Urostomides minimus ( Kahl, 1927 ) Feng, Xiaochen, Zhuang, Wenbao, Li, Ran & Hu, Xiaozhong, 2025

Urostomides minimus ( Kahl, 1927) View in CoL comb. nov.

( Figs 1–7 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 ; Table 1 View Table 1 )

1927 Metopus bacillatus var. minima var. n. —Kahl, Arch. Protistenkd., 57, fig. 17 on p. 152.

1932 Metopus minimus View in CoL sp. nov. ( Metopus bacillatus var. minima Kahl, 1927 )—Kahl, Tierwelt Dtl., 21, fig. 70 on p. 414 (justified emendation of spelling to agree with genus gender when elevated to species’ rank, erroneous designation as ‘spec. n’.).

1962 Metopus minimus View in CoL —Vuxanovici, Nota II. Stud. Cercet. Biol., Ser. biol. anim., 14, fig. 31 on p. 343 (misdated 1935).

Remarks

Kahl (1927, 1932) briefly described the live morphology of this species. Additionally, Vuxanovici (1962) illustrated M. minimus witharoundposteriorbodyend.Subsequently, Jankowski(1964) regarded it as a junior synonym of M. striatus . Nevertheless, due to lack of detailed morphological and molecular data, the taxonomic classification of this species remained unresolved.

Improved diagnosis

Cells 40–70 × 25–40 μm in vivo, shape typically obpyriform with a round posterior end. Preoral dome massive, approximately three-quarters of the length of body. Usually five elongated caudal cilia. Cortical granules colourless, globular, densely spaced in rows. Four perizonal stripe rows of which rows 1–3 arranged in about 52 false kineties. A total of 16–20 somatic kineties, usually including seven dome kineties. Adoral zone composed of 28 membranelles on average. Paroral membrane diplostichomonad, left file shortened anteriorly. Sapropelic freshwater habitat.

Voucher slides

Two slides with protargol-stained voucher specimens (registration numbers: FXC2021043001-01 , -02 ) are deposited in the Laboratory of Systematic Taxonomy at the Ocean University of China, China . Another slide with voucher specimens (registration numbers: FXC2021043001 - 03 ) is deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China .

Description of Chinese population

Body size 40–70 × 25–40 μm, 53 × 31 μm on average in vivo (N = 17); in protargol-stained 57–95 × 32–60 μm, about 76 × 44 μm on average in protargol-stained specimens. Length:width ratio 1.7: 1 on average in vivo, ranging from 1.4: 1 to 2.4: 1. Body shape obpyriform in ventral view, slightly dorsoventrally flattened, posterior body end rounded ( Figs 1A View Figure 1 , 2A, B View Figure 2 ). Preoral dome massive, thick, overhangs left cell margin, occupies about three-quarters of body length in vivo ( Figs 1A View Figure 1 , 2A, B View Figure 2 ). Macronucleus single, globular to ellipsoidal, usually in preoral dome, 18 × 15 μm on average in vivo and 24 × 17 μm on average in protargol-stained specimens, containing 0.4–3 μm diameter nucleoli; one globular micronucleus, about 4 μm in diameter both in vivo and in protargol-stained specimens, usually adjacent to macronucleus ( Figs 1C View Figure 1 , 2H, F View Figure 2 ). Contractile vacuole round, terminal or sub-terminal ( Figs 1A View Figure 1 , 2D View Figure 2 ). Cytoplasm colourless, containing numerous globular (about 1.5 μm across) or polygonal (about 2 μm long) granules ( Fig 2A, B, E View Figure 2 ). Cortical granules globular and colourless in vivo, about 0.8 μm across, densely arranged in rows ( Figs 1E View Figure 1 , 2C, E View Figure 2 ). Swims slowly while rotating on long axis.

Ordinary somatic cilia 6–9 μm long, and perizonal stripe cilia only slightly longer, 8–12 μm long. Excluding perizonal stripe rows, 16–20 somatic kineties in total, including six to eight dome kineties; all somatic kineties composed of dikinetids with both basal bodies bearing cilia, kineties converge posteriorly on a small terminal glabrous area ( Figs 1D View Figure 1 , 2I View Figure 2 ). Approximately five elongated caudal cilia, 25–36 μm long ( Figs 1A View Figure 1 , 2A View Figure 2 ). Perizonal stripe composed of four rows of densely spaced dikinetids with both basal bodies ciliated, dikinetids in rows 1–3 arranged in about 52 false kineties; row 4 shorter than other rows at both ends; rows 1–3 narrowly-spaced, forming a wider gap with row 4 ( Figs 1B, G View Figure 1 , 2F, L, M View Figure 2 ). The space between dome kineties 1 and 2 distinctly wider than the space between dome kinety 1 and perizonal stripe row 4, 10.4 μm and 4.2 μm apart on average, respectively ( Figs 1F View Figure 1 , 2F View Figure 2 ).

Adoral zone comprises about 28 (range 24–35) membranelles, begins at mid-line of ventral surface and extends spirally and anteriad to left margin of dorsal surface, making a <180° turn, adoral membranelles typically rhomboidal, those at mid-portion widest, about 6.4 μm long, composed of three or four rows of basal bodies ( Figs 1C, F, G View Figure 1 , 2F, J, M View Figure 2 ). Paroral membrane diplostichomonad, viz.twoparallelunequalfilesofmonokinetids, commences at the proximal end of the adoral zone, the right file about 25 μm long, the left file about half the length of the right one in protargol-stained specimens ( Figs 1C View Figure 1 , 2G View Figure 2 ).

Morphogenesis ( Figs 3–7 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 )

Temporal relationship of divisional processes We separate the divisional stages mainly in accordance with Vďačný and Foissner (2017) viz., early dividers ( Figs 3 View Figure 3 , 4 View Figure 4 , 5A–D View Figure 5 ), middle dividers ( Figs 5E, F View Figure 5 , 6A, B View Figure 6 ), late dividers ( Figs 5G, H, J–L View Figure 5 , 6C–H View Figure 6 ), and post-dividers.

Body variations Dividers in most stages can be easily distinguished by their different body shapes and decreased swimming speed. Kineties proliferate and the preoral dome changes during the divisional process, with the alteration of body shape from obpyriform in early dividers to elliptical in middle dividers ( Figs 3 View Figure 3 , 4 View Figure 4 , 5A, B, D–F View Figure 5 , 6A, B View Figure 6 ). Late dividers first undergo a period when the opisthe is slightly longer than the proter, then both become nearly the same size before cell division ( Figs 5G, H, J–L View Figure 5 , 6C–F View Figure 6 ).

Development of adoral zone of membranelles Stomatogenesis is pleurotelokinetal, i.e. partial somatic kineties, especially including all four perizonal stripe rows for this species, contribute to the formation of new adoral membranelles. The process initiates in the posterior body, where some somatic kineties, at least including almost all dome kineties, duplicate to form opisthe’s oral primordium 1 (hereafter designated as OOP1), serving as the precursor of the proximal part of new adoral zone of membranelles (less than 10 in number) ( Figs 3D View Figure 3 , 4C, D View Figure 4 ; left circle in Fig. 3E View Figure 3 ). Subsequently, the proliferation of basal bodies occurs in perizonal stripe rows 1–4, and basal bodies in the posterior portion of each row align longitudinally, forming four conspicuous and relatively ordered ‘columns’ ( Figs 3C View Figure 3 , 4C, E View Figure 4 ), and then these four ‘columns’ gradually disengage the parental perizonal rows 1–4, forming a separate field (right circle in Fig. 3E View Figure 3 ; Figs 3G–K View Figure 3 , 4F–I View Figure 4 ). Remarkably, these four columns, a substantial part of which develop into opisthe’s oral primordium 2 (hereafter designated as OOP2), play a vital role in shaping the adoral membranelles of the opisthe.

With the preliminary completion of assembly of the posterior adoral membranelles, OOP2, mainly composed of dikinetids and triads, starts contributing to the formation of anterior adoral membranelles of the opisthe. Initially, the basal bodies in the anterior end of the rightmost column, derived from perizonal row 4, assemble more than two membranelle-like structures. Simultaneously, remaining basal bodies migrate posteriorly and integrate into elongating existing membranelles ( Figs 3E, G, H, I, K View Figure 3 , 4C, E–I View Figure 4 ).

Notably, in later stages, at least three columns of OOP2 reorganize, forming a distinct anarchic field filled with disordered kinetosomes ( Figs 4K View Figure 4 , 5A View Figure 5 ). Moreover, in late early dividers and subsequent middle dividers, some kinetosomes undergo rearrangement, resulting in an increase in the number of opisthe’s adoral membranelles to reach the average number at the interphasic stage (about 28). Also, a proportion of kinetosomes migrate in different directions, with two results: elongation of the newly formed membranelles to the species-specific length and generating two opisthe’s postoral kineties (ventral kinety terminating anteriorly at the posterior border of the general oral region). This process maintains the number of interphase opisthe’s somatic kineties, as two parental somatic kineties have been involved in the formation of opisthe’s perizonal stripe rows 4 and 5. ( Figs 4M View Figure 4 , 5D, E View Figure 5 , 6A View Figure 6 ).

The dedifferentiation of the parental adoral zone commences in the late early dividers, completing as division progresses, with the reorganization of its kinetosomes occurring concomitantly ( Figs 4K, M View Figure 4 , 5A, D, E, G, J, K View Figure 5 , 6A, C–E View Figure 6 ). Additionally, there are late stages during which a number of kinetidal fragments exist near the proximal portion of proter’s adoral membranelles ( Figs 5G, J View Figure 5 , 6C–E View Figure 6 ). Before cytokinesis, the proter’s and opisthe’s adoral membranelles are rectangular, rather than rhomboidal at interphase ( Figs 5G, H, J–L View Figure 5 , 6C–F View Figure 6 ).

Development of perizonal stripe and paroral membrane The anlage of opisthe’s paroral membrane emerges in late early dividers when four perizonal stripe rows lengthen to the right of the newly formed adoral membranelles ( Figs 4L View Figure 4 , 5A View Figure 5 ). Its posterior part is formed by several dome kineties (at least DK1–3) and parental perizonal rows 3 and 4 ( Figs 4L View Figure 4 , 5A View Figure 5 ). And then in middle dividers ( Figs 4L, M View Figure 4 , 5A, D, E View Figure 5 , 6A View Figure 6 ), its anterior part is derived from the kinetids in the posterior half of parental perizonal rows 1 and 2, which finally form the majority of the new paroral membrane. Regarding the parental paroral membrane, it begins to dedifferentiate in early dividers to form the anlage of proter’s paroral membrane ( Figs 3I View Figure 3 , 4I View Figure 4 ), and subsequently maintains two loose rows of kinetids ( Figs 3K View Figure 3 , 4K View Figure 4 , 5A, D, E, G View Figure 5 , 6A, C, D View Figure 6 ). Till late dividers, the anlagen of both proter and opisthe transform into a single long row of monokinetids (about 40 μm) successively ( Figs 5J View Figure 5 , 6E View Figure 6 ), and then both become a single row of dikinetids ( Fig. 5K View Figure 5 ). In very late dividers, proter’s paroral membrane comprises two rows of monokinetids with similar lengths ahead of opisthe’s paroral, while opisthe’s paroral still looks like a single row of dikinetids ( Fig. 6G View Figure 6 ). It is equally reasonable that the special configuration of paroral membrane of daughter cells, i.e. two rows of monokinetids, is entirely obtained after cytokinesis.

The proliferation of the parental perizonal stripe first occurs in early dividers, slightly later than that of other somatic kineties ( Figs 3C View Figure 3 , 5C View Figure 5 ). As above mentioned, the four stripe rows perform two functions, i.e. contributing to the formation of the new adoral membranelles and the new paroral membrane. Besides, after these stripe rows participate in the formation of oral apparatus, integrated perizonal rows 3 and 4 migrate leftward to become opisthe’s perizonal rows 1 and 2. And then the posterior half of the original dome kineties 1 and 2 shift and transform into opisthe’s perizonal rows 3 and 4. Eventually, in later stages, these newly formed four stripe rows completely disconnect from the proter’s rows 3–4 and dome kineties 1–2 ( Figs 5E, H, J View Figure 5 , 6A, C–E View Figure 6 ). Specifically, the number of the opisthe’s somatic kineties remains unchanged, maybe because the kinetofragments beside the opisthe’s adoral zone generate new postoral kineties ( Figs 4M View Figure 4 , 5D, E View Figure 5 , 6A View Figure 6 ). The proter’s perizonal stripe undergoes replication and rearrangement at its initial position. Before separation, basal bodies of both proter’s and opisthe’s perizonal stripe transform into normal dikinetids ( Figs 5K View Figure 5 , 6H View Figure 6 ).

Development of remaining somatic kineties The replication of somatic kineties, excluding the perizonal stripe, commences in very early dividers, and each pair of dikinetids forms a triad or tetrad ( Figs 3A, B View Figure 3 , 4A, B View Figure 4 ). Meanwhile, these triads and tetrads are arranged longitudinally, without equidistant spaces; even sometimes, several dikinetids, triads, or tetrads are aligned much more closely ( Figs 3C–G, I, J View Figure 3 , 4C, D View Figure 4 ). Intriguingly, somatic kineties (excluding perizonal rows) of early late dividers, appear to form monokinetidal kineties ( Figs 5G, H, J View Figure 5 , 6D, E, F View Figure 6 ). Before the cytokinesis, these kinetids become dikinetids via proliferation ( Figs 5K, L View Figure 5 , 6H View Figure 6 ).

Nuclear division The macronucleus gradually increases in size in early dividers ( Figs 3J View Figure 3 , 4J View Figure 4 , 5C View Figure 5 ), assuming a conspicuous oblong shape in middle division ( Figs 5F View Figure 5 , 6B View Figure 6 ). In late dividers, it becomes dumb bell-shaped ( Figs 5H View Figure 5 , 6C–F View Figure 6 ). Finally, it divides into two new macronuclei during cytokinesis.

The micronucleus was observed in only a few dividers. In early division, the micronucleus divides into two daughter micronuclei united by a thin strand ( Figs 4J View Figure 4 , 5C View Figure 5 ).

Reorganization ( Figs 5I, M View Figure 5 , 6I View Figure 6 )

Only a limited number of reorganizers were observed; however, it is evident that reorganization occurs in a manner similar to that of the proter during binary fission, i.e. (i) the paroral membrane reconfigures, and its species-specific pattern, viz. two single rows of monokinetids, is obtained from a single row of dikinetids at early stages of this process; and (ii) the perizonal stripe and adoral zone of membranelles are renewed ( Figs 5I, M View Figure 5 , 6I View Figure 6 ).