, only some of the old structures are resorbed, and most persist unchanged (Fig. 5I), which could explain why there are some extra dikinetids arranged along the normal dorsal kineties in some interphase specimens (Figs. 2G, 3C).
Nuclear apparatus. In the early stage, replication bands are present in each macronuclear nodule (Fig. 4C, Supplementary Fig. S1B). Division of the nuclear apparatus proceeds in the usual way for most hypotrichs, that is, at the middle stage, macronuclear nodules completely fuse into a single mass which then lengthens and splits, and the micronuclei also fuse into a single mass in middle dividers (Fig. 4G, I, Supplementary Fig. S1G).
Reorganizers. One stage of physiological reorganization was found in SEM preparations, which shows cortical developmental features similar to those in the proter (Fig. 5H).
18S rRNA gene sequence and phylogenetic analyses (Fig. 6)
Figure
6.A Maximum likelihood (ML) tree based on the 18S rRNA gene, showing the phylogenetic position of Hypotrichidium tisiae (in bold). Numbers at nodes denote ML bootstrap values/Bayesian inference (BI) posterior probabilities. Asterisks (*) indicate mismatch in topologies between BI and ML analyses. Fully supported (100%/1.00) nodes are marked with red solid circles. The scale bar corresponds to two substitutions per 100 nucleotide positions. B The sampling site of the Boise population of Hypotrichidium tisiae
The 18S rRNA gene sequences of Hypotrichidium tisiae were obtained from three single cells, which have been deposited in the GenBank database (Accession numbers ON117314–ON117316). The three new sequences are identical to each other and have a length (excluding primers) of 1723 bp and a GC content of 45.04%. The most similar sequence to the new species is that of Hypotrichidium paraconicum (GenBank number: JQ918371, sequence similarity of 99.5%, eight nucleotide differences). There is another Hypotrichidium sequence, that is, Hypotrichidium conicum (GenBank number: MW830115), available in GenBank database, which differs from Hypotrichidium tisiae by 13 nucleotides (sequence similarity of 99.2%).
Phylogenetic trees based on 18S rRNA gene sequences using Bayesian inference (BI) and maximum likelihood (ML) analyses have almost identical topologies, therefore, only the ML tree with support from both methods is shown. The monophyly of the genus Hypotrichidium is moderately to fully supported in the phylogenetic trees (ML/BI, 92%/1.00). The three Hypotrichidium species cluster together with Notohymena gangwonensis (MN977117) and are sister to the clade containing Neokeronopsis asiatica (KM061386), Afrokeronopsis aurea (EU124669 and KY968741), Apoterritricha lutea (KJ619458), and Australocirrus shii (JQ513386).
Discussion
Identification of the Boise population of Hypotrichidium tisiae
Based on a population collected from the shallow marshes fed by the Tisza River in Hungary, this species was originally reported under the name Spirofilum tisiae by Gelei (1929). Later, Gelei himself transferred the species to Hypotrichidium and described another Hungarian population (Gelei 1954). The Boise population shares most of the typical features with the populations described by Gelei (1929, 1954): (1) a pyriform body shape, some specimens more slender, most cells possess a short acutely pointed (pin-like) posterior tail; (2) buccal apparatus occupies flattened left anterior ventrolateral area, buccal cavity itself slightly vaulted dorsally, posterior half of cell almost circular in cross-section; (3) conspicuous buccal region, occupies about half of body length, with an especially distinct buccal depression located in the anterior part of the buccal cavity; (4) paroral membrane well developed and strongly curved but not recurved; (5) single frontal cirrus and four meridional rows, with the rightmost one on the right body margin; (6) the cirral rows below the buccal vertex not connected with the meridional rows; (7) cortical granules prominent, densely packed, distributed over the whole body; (8) the location of contractile vacuole and the arrangement of dorsal kineties. It is noteworthy that there are two types of cortical granules in the Boise population, while Gelei (1929, 1954) did not mention the types of cortical granules. The difference is probably due to the fact that Gelei's descriptions (1929, 1954) were based only on impregnated cells (determination of cortical granules requires examination of living cells). Thus, we consider the Hungarian populations and the Boise population to be conspecific.
