Ultrastructural aspects of oocytes and vitellocytes in Syndesmis patagonica (Platyhelminthes, Rhabdocoela, Umagillidae)

FALLENI, A.; LUCCHESI, P.; GHEZZANI, C.; BROGGER, MARTÍN IGNACIO

Urbino

Congreso; 10th Multinational Congress on Microscopy; 2011

Umagillid flatworms are endosymbionts of echinoids. Specimens of the new, recently described species Syndesmis patagonica were collected from the intestine of the sea urchin Arbacia dufresnii in Patagonia, Argentina. Among Rhabdocoela the Umagillidae and the Pterastericolidae are of special interest because, sharing certain characteristics with some Neodermata, they are tentatively considered to be potential ancestors of the Monogenea. To date little is known about the female gonad ultrastructure of the umagillids: the only data available concern the eggshell formation of Syndisyrinx franciscanus, while several studies have been performed on the female gonad of free-living rhabdocoels. We examined the female gonad of S. patagonica by means of TEM and cytochemical techniques focusing on the genesis, structure and composition of oocyte and vitellocyte inclusions in order to compare them with those from other platyhelminths in particular Rhabocoela and Neodermata. The female gonad of S. patagonica consists of well-separated germaria and vitellaria, both enveloped by an extracellular lamina and an inner sheath of accessory cells. Oocyte differentiation is characterized by the development of RER and Golgi complexes. The latter organelles appear to be involved in the production of round granules, about 2 ìm in diameter, whose substructure shows a homogeneous electron-dense, sub-central/central core surrounded by a less dense granular component and a translucent halo delimited by a wavy membrane. These egg granules are positive to the cytochemical test for polyphenol detection, are unaffected by protease and have been interpreted as residual eggshell granules. Mature oocytes also contain a small amount of yolk and lipids. The vitellaria consist of oval follicles containing vitellocytes at different stages of maturation. Developing vitellocytes contain well-developed RER and small Golgi complexes involved in the production of eggshell and yolk globules. Eggshell globules exhibit a round shape, measure 3-4 !m in diameter and have a multigranular/mosaic-like pattern of the content containing polyphenols. Yolk globules (1,5-2 ìm in diameter) show a homogeneous protein content of medium electron density which is negative to the test for polyphenols. In conclusion our preliminary investigation shows that the female gonad of S. patagonica possesses ultrastructural features typical of the basic pattern of Rhabdocoela: a) presence of a tunica (extracellular lamina/matrix + cellular sheath) which surrounds the gonad; b) oocytes containing egg granules with a polyphenolic content; c) vitellocytes with polyphenolic eggshell globules with a multigranular/mosaic-like pattern. The presence of polyphenolic egg granules and of polyphenolic eggshell globules with a multigranular/mosaic-like pattern in the vitellocytes have been considered apomorphic features of the taxon Rhabdocoela. These ultrastructural features are shared with the taxon Prolecithophora and can be considered synapomorphies of the two taxa.