Ricksol, Shɨshkin-Skarð & Drachko & Zlatogursky, 2023

Shɨshkin-Skarð, Yegor, Drachko, Daria & Zlatogursky, Vasily V., 2023, Shedding light on the origin of Acanthocystidae: Ricksol blepharistes gen. n., sp. n. (Ricksolidae fam. n., Panacanthocystida, Centroplasthelida), with notes on the evolution of the genera Acanthocystis, Ozanamia gen. n. (Ozanamiidae fam. n.), and “ Heterophrys-like organisms ”, Organisms Diversity & Evolution (New York, N. Y.) 23 (2), pp. 263-274 : 270

Ricksol gen. n., cardioid plate morphotype, and the origin of Acanthocystis

Mikrjukov (1997, Fig. 1 ; 2002, Fig. 8) drew the stage scheme of centrohelid evolution where Acanthocystis originated from the ancestors having feather-shaped external scales through the ancestors having external scales with the cardioid plate: lateral wings located at one end of the scales reduced to the small plate (1); then, the shaft bent for 90° to the plate leaving the incision on it (2); and finally, edges of the incision fused, and the Acanthocystis spine scale with the circular basal plate and the vertical shaft was formed (3). Later, the SSU rRNA genes of Pterocystis (feather-shaped scales) and Ozanamia curvata comb. n. and O. symna comb. n. (cardioid basal plates) were sequenced and placed far from these of Acanthocystis in phylogenetic analyses—in another major clade of centrohelids ( Cavalier-Smith & von der Heyden, 2007; Zlatogursky, 2014; Shɨshkin et al., 2018). Mikrjukov’s idea of the Acanthocystis origin began to seem unlikely—another morphological scheme destroyed by the molecular era in systematics.

Now, we found R. blepharistes (feather-shaped external scales with wings reduced to a small plate) branching before Acanthocystis . Previously, it was shown that the cardioid plate morphotype also occurs in Panacanthocystida ( Fukuda, 2014). So, the main “missing links” (stages 1 and 2 outlined above) returned to the scheme instead of Pterocystis and Ozanamia , which fell out of Panacanthocystida.

Figures

Fig. 1 Morphology of Ricksol blepharistes gen. n., sp. n. a, b General view of the undis- turbed cell in a Petri dish, IMC. c The cells, squished under a coverslip, DIC. d Single scales detached from the cell depicted in (c). Collage (scales from 3 separate images were com- bined in one view; original raw images are available in Figs. S1, S2, and S3) (PC). e General view of the air-dried spine and plate scales. SEM. f Close-up of the air-dried spine and plate scales (SEM). g Close-up of the single plate scale. SEM. h A crenate margin of the plate scale (SEM). i A fragment of the spine scale from a basal plate end (SEM). Abbreviations: a, axopodia; ar, axial rib; bp, basal plate; g, granules (kinetocysts, extrusomes); hp, hemispherical papillae; m, margin; ps, plate scales; se, conical shaft ending; sh, shaft; ss, spine scales. Scale bars: a–e (a and b are in the same scale to show size vari- ability) 10 µm; f 2 µm; g–i 1 µm