Oogenesis characteristics in the hydrothermal vent polychaeteAlvinella pompejana

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Abstract
The morphology of the female genital tract and mechanisms of oogenesis were investigated through light and transmission electron microscopy in the vent polychaete Alvinella pompejana. We showed that the genital pore exhibits different morphologies in males and females and can be used for sex identification. The female genital tract consists of two oviducts that contain mature oocytes and spermathecae, which may contain a few unfertilised oocytes, and simultaneously spermatozoa. Ultrastructural analysis of both coelomic and genital tract oocytes showed that vitellogenesis is mostly achieved in the coelomic cavity, apparently without helper cells, and involves autosynthetic mechanisms of yolk production. Such a mechanism suggests that egg growth is slow. It is commonly admitted that hydrothermal environments are unpredictable and highly variable, and thus, may favour species that are able to produce eggs rapidly facing environmental changes. As Alvinella pompejana does not seem to follow such a reproductive pattern, we hypothesised that the reproductive process may be considered as a two-step process where only the second one would be directly influenced by the environment. First, coelomic vitellogenesis would be a relatively slow process, regulated physiologically independently of abrupt environmental changes. In the second step, mature eggs would be selected and stored for further spawning and fertilisation, at any time triggered by environmental cues or biological signal such as sperm transfer.

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