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We have cloned and characterized a cDNA encoding Cg-tal in the Pacific oyster Crassostrea gigas. The isolated cDNA encodes a 219 amino acids protein that contains the basic helix-loop-helix (bHLH) domain homologous to that of vertebrate and invertebrate Tal1/SCL. Phylogenetic analyses sustained that Cg-Tal belongs to this family of bHLH transcription factors. Northern blot analysis of Cg-tal mRNA expression in adult oyster tissues indicated that Cg-ta1 was specifically expressed in hemocytes, in a constitutive manner. In vertebrates, activation of Tal1/SCL expression is essential for the initiation of hematopoiesis and the formation of hematopoietic stem cells. Considering Tal1/SCL function in vertebrates, Cg-Tal is likely to constitute a promising tