Hemiholosticha pantanalensis, Vďačný & Foissner, 2019

Vďačný, Peter & Foissner, Wilhelm, 2019, Morphology and Ontogenesis of Hemiholosticha pantanalensis nov. spec. (Ciliophora, Hypotrichia, Psilotrichidae), Acta Protozoologica 58 (3), pp. 93-113 : 103-106

Ontogenesis of Hemiholosticha pantanalensis

Division mode: Binary fission is homothetogenic and occurs in freely motile condition. Stomatogenesis is apokinetal and the parental oral structures are not involved in the formation of the daughter oral ciliature, although the parental undulating membranes are reorganised.

Adoral zone of membranelles and undulating membranes: Stomatogenesis commences with de novo (apokinetal) proliferation of basal bodies close to the posterior postoral cirrus, i.e., very near to the buccal vertex ( Figs 6A, B , 8A, B ). The growing anarchic field soon invaginates into a deep pouch which contains densely and irregularly arranged protomembranellar basal bodies. The growing cilia of the protomembranelles are thus covered by the cortex and only some are visible through the ovate opening of the pouch in the SEM micrographs ( Fig. 8C, D ). However, the parental postoral cirrus and some densely arranged basal bodies bearing short cilia do not sink into the pouch but remain on its right margin ( Fig. 8D , arrow). According to protargol preparations, these narrowly spaced basal bodies are very likely derived from the right anterior portion of the oral primordium and will form a long dikinetal streak, i.e., the anlage for the undulating membranes of the opisthe ( Fig. 6C, D, F , arrows). In the meantime, the protomembranelles start to differentiate from right to left in a posteriad direction. The new adoral zone extends anteriorly between the parental adoral zone and the left marginal cirral row ( Fig. 6C, D, F ). The distal adoral membranelles evaginate on the cell surface while the proximal membranelles are still forming inside the pouch ( Fig. 9A–D). The opisthe’s adoral zone is fully developed before macronuclear fusion in mid-dividers ( Fig. 6F ). Formation of the new paroral and the new endoral membrane is, however, accomplished in mid-dividers by splitting the opisthe’s undulating membrane anlage ( Fig. 7A, C ). Both membranes attain their specific morphology and position in late dividers and/or post-dividers ( Figs 7E, G , 9C, D, 10A, B ).

The parental adoral zone does not show any sign of reorganization. On the other hand, reorganization of the parental undulating membranes commences already in early dividers, i.e., after evagination of opisthe’s anteriormost adoral membranelles ( Figs 6F , asterisk, 9A, B, arrows). In mid-dividers, the new proter’s undulating membranes are arranged in parallel whereby the endoral membrane begins in mid of buccal cavity resembling the Stylonychia pattern ( Fig. 7A, C ). The species specific Cyrtohymena -like pattern is obtained in late dividers ( Fig. 7G ).

Cirral streaks: Five streaks of basal bodies develop both in proter and opisthe. In the proter, streak I forms near the posterior cirrus of parental ventral row R1; streak II appears near the posterior cirrus of parental ventral row R2; and streaks III–V develop within parental ventral row R3. In the opisthe, streaks I and II appear anterior to the opisthe’s undulating membrane anlage and are possibly derived from the oral primordium ( Figs 6C, D , 8D ); streak III forms very likely by the dedifferentiated parental anterior postoral cirrus; and streaks IV and V develop within parental ventral row R3 slightly posterior to proter’s streaks III–V. All streaks elongate posteriorly and start to produce cirri already in early dividers ( Fig. 6C, D, F ). Streak I generates two cirri of ventral row R1; streak II forms two cirri of ventral row R2; streak III produces two cirri migrating leftwards and posteriorly to form the postoral row; streak IV forms three cirri of the anterior portion of ventral row R3; and streak V forms the remainder cirri of the posterior portion of row R3, as indicated by a break in this row in some mid-dividers ( Fig. 7A , arrows) as well as by the different spacing of the three or four anteriormost cirri in vegetative cells ( Fig. 1B, E ). The new cirral rows migrate to their species-specific positions in late mid-dividers and late dividers. The majority of parental cirri becomes resorbed in late dividers and early post-dividers ( Fig. 7A, C, E, G ).

Marginal and dorsal anlagen: Formation of new marginal cirral rows and dorsal kineties proceeds as usual for psilotrichids. The proter’s left and right marginal row anlagen develop at the anterior end of the parental marginal cirral rows, very likely by dissociation of their anteriormost cirri. The opisthe’s left and right marginal row anlagen are formed within the parental marginal cirral rows at about level of the oral primordium ( Fig. 6C, D ). The marginal row anlagen extend posteriorly, gradually producing new cirri already in early dividers ( Fig. 6F ). The parental marginal cirri are almost completely resorbed and replaced by new cirri during the mid- and late division stages ( Fig. 7A, C, G ).

Morphogenesis of the dorsal side ciliature begins already in early dividers. Within-row primordia first appear in the third dorsal kinety at two sites, viz., anterior and posterior to the prospective fission area, separated by two parental bristles that are resorbed later ( Fig. 6E ). The primordia of the first and second dorsal kineties develop later on and are produced in a similar way, i.e., by proliferation of basal bodies at two sites ( Fig. 6G ). The new dorsal kineties get their final length in mid-dividers when most parental dorsal bristles are already resorbed ( Fig. 7B, D ).

Nuclear division: The nuclear apparatus divides as typical for bimacronucleate hypotrichs. Specifically, each macronuclear nodule shows a replication band during the early stages of ontogenesis. In mid-dividers, both nodules fuse into a centrally located mass ( Fig. 7B ) which elongates ( Fig. 7D ) and divides into two oblong and pointed pieces ( Fig. 7H ). Each piece divides once in post-dividers, producing two macronuclear nodules ( Fig. 7F ).

The micronucleus divides only once after fusion of the macronuclear nodules during the middle divisional stages. The daughter micronuclei are connected by a fibre bundle that elongates in late mid-dividers ( Fig. 7B, D ) and disappears in late dividers ( Fig. 7H ). The micronucleus moves to the species-specific position in post-dividers when the two macronuclear nodules have settled ( Fig. 7F ).

Figures

Fig. 6. A–G. Hemiholosticha pantanalensis nov. spec., ciliary pattern and nuclear apparatus of early dividers after protargol impregnation. Parental cirri depicted by contour, new ones shaded black. Arrow in (C, D, F) marks the undulating membrane anlage; arrowhead in (A, F) denotes the posterior postoral cirrus, and asterisk in (F) marks the reorganizing parental undulating membranes. A. Ventral view of a very early divider, showing the oral primordium close to the posterior postoral cirrus (arrowhead), i.e., just posterior of the buccal vertex. B. Detail of the oral primordium growing around the posterior postoral cirrus. The oral primordium develops in a deep pouch, as typical of psilotrichids, and consists of densely and irregularly arranged basal bodies that will produce the opisthe’s adoral zone of membranelles. C. Ventral view, showing that the opisthe’s undulating membrane anlage (arrow) develops from the oral primordium. Five cirral streaks appear in the opisthe: the first two are possibly derived from the oral primordium, the third streak forms near the anterior cirrus of the post- oral row, and streaks IV and V develop within ventral cirral row R3. At this stage, the proter’s right marginal cirral row anlage appears at the anterior end of the parental row, very likely by dissociation of its anteriormost cirri. The opisthe’s right marginal cirral row anlage is formed within the parental row at the level of the oral primordium. D–G. Ventral and dorsal views, showing five cirral streaks developing both in proter and opisthe. Formation of new marginal cirral rows and of dorsal kineties proceeds as usual for psilotrichids. Specifically, the proter’s left and right marginal cirral row anlagen develop at the anterior end of the parental marginal cirral rows. The opisthe’s left and right marginal cirral row anlagen are formed within the parental marginal cirral rows at the level of the oral primordium. The marginal cirral row anlagen gradually produce new cirri posteriorly. Morphogenesis of the dorsal ciliature begins by formation of within-row primordia in the third dorsal kinety at two sites, anterior and posterior to the fission area, separated by two parental bristles (E). The primordia of the first and second dorsal kinety develop later on and are produced in a similar way, that is, by proliferation of basal bodies at two sites which are separated by some parental bristles. The new dorsal kineties reach their final length and structure in mid-dividers (cp. Fig. 6G with Fig. 7B, D). I–V – cirral streaks; ADK3 – anlagen of dorsal kinety 3; ALMR – anlagen of left marginal cirral row;ARMR – anlagen of right marginal cirral row; DK1–3 – dorsal kineties; MA – macronuclear nodules; MI – micronucleus; OP – oral primordium. Scale bars: 20 µm.

Fig. 8. A–D. Hemiholosticha pantanalensis nov. spec., early dividers in the SEM. A, C. Ventral overviews. B, D. Details of specimens shown in (A, C). The oral primordium develops near the posterior postoral cirrus and soon invaginates into a pouch. Arrow in (D) marks some short cilia that do not sink into the pouch but remain on its right margin very likely forming the opisthe’s undulating membranes.AZM – adoral zone of membranelles; CS I – cirral streak I; LMR – left marginal cirral row; OP – oral primordium; PM – paroral membrane; POC – posterior postoral cirrus; R3 – ventral cirral row 3; RMR – right marginal cirral row. Scale bars: 5 µm (B, D) and 20 µm (A, C).

Fig. 7. A–H. Hemiholosticha pantanalensis nov. spec., ciliary pattern and nuclear apparatus of mid-dividers (A–D), of a late divider (G, H), and of a post-divider (E, F) after protargol impregnation. Arrow in (A) indicates that ventral cirral row R3 forms from anlagen IV and V. I–V – cirral streaks; DK1–3 – dorsal kineties; F – fibres; LMR – left marginal cirral row; MA – macronucleus; MI – micronucleus; POR – postoral cirral row; R1–3 – ventral cirral rows; RMR – right marginal cirral row. Scale bars: 20 µm.

Fig. 10.A, B. Hemiholosticha pantanalensis nov. spec., a late divider in the SEM. A. Ventral overview. B. Detail of opisthe’s oral apparatus. The adoral zone is already evaginated and almost fully developed, i.e., consists of long new anterior membranelles and of short posterior membranelles. The paroral membrane has, however, not got the cyrtohymenid C-shape. AME – anterior membranelles; OAZM – opisthe’s adoral zone of membranelles; OLMR – opisthe’s left marginal row; OPM – opisthe’s paroral membrane; PAZM – proter’s adoral zone of membranelles; PME – posterior membranelles. Scale bars: 10 µm (B) and 20 µm (A).

Fig. 1. A–K. Hemiholosticha pantanalensis nov. spec. from life (A, G–K) and after protargol impregnation (B–F). Arrowhead in (A, B, E, G, H) marks the anterior body projection. A. Ventral view of a representative specimen. B, C, E, F. Ventral and dorsal view of ciliary pattern and nuclear apparatus in holotype (B, C) and in a paratype (E, F) specimen. D. Semi-schematic diagram of a dorsal kinety. G, H. Shape variants. I. Cross section, showing three prominent dorsal ribs (arrows). J. Intracellular green algae contain a subapical red eyespot, starch granules, and a big chloroplast. K. Division of an intracellular green alga yields four tightly packed daughter cells. AZM – adoral zone of membranelles; CH – chloroplast; DB – dorsal bristles; DK1–3 – dorsal kineties; DI – dorsal dikinetids; EM – endoral membrane; LMR – left marginal cirral row; MA – macronuclear nodules; MI – micronucleus; PM – paroral membrane; POR – postoral cirral row; R1–3 – ventral cirral rows; RMR – right marginal cirral row; SG – starch granules; ST – stigma. Scale bars: 20 µm (B, C, E, F) and 40 µm (A).