Their timing of onset into meiosis, regulated by retinoic acid. Similarly, in oysters, overlapping pathways may control mitosis/meiosis and sperm/oocyte switches. In oysters, the gonad consists of numerous tubules, invaginated in storage tissue and mantle. During gonad development, tubules develop at the expense of the storage tissue. When dissecting gonad at the earlier stages, we collected a tissue commonly designed “Benzethonium Chloride mantle-gonad”, surrounding the visceral mass and consisting in a mix of conjunctive, germinal and other somatic cells. The low number of germ cells present in stages 0 and 1 samples renders the identification of genes specifically expressed early in gonad development difficult. Therefore, a large part of the genes more expressed in stage 0 and stage 1 gonads was likely to represent somatic cells. Indeed, numerous muscle fibers specifying genes, such as those encoding the calcium binding proteins Calponin and Transgelin, the actin-depolymerising factor and the molecular spring Titin, were found highly expressed in mantle and adductor muscle. One hundred and twelve genes from cluster 1 were significantly less expressed in stripped oocytes, further suggesting that they were expressed by somatic cells. Only two genes from this cluster were significantly more expressed in oocytes than in somatic cells. The germ cell specific expression makes them extremely interesting despite the lack of homologies to any other known genes. Identifying genes specifically expressed by germ cells at the beginning of gametogenesis and sex differentiation will require to enrich samples in germ cells, which might be solved by gene expression analysis of laser capture microdissected cells. Ubiquitin targeting and proteasome take an important place in late spermatogenesis stages. Indeed, an ubiquitin-dependent sperm quality control has been observed in mammals. A similar process may take place in oysters before sperm is released into the ocean. Investigation of tissue expression of the genes involved in spermatogenesis revealed that a high proportion of these genes were also highly expressed in gills and labial palps, suggesting functional similarities between these tissues. The functions of these genes were related to flagella and cilia structure and movement reflecting the common transcriptomic features of spermatozoa flagella with gills and labial palps ciliated epithelia. For example, genes encoding Dynein and Kinesin-like proteins were found. The sperm surface protein sp17 that used to 
be described as a sperm specific protein, plays also a Butenafine hydrochloride regulatory role in human somatic ciliated cells. The most significant outcome of our study is the identification of transcripts that improve our understanding of gametogenesis in the Pacific oyster and produce lists of relevant candidate genes for further studies. Here we report temporal variation of gene expression during oyster gonad differentiation and development. In addition to genes preferentially expressed at each differentiation stage and for each sex, we compared the data with a dataset of gene expression in somatic tissues and in oocytes and identified subsets of genes specifically expressed in oocytes, somatic cells and in flagella and cilia structures. Furthermore, to reveal new clues in determining the pathways involved in sex differentiation in C. gigas, a facultative protandrous alternative hermaphrodite, we identified genes specifically expressed in either males or females.