Lepidometopus platycephalus, Vď’Ačný & Foissner, 2017

Vď’Ačný, Peter & Foissner, Wilhelm, 2017, A Huge Diversity of Metopids (Ciliophora, Armophorea) in Soil from the Murray River Floodplain, Australia. II. Morphology and Morphogenesis of Lepidometopus platycephalus nov. gen., nov. spec., Acta Protozoologica 56 (1), pp. 39-57 : 47-51

Morphogenesis of Lepidometopus platycephalus

Division mode: Binary fission is homothetogenic and occurs in freely motile (non-encysted) condition. Body shape changes drastically. Stomatogenesis is pleurotelokinetal. The parental oral structures are reorganized but are not involved in the formation of the daughter oral ciliature.

Body changes: Early dividers are 48–55 × 23– 25 µm in size, i.e., they are slightly larger than mor- phostatic specimens and gradually loose the reniform and rhomboid appearance, becoming Metopus -shaped ( Fig. 6A–H). The cell portion carrying the prospective adoral polykinetids transforms into a small bulge recognizable in lateral view ( Fig. 6E, F). On the other hand, the parental oral area is still unchanged, i.e., the preoral dome is strongly flattened and distinctly projects from body proper ( Fig. 6C).

In mid-dividers, the body slightly shortens and conspicuously broadens to 45–50 × 30–38 µm. These cells are thus the stoutest and shortest dividers. Body shape drastically changes: (1) the outline becomes broadly elliptic, (2) the ventral side becomes strongly inflated, and (3) the preoral dome turns into a small, rounded protuberance projecting from the left anterior body margin ( Fig. 7A–F).

Just before separation, the daughter cells are broad- ly ellipsoid without any sign of a preoral dome ( Fig. 8A, B). However, fundamental changes in cell shape and size take place after division: (1) the body intensively grows from about 33 × 23 µm to about 55 × 22 µm, (2) the anterior portion of the cell flattens, and (3) the preoral dome pulls out of the cell to roof the adoral zone by twisting leftwards taking along the perizonal stripe which thus obtains the typical Γ-shaped pattern ( Fig. 8C–H). Nevertheless, late post-dividers are still very dissimilar from morphostatic cells. They are oblong, not or only slightly twisted anteriorly ( Fig. 8G, H), and longer than morphostatic specimens (on average 55 × 22 µm vs. 38 × 20 µm in protargol preparations). This indicates that their further development must be associated with body shortening, possibly as a consequence of massive remodelling to a reniform/rhomboid shape.

Development of adoral zone: The formation of the opisthe’s adoral zone is associated with two concomitantly proceeding events taking place in early dividers: (1) proliferation of dikinetids (protopolykinetids) in the posterior portion of about six dorsal and dorsolateral kineties ( Fig. 6A–G, arrowheads) and (2) the production of dikinetids in the anterior portion of about three postoral kineties, i.e., slightly posterior to the proximal end of the perizonal stripe ( Fig. 6A, C, E, G, asterisks). In early mid-dividers, the newly formed dikinetids detach from the somatic ciliary rows and migrate to assemble the opisthe’s adoral zone ( Fig. 6I). The new adoral pol- ykinetids are thus migrating kinetofragments composed of two long rows of basal bodies. The third long row and the short row making the polykinetids L-shaped are mostly added in post-dividers ( Fig. 8C, G).

Reorganization of the parental adoral zone begins in early mid-dividers. The polykinetids of the proter become smaller and loose the interphase L-shaped pattern, very likely due to the resorption of one long row and the short row of basal bodies ( Fig. 6I). Taking into account that the adoral zone of proter and opisthe has a similar morphology in mid-dividers and late dividers ( Figs 7A, C, E, 8A), the proter’s adoral zone must ob- tain the species-specific pattern also post-divisionally.

Development of perizonal stripe and paroral membrane: During the formation of the new adoral polykinetids, an intrakinetal proliferation of kinetids commences in the parental perizonal rows which thus elongate posteriorly into the glabrous area between the opisthe’s adoral zone and the posterior end of the split parental somatic kineties ( Fig. 6E, G, I). In mid-dividers, the posterior portion of perizonal rows 1 and 2 disorders and the resulting dikinetids migrate along the new adoral zone to assemble the opisthe’s paroral membrane ( Fig. 7A, C, E, asterisks); perizonal rows 3–5 remain ordered and their posterior portion becomes perizonal rows 1’–3’ in the opisthe. Opisthe’s perizonal rows 4’ and 5’ are formed from the posterior half of the first two dorsolateral kineties which migrate towards perizonal rows 1’–3’ in very late mid-dividers and late dividers ( Figs 6I, 7A, C, E, arrows). The loss of these two dorsolateral kineties is compensated by the formation of one or two ciliary rows left of the opisthe’s adoral zone ( Fig. 7A, C, E, arrowheads).

In connection with body re-shaping, the parental perizonal stripe looses the Γ-shape and moves from dorsolateral to the right margin of the ventral side; the species-specific pattern is obtained post-divisionally. The parental paroral membrane is entirely disordered in mid-dividers ( Fig. 7A, C, E) and re-assembled in late dividers ( Fig. 8A).

Development of somatic ciliature: After formation of the prospective adoral polykinetids, an intense proliferation of basal bodies begins in all somatic ciliary rows. First, the basal bodies of the individual dikinetids go apart more or less distinctly. Then a new basal body develops in front of the anterior basal body generating a triad. Subsequently, a fourth basal body forms ahead of the posterior basal body producing a tetrad. Eventu- ally, the tetrad splits into two pairs of dikinetids. The replication of dikinetids finishes in mid-dividers, i.e., when all somatic ciliary rows are arranged meridionally and composed of comparatively narrowly spaced dikinetids ( Fig. 7C–F). In late dividers, the somatic ciliary rows split in the middle leaving a barren area at the posterior region of the proter and at the anterior region of the opisthe ( Fig. 8A, B, arrowheads). The somatic diki- netids are still narrowly arranged in early post-dividers ( Fig. 8C, D). Their loose spacing is obtained gradually by growth and patterning of the body ( Fig. 8G, H).

Nuclear division: In early dividers, the macronucleus is as in morphostatic cells, i.e., it is oblong and situated between the anterior and posterior end of the adoral zone ( Fig. 6B, D, F, H). Later on, the macronucleus be- comes larger, rounds up, and migrates to mid-body ( Fig. 6J). In mid-dividers, the centrally located macronucleus becomes broadly fusiform and begins to divide ( Fig. 7B, D, F). In late dividers, the macronucleus is dumbbell-shaped with a conspicuous constriction in the fission area ( Fig. 8 B). After binary fission, the macronucleus is lenticular and pointed ( Fig. 8D). During post-divisional growth, the macronucleus becomes oblong and moves to the species-specific interphase position ( Fig. 8F, H).

When division commences, the micronucleus increases in size from 2.5 µm to 4.0 µm, showing fibrous structures, possibly prophasic chromosomes ( Fig. 6B). The micronucleus further enlarges to about 6 µm and impregnates heterogeneously, which is indicative of the formation of spindle microtubules ( Fig. 6D, H, J). Thus, the division spindle and the metaphase plate made of about eight chromosomes, become distinct already in early dividers ( Fig. 6F). When the macro- nucleus becomes globular, the micronucleus begins to divide ( Fig. 7B). The daughter micronuclei impregnate homogenously and are connected by a fiber bundle that conspicuously elongates in mid-dividers ( Fig. 7D, F). The bundle disappears and the micronucleus achieves the species-specific size already in late dividers ( Fig. 8B). During post-divisional patterning, the micronucle- us moves to the macronucleus ( Fig. 8D, F, H).