Gametogenic cycle and gonadal apoptotic cell detection in the semelparous side-gilled sea slug Pleurobranchaea maculata (Quoy & Gaimard, 1832)

BÖKENHANS, VERENA; AVERBUJ, ANDRÉS ; GUILIANELLI, SEBASTIÁN

Munich

Congreso; World Congress of Malacology Munich 2022; 2022

Ludwig Maximilians Universität München

Most species belonging to the Superorder Nudipleura (Heterobranchia) have been described as semelparous reproductive strategists, mainly based on changes observed in population density. However, their gametogenic cycles have rarely been studied. This work describes the gametogenic cycle of the simultaneous hermaphrodite sea slug Pleurobranchaea maculata (Pleurobranchaeidae) in Patagonia, Argentina. The gametogenic cycle was studied by classical histology. Additionally, apoptosis during the gametogenic cycle was observed using the immunohistochemical TUNEL technique. P. maculata exhibited sperm and oocytes development occurring asynchronously in separate gametogenic acini. Sperm development began in February while oocyte development began in April. The highest spermatogenic and oogenic activity was observed from July to January characterized by long periods of partial evacuations. In February and March, a high percentage of individuals with deteriorated acinar structure (post-reproductive) were found. Maturity calculated from oocyte size and presence of mature sperm showed that immature and post-reproductive individuals coexisted during these months, while mature individuals were rarely observed. This finding suggests that P. maculata is a semelparous species with an annual life cycle, with a single spawning season extending from July to January (winter to summer southern hemisphere). The use of TUNEL to study apoptosis during the gametogenic cycle, as a novel approach for heterobranchs, evidenced that during the different reproductive stages apoptotic processes occur in different cell types. At the beginning of the reproductive cycle (mature spermatogenic acini and inmature oogenic acini), positive apoptotic signals are located mainly in spermatogonia, probably to maintain homeostasis in reproductive cells. Reproductively active individuals showed a lack of apoptotic nuclei in all types of gonadal cells. However, in post-reproductive individuals, apoptotic nuclei were visualized in the structural cells of the basal membrane of both gametogenic acini, compromising gonadal viability. This outcome evidences that apoptotic processes are involved in the gonadal deterioration and could be responsible for the mechanisms underlying death in this species.