Ultrastructural analysis of the ovary and oogenesis in Spinicaudata and Laevicaudata (Branchiopoda) and its phylogenetic implications

Recent molecular studies have indicated a close relationship between Crustacea and Hexapoda and postulated their unification into the Pancrustacea/Tetraconata clade. Certain molecular analyses have also suggested that the crustacean lineage, which includes the Branchiopoda, might be the sister group of Hexapoda. We test this hypothesis by analyzing the structure of the ovary and the ultrastructural features of oogenesis in two branchiopod species, Cyzicus tetracerus and Lynceus brachyurus, representing two separate orders, Spinicaudata and Laevicaudata, respectively. The female gonads of these species have not been investigated before. Here, we demonstrate that in both studied species the ovarian follicles develop inside characteristic ovarian protrusions and comprise a germline cyst surrounded by a simple somatic (follicular) epithelium, supported by a thin basal lamina. Each germline cyst consists of one oocyte and three supporting nurse cells, and the oocyte differentiates relatively late during ovarian follicle development. The synthesis of oocyte reserve materials involves rough endoplasmic reticulum and Golgi complexes. The follicular cells are penetrated by a complex canal system and there is no external epithelial sheath covering the ovarian follicles. The structure of the ovary and the ultrastructural characteristics of oogenesis are not only remarkably similar in both Cyzicus and Lynceus, but also share morphological similarities with Notostraca as well as the basal hexapods Campodeina and Collembola. Possible phylogenetic implications of these findings are discussed.