Redefinition of the ciliary pattern based on binary division
The development of the ciliary during binary division of Hypotrichidium tisiae can be summarized as follows: (1) The unchanged parental adoral zone of membranelles is inherited by the proter. (2) The parental endoral membrane differentiates into the undulating membranes anlage for the proter. (3) The oral primordium in the opisthe arises de novo in a deep pouch. (4) Five frontoventral cirral anlagen (FVA) develop in both proter and opisthe, the leftmost one (FVA Ⅰ) generates the undulating membranes and a single frontal cirrus, the three middle ones (FVA Ⅱ–Ⅳ) contribute to the three frontoventral cirral rows, the rightmost one (FVA Ⅴ) migrates posteriorly and forms the postoral ventral cirri. (5) Two left marginal row anlagen develop de novo and give rise to the two left marginal cirral rows. (6) A single right marginal row anlage develops de novo and fragments to form the anterior and posterior parts of the right marginal cirral row. (7) Two dorsal kinety anlagen occur de novo, the right one of which fragments. (8) A long row of caudal cirri is formed at the end of dorsal kineties 2 and 3, respectively.
Even though Hypotrichidium has been studied for over a century and dozens of populations have been described (for a review, see Chen et al. 2013), the ciliary pattern of the genus remains to be elucidated definitively (e.g., four cirral rows above meridian were treated as meridional rows, the others were simply labeled as cirral rows, Fig. 1B, C), mainly due to incomplete morphogenetic information. Based on the detailed morphogenetic data obtained in this study, we give a redefinition of the ciliary pattern for the genus Hypotrichidium. The single cirrus located above the paroral membrane originated from the undulating membranes anlage, suggesting that it is a frontal cirrus. The three left meridional rows and cirral row 1 originate from the frontoventral cirral anlagen indicating that the three left meridional rows are homologous to the frontoventral cirral rows and cirri in cirral row 1 are postoral ventral cirri. The right marginal cirral row anlage fragments to form the rightmost meridional row (MR4) and cirral row 2, contributing to the anterior and posterior parts of the right marginal cirral row. Cirral rows 3 and 4 originate from the ends of dorsal kineties, indicating that they are two caudal cirral rows. The origin and location of cirral rows 5 and 6 suggest that both are left marginal cirral rows. There are initially two dorsal kineties with the third formed by fragmentation of the right one. In brief, the ciliary pattern of Hypotrichidium is: single frontal cirrus, three frontoventral cirral rows, a row of postoral ventral cirri, the long right marginal cirral row separated into anterior and posterior parts, two left marginal cirral rows, two caudal cirral rows, and three dorsal kineties, with the rightmost kinety fragmented from the middle one.
Morphogenetic features and phylogenetic analyses indicate Hypotrichidium close to Postoralida
In the phylogenetic trees, Hypotrichidium is clustered in a main clade of Postoralida, which is characterized by a strong synapomorphy, namely the presence of postoral cirri (i.e., non-transverse cirral products of FVA Ⅳ that have migrated to the postoral region). The morphogenetic pattern shows that Hypotrichidium possesses a row of postoral ventral cirri generated from FVA Ⅳ. Another key characteristic shared by Hypotrichidium and Postoralida is the oxytrichid fragmentation of the rightmost dorsal kinety anlage, a phenomenon which appears to occur only within the Postoralida (Berger 1999; Paiva 2020). Outside the Postoralida, other types of dorsal anlagen fragmentation occur, such as the Hemigastrostyla type and Tachysoma type (Chen et al. 2017; Song and Shao 2017). Also, there are additional morphogenetic features shared by Hypotrichidium and members of Postoralida: (1) the retention of the parental adoral zone during divisional morphogenesis; and (2) three or fewer dorsal anlagen, both of which features are defining characteristics of the higher taxon Diatirostomata, which includes Postoralida. In brief, the inclusion of Hypotrichidium within the Postoralida is strongly supported by the conserved morphogenetic features (e.g., number of dorsal kinety anlagen, dorsal kinety fragmentation pattern, presence of postoral cirri, and the retention of the parental adoral zone), all of which carry potent phylogenetic signals (Paiva 2020).
