Kulikovia spp

Yurchenko, Olga V. & Chernyshev, Alexey V., 2022, Sperm morphology and some aspects of acrosomal complex development in four species of Heteronemertea (Pilidiophora, Nemertea), Zoologischer Anzeiger 299, pp. 38-48 : 42

3.2. Sperm morphology in Kulikovia spp .

Unlike M. bella , the spermatozoa of the studied species of the genus Kulikovia have shorter bullet-like sperm heads of the ect-aquasperm type ( Fig. 2A, C, E ). In sections, each spermatozoon contains an acrosomal complex, a nucleus, and a short midpiece with mitochondria ( Fig. 2B, D, F ). A morphometric study of the spermatozoal organelles has revealed different sizes of nucleus and acrosomal complex between the studied species and shown their species-specific shapes ( Table 2). Nevertheless, in all studied Kulikovia species, several common general features in sperm organization have been identified: a small, single-vesicle, anteriorly located acrosomal complex, a straight, cone-shaped nucleus, five mitochondria in the posterior region of the nucleus, and a posteriorly oriented flagellum ( Fig. 2A–F ).

The acrosomal complex in all the species is organized rather simply, and consists in longitudinal sections of a single cone, an electron-dense acrosomal vesicle, and subacrosomal space limited by invagination of the basal part of the acrosomal vesicle and the apical part of the nucleus ( Fig. 2G–I ). The acrosomal vesicle in all the studied Kulikovia spp . is electron-dense with some heterogeneity in the content. In Kulikovia sp. , it is more visible and represented by the slightly lighter apical part and the electron-denser basal part with a lamellar pattern ( Fig. 2I ). According to previous studies (Franz´en 1983a; Dohren ¨et al. 2010), the posterior extension of the acrosomal vesicle is an acrosomal ring component that we observed in all the studied Kulikovia species ( Fig. 2G–I ). In all of the three species, the subacrosomal space has a shape close to a trapezium and is filled with subacrosomal substance of moderate electron density ( Fig. 2G–I ). In transverse sections, the acrosomal complex has a rounded shape with an electron-dense ring (acrosomal ring component) surrounding the subacrosomal space of moderate electron density ( Fig. 2J ).

The nucleus contains condensed electron-dense chromatin ( Fig. 2B, D, F ) but sporadic nuclear lacunae (small regions with diffuse chromatin) can be found ( Fig. 2F, K ). Despite the different length/width ratios of nucleus in Kulikovia spp . ( Table 2), the nuclear shape is always cone-like; a slight convexity is visible on the anterior surface and several invaginations on the posterior part (posterior nuclear fossa and invaginations hosting mitochondria) ( Fig. 2K ).

The midpiece in all the studied species consists of five rounded mitochondria ( Fig. 2L ) with a diameter of ca. 0.9 μm. In all Kulikovia spp ., spermatozoa with six mitochondria were also found ( Fig. 2M ), but their proportion in the sperm population was no more than 1%. Mitochondria surround the proximal and distal centrioles located perpendicularly to each other ( Fig. 2K ). The distal centriole has a pericentriolar complex organized by nine satellite fibers ( Fig. 2P, Q ). Particles of presumptive glycogen are often visible in cytoplasm. Moreover, in Kulikovia sp. , an electron-dense vesicle is often present in the apical cytoplasm near the acrosomal complex ( Fig. 2P ) or in the midpiece area ( Fig. 2Q ).

The axoneme in Kulikovia spp . has a typical 9 × 2 + 2 microtubular organization ( Fig. 2M ), but the length of its flagellum differs from the longest one in Kulikovia sp. by more than 10 μm ( Table 2).

Figures

Fig. 2. Fine organization of spermatozoon in Kulikovia spp (A, C, E) SEM (B, D, F-Q) TEM (A) Bullet-like sperm head of K. alborostrata with midpiece (mp) and flagellum (f) (B) Longitudinal section along sperm head of K. alborostrata with acrosomal complex (ac), nucleus (n), and mitochondria (m) in the midpiece. Similar structures in K. torquata (C, D) and Kulikovia sp (E, F). Longitudinal sections along acrosomal complex in K. alborostrata (G), K. torquata (H), and Kulikovia sp (I), consisting of acrosomal vesicle (av) and subacrosomal material (asterisks). Arrowheads indicate acrosomal ring component (J) Transverse section across acrosomal complex of K. alborostrata on its basal part with acrosomal ring component (arrowheads) and subacrosomal material (asterisk) within it (K) Longitudinal section along midpiece of Kulikovia sp. with proximal (pc) and distal centrioles (dc) and mitochondria. Nucleus contains nuclear lacuna (nl) and invaginations: posterior nuclear fossa (pnf) and invaginations hosting mitochondria (arrows). Midpiece with five (L) and six (M) mitochondria around a centriole in Kulikovia sp (N) Satellite fibers (sf) as part of pericentriolar complex in a longitudinal section along spermatozoon of Kulikovia sp. Asterisk indicates glycogen-like particles (O) Transverse section across pericentriolar complex in Kulikovia sp consisting of nine satellite fibers. Electron-dense globule (arrow) near acrosomal vesicle, in the anterior part (P), and near mitochondria (Q), in the posterior part of spermatozoon of Kulikovia sp. Scale bars: A-F = 1 μm, G-J, O, Q = 0.2 μm, K–N, P = 0.5 μm.