Interphasic morphological features with meaningful phylogenetic signals
Hypotrichidium nests with Notohymena gangwonensis, Neokeronopsis asiatica, Afrokeronopsis aurea, Apoterritricha lutea, and Australocirrus shii in the phylogenetic trees. In addition to the above-mentioned morphogenetic features, the close relationship among them is also supported by the following interphase morphological characteristics: (1) number of macronuclear nodules (invariably two); (2) flexible body; (3) the presence of cortical granules, usually yellowish in color; and (4) paroral membranes well developed and strongly curved (Chen et al. 2013; Foissner and Stoeck 2008; Foissner et al. 2010; Jung et al. 2015; Kim et al. 2014, 2019; Kumar and Foissner 2015).
The features of the nuclear apparatus certainly contain important taxonomic information, strongly supported by several hypotrich groups, such as the arcuseriids (numerous macronuclear nodules), Kentrurostylida (a high number of macronuclear nodules), and Diatirostomata (most species with two to four macronuclear nodules) (Paiva 2020). The number of macronuclear nodules is a key morphological feature supporting the classification of Hypotrichidium within Postoralida, Diatirostomata.
For most of the past century, two kinds of hypotrichs have been recognized: those with a flexible cortex and those with a rigid one (Kahl 1932). Although the structural basis of this difference is not known, the flexibility/rigidity paradigm is a main feature widely used in the classification of hypotrichs (i.e., all the rigid oxytrichids belong to the subfamily Stylonychinae Berger & Foissner, 1997, all the kentrurostylids are flexible) (Berger 1999, 2006; Paiva 2020). The exception of cortical rigidity of Rigidothrix Foissner & Stoeck, 2006, a member of the Uroleptida, represents a convergence (Paiva 2020). However, as cortical flexibility could be used to distinguish species, genera, or even higher taxa (such as subfamilies/families), the extent to which such flexibility provides a phylogenetic signal remains problematic.
As proposed in previous studies, the presence or absence, morphology (e.g., size, color, ultra-structure), arrangement pattern, and function of cortical granules are all of significant phylogenetic utility in the hypotrichs (Berger 1999; Paiva 2020; Xu et al. 2020). Nevertheless, the functional features and detailed morphology (such as the ultrastructure) of cortical granules are not well known for most hypotrich species. Thus, more studies are necessary to evaluate their systematic relevance.
Another important morphological feature, the oral apparatus, also has phylogenetic significance and provides taxonomic information in hypotrichs. For example, mainly because of the presence of the buccal depression (Foissner and Stoeck 2008), Foissner et al. (2010) upgraded Afrokeronopsis from subgenus to genus level. Using the structure of the oral apparatus, the location of the contractile vacuole, and three ontogenetic features, Heber et al. (2014) revised four distinct psilotrichid genera. Based on the refined interpretation of the shape of the paroral membrane, Kumar and Foissner (2015) proposed the Australocirrus pattern of undulating membranes and suggested synonymy of Cyrtohymenides and Australocirrus. Although the classification based on the Gonostomum-patterned oral structure is widely accepted, phylogenetic relationships are largely unresolved due to under-sampling. To verify the phylogenetic signal of a Gonostomum-patterned oral apparatus, Wang et al. (2021b) increased taxon sampling and provided further analyses.
Despite the distinctive above-mentioned morphological features, in molecular phylogenetic analyses, the support values for the Hypotrichidium-Notohymena-Apoterritricha- Afrokeronopsis-Neokeronopsis-Australocirrus shii clade are low. Thus, as discussed above, more extensive sampling and further investigations are required to more confidently elucidate the evolutionary relationships within this group.
Phylogenetic analyses support the erection of separate families for spirofilids
The phylogenetic trees show that the Hypotrichidium species cluster together, indicating the genus Hypotrichidium is monophyletic at the present state of knowledge, consistent with the unique morphological and morphogenetic characteristics they share. While far from the taxa that used to be classified in Spirofilidae, that is, the slender "tubicolous" spirofilids (Chaetospira Lachmann, 1856, and Stichotricha Perty, 1852) and the highly helical spirofilids (Atractos Vörösváry, 1950), the Hypotrichidium spp. are placed in a main clade of Postoralida, which confirms the establishment of separate families for spirofilids as previously proposed (Bourland 2015; Song et al. 2022).
Systematic positions of Hypotrichidium and Spirofilidae
The type genus of Spirofilidae, Hypotrichidium Ilowaisky, 1921, was established with Hypotrichidium conicum Ilowaisky, 1921 as the type species. In the original description of Hypotrichidium conicum, Ilowaisky (1921) placed it in the hypotrichs, although he realized that the peculiar pyriform body shape and spiral cirral rows distributed around the whole body did not conform to the characteristics commonly used in defining hypotrichs, i.e., body dorsoventrally flattened and cirri only present on ventral side.
Gelei (1929) described a new genus and new species under the name Spirofilum tisiae Gelei, 1929. Rossolimo (1930) was the first to recognize the similarity of the two forms described by Ilowaisky (1921) and Gelei (1929), and concluded that these two forms are conspecific, even though there were minor discrepancies in detail. Therefore, according to the rule of priority, Spirofilum became a junior synonym of the genus name Hypotrichidium. Gelei (1954) recognized this in his later paper and agreed that the two forms belong to the same genus, but still defended the form described in 1929 as a separate species of the genus, namely as Hypotrichidium tisiae.
Limited morphological features and the lack of morphogenetic information has led to a very confused systematic history for Hypotrichidium, that is, the genus has been classified by various authors in different families and higher taxa. Hypotrichidium was originally placed in the family Oxytrichidae Ehrenberg, 1830 by Ilowaisky (1921) without explaining his reasoning. As the genus Spirofilum (synonym of Hypotrichidium) showed the features of spiral cirral rows, and neither as many cirri as in Oxytrichidae nor so few cirri as in Euplotidae, a new family, Spirofilidae, was established by Gelei (1929) for the genus and the following diagnosis was provided: body ovate in shape; cirri distributed both on frontal and middle body; only right marginal cirri present; cirri of the rear part in spiral rows with some on dorsal side; free swimming, not benthic; two macronuclear nodules.
Kahl (1932) and Dingfelder (1962) placed Hypotrichidium in the Oxytrichidae. Corliss (1961) classified Hypotrichidium in Oxytrichidae, but he held the opinion that Spirofilidae was possibly a valid family name. Fauré-Fremiet (1961) erected two new suborders, namely Stichotrichina and Sporadotrichina (elevated to orders Stichotrichida and Sporadotrichida, respectively according to Lynn [2008]). Fauré-Fremiet (1961) also established a new stichotrichid family Strongylidae (emended by Tuffrau [1972] to Strongylidiidae) with Hypotrichidium included, which was accepted by Tuffrau (1972) and Stiller (1975). However, Borror (1972) recognized the family Spirofilidae (including Hypotrichidium) and considered Strongylidiidae to be a junior synonym. Jankowski (1975) established a new family Hypotrichidiidae for Hypotrichidium, followed by his later revision of the order Hypotrichida (Jankowski, 1979). Corliss (1979) accepted both Spirofilidae and Strongylidiidae, and stated that "although the generic name Spirofilum must, by the law of priority, fall as a junior synonym of the older Hypotrichidium, it is perfectly legitimate to maintain the familial name founded on it: thus Spirofilidae is correct, and Jankowski's (1975) recent "replacement" name of Hypotrichidiidae must be treated as a (junior) synonym." The classification of Hypotrichidium by Corliss (1979) was subsequently accepted by most subsequent studies, such as Dragesco and Dragesco-Kernéis (1986), Tuffrau (1987), Tuffrau and Fleury (1994), Lynn and Small (2002), Jankowski (2007), and Lynn (2008).
The Spirofilidae has a confused nomenclatural history as mentioned above. The genera assigned to the family have also been changing since it was established. Gelei (1929) erected the family Spirofilidae as monotypic. In the first revision which recognized the Spirofilidae, seven genera were assigned in the family (Borror 1972). Subsequently, Corliss (1979) classified nine genera to the Spirofilidae, including Kahliela Tucolesco, 1962 as incertae sedis therein. According to Tuffrau (1987) and Tuffrau and Fleury (1994), eight and nine genera were assigned to the Spirofilidae, respectively. Lynn and Small (2002) placed eight genera in the Spirofilidae, while Jankowski (2007), apparently accepting Corliss's previously mentioned synonymization of Hypotrichidiidae, placed ten genera in the family Spirofilidae.
In the most recent and extensively accepted revision of ciliates, Lynn (2008) retained the placement of Hypotrichidium in Spirofilidae, Stichotrichida, together with another 11 genera (Atractos Vörösváry, 1950; Chaetospira Lachmann, 1856; Microspiretta Jankowski, 1975; Mucotrichidium Foissner et al., 1990; Parastrongylidium Fleury & Fryd-Versavel, 1985; Pelagotrichidium Jankowski, 1978; Planitrichidium Jankowski, 1979 [nomen nudum]; Spirofilopsis Corliss, 1960; Stichotricha Perty, 1849; Strongylidium Sterki, 1878; Urostrongylum Kahl, 1932) and included one genus incertae sedis (Kahliela). Because detailed morphological, morphogenetic, and molecular data are not available for most of the spirofilid species, including the type species of Hypotrichidium (H. conicum), the systematics and phylogeny of Spirofilidae remain unresolved. Recent investigations have questioned the monophyly of Spirofilidae and some modifications of the family have been made. Xu and Lei (2007) added a new spirofilid genus Metastrongylidium Xu & Lei, 2007, which is considered as a junior synonym of Spiroamphisiella Li et al., 2007 and was assigned to the family Amphisiellidae Jankowski, 1979 by Berger (2008). The genus Pseudouroleptus Hemberger, 1985, originally included in the Amphisiellidae Jankowski, 1979 by Hemberger (1985), was assigned to the Spirofilidae by Chen et al. (2015), but was transferred to the reactivated family Strongylidiidae, together with the genus Strongylidium Sterki, 1878, in the recent study of Luo et al. (2018). Bourland (2015) proposed a new family Atractosidae for the genus Atractos based on its distinctive morphological features (i.e., the presence of a dorsomarginal kinety) and morphogenetic characteristics (i.e., the unusual de novo mode of dorsal kinety replication), together with the phylogenetic analyses. In the same study, Bourland (2015) also stated that the study clearly separated Stichotricha aculeata from other spirofilids. Most recently, Song et al. (2022) removed Chaetospira and Stichotricha to the family Chaetospiridae Jankowski, 1985 based on their shared morphological features (i.e., flask-shaped body, oral region extending along narrow anterior neck region, two spiral ventral and marginal cirral rows) and their close relationship in phylogenetic trees. Therefore, eight genera (Hypotrichidium, Microspiretta, Mucotrichidium, Parastrongylidium, Pelagotrichidium, Planitrichidium, Spirofilopsis, Urostrongylum) together with Kahliela, an incertae sedis genus, are still included in the Spirofilidae. However, only two sequences from one genus, i.e., Hypotrichidium paraconicum (reported by Chen et al. 2013) and Hypotrichidium conicum (submitted directly to GenBank database without morphological description), are available for the eight above-mentioned spirofilid genera. The position of Hypotrichidium was rather poorly resolved, reflected by the low support for nodes and partially incongruent topologies in trees constructed by different methods. Nevertheless, the phylogenetic analyses suggested some relationship between Hypotrichidium and the oxytrichids (Chen et al. 2013).
Our present study reveals a similar phylogeny to that shown in Chen et al. (2013), i.e., a close relationship between Hypotrichidium (the type genus of Spirofilidae) and the oxytrichids (the main group of Postoralida). Based on integrative analyses of morphology, morphogenesis, and phylogeny, the inclusion of Hypotrichidium within the Postoralida is strongly supported (for details, see above sections). Moreover, as none of the three orders, Stichotrichida, Sporadotrichida, or Urostylida Jankowski, 1979 (the basic delineation of Lynn's system), accurately reflects the evolutionary history of the hypotrichs as recovered by previous studies (Ma et al. 2021a, b; Paiva 2020; Vďačný and Foissner 2021), we prefer to follow the system recently proposed for Hypotrichia by Paiva (2020). Therefore, the Spirofilidae should be classified in Postoralida instead of Stichotrichida.
Materials and methods
Sample collection, observation, and terminology
The species Hypotrichidium tisiae was isolated from a freshwater sample which was collected on 23 May 2017 from a small temporary puddle in Ann Morrison Park (43°36′25″N; 116°12′57″W), Boise, Idaho, USA. Specimens were cultivated according to Chen et al. (2021). Although attempts at establishment of clonal cultures were unsuccessful, no other hypotrichs of a similar morphotype were present in the protargol or SEM preparations, indicating that our morphological, morphogenetic, and molecular studies deal with one and the same species.
Studies of live cells, counts and measurements of protargol-stained specimens, and drawings were carried out according to Bourland (2015). Protargol staining used to display the ciliature and nuclear apparatus was according to Wilbert (1975). Specimens for SEM were prepared according to Luo et al. (2021). Terminology is mainly according to Berger (1999).
DNA extraction, gene sequencing, and phylogenetic analyses
Single cells (three in total) of Hypotrichidium tisiae were selected from raw cultures, washed with filtered (0.22 µm) habitat water, and placed in separate PCR tubes containing 25 μl EB buffer (Qiagen, Valencia, CA, USA). DNA extraction, PCR and sequencing of purified PCR products were carried out according to Luo et al. (2021). Contigs were assembled using Seqman V. 7.1.0 (DNAStar).
The 18S rRNA gene sequences of the hypotrich representatives and sequences of four euplotids as outgroup taxa were selected and downloaded from the National Center for Biotechnology Information (NCBI) Database (https://www.ncbi.nlm.nih.gov/). Taxon names and accession numbers are given in Fig 6 and Supplementary Table S1. The sequence alignment, Bayesian inference (BI) analysis, and maximum likelihood (ML) analysis were carried out according to Xu et al. (2022). The final alignment used for phylogenetic analyses included 99 sequences and 1782 positions. The phylogenetic trees were visualized with MEGA 7.0 (Kumar et al., 2016). According to Hillis and Bull (1993), bootstrap values ≥ 95%, 70–94%, and < 70% were considered as high, moderate, and low, respectively. While for posterior probabilities, values ≥ 0.95 are regarded as high and ≤0.94 as low (Alfaro et al. 2003). The systematic classification follows Adl et al. (2019) and Paiva (2020).
This work was supported by the National Natural Science Foundation of China (Nos. 31900319, 32030015, 32070432), the National Key Research and Development Program of China (No. 2018YFD0900701), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2019333). We thank Prof. Xiaozhong Hu, Ocean University of China (OUC), for his helpful suggestions regarding species identification, and Prof. Weibo Song, OUC, for his comments on the manuscript.
Author contributions
W.A.B: conceived and guided the study. X.Luo: collected samples and carried out the laboratory work. X.Lu: was responsible for the molecular phylogenetic analyses. X.Luo: drafted the manuscript. J.H, H.M, Y.L, X.Lu, and W.A.B: made further revisions. All authors read and approved the final version of manuscript
Data availability
The small subunit ribosomal RNA gene sequences generated during the present study have been deposited to GenBank with accession number: ON117314–ON117316 (https://www.ncbi.nlm.nih.gov/nuccore). The other data generated or analyzed during this study are included in this manuscript and supporting files.
Declarations
Conflict of interest
The authors declare no conflicts of interests.
Animal and human rights statement
No animal and human rights are involved in this article.
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Table
1.
Morphometric data for Hypotrichidium tisiae based on the Boise population
Characteristics
Min
Max
Mean
Med
SD
CV
n
Body length
107
142
124.4
126.0
9.2
7.4
25
Body width
77
100
87.4
87.0
5.4
6.2
25
Body width: body length (%)
63
80
70.4
69.8
4.1
5.8
25
Buccal cavity, length
53
77
64.8
64.0
6.0
9.2
25
Buccal cavity length: body length (%)
44
62
52.2
51.6
4.5
8.6
25
Adoral membranelles, number
32
44
39.6
40.0
2.8
7.1
25
Frontal cirrus, number
1
1
1.0
1.0
0.0
0.0
25
Frontoventral cirral rows, number
3
3
3.0
3.0
0.0
0.0
25
Cirri in frontoventral cirral row 1, number
8
17
10.7
10.0
2.8
26.1
23
Cirri in frontoventral cirral row 2, number
10
18
13.1
13.0
2.3
17.6
24
Cirri in frontoventral cirral row 3, number
11
19
15.5
15.0
1.6
10.4
25
Extra cirral row, number
0
1
0.0
0.0
0.2
500.0
25
Cirri in extra cirral row, number
9
9
9.0
9.0
0.0
0.0
1
Postoral ventral cirri, number
19
26
22.9
23.0
1.5
6.5
24
Cirri in anterior part of right marginal cirral row, number
12
18
15.3
15.0
1.4
9.0
25
Cirri in posterior part of right marginal cirral row, number
20
29
24.9
25.0
2.5
9.9
25
Left marginal cirral rows, number
2
2
2.0
2.0
0.0
0.0
19
Cirri in left marginal cirral row 1, number
17
23
19.9
20.0
1.6
8.0
10
Cirri in left marginal cirral row 2, number
23
28
25.7
26.0
1.6
6.2
19
Caudal cirral rows, number
2
2
2.0
2.0
0.0
0.0
25
Cirri in caudal cirral row 1, number
26
34
29.0
29.0
2.3
8.1
21
Cirri in caudal cirral row 2, number
23
32
27.6
28.0
2.4
8.8
25
Dorsal kineties, number
3
3
3.0
3.0
0.0
0.0
25
Dikinetids in dorsal kinety 1, number
10
20
16.4
17.0
2.4
14.5
21
Dikinetids in dorsal kinety 2, number
13
20
15.9
16.0
2.1
13.3
21
Dikinetids in dorsal kinety 3, number
12
18
15.0
15.0
1.7
11.5
21
Macronuclear nodules, number
2
2
2.0
2.0
0.0
0.0
25
Anterior macronuclear nodule, length
23
35
27.4
28.0
3.1
11.2
25
Anterior macronuclear nodule, width
13
20
16.0
15.0
1.7
10.9
25
Posterior macronuclear nodule, length
22
37
29.8
30.0
4.3
14.3
25
Posterior macronuclear nodule, width
14
23
19.2
19.0
2.1
10.7
25
Micronuclei, number
2
3
2.4
2.0
0.5
20.8
20
Micronuclei, length
3.6
5
4.3
4.3
0.3
7.0
25
Micronuclei, width
3
4.9
3.9
4.0
0.5
12.1
25
All data are based on randomly selected protargol-stained specimens. Measurements in μm CV coefficient of variation in %, Max maximum, Mean arithmetic mean, Med Median, Min minimum, n number of specimens observed, SD standard deviation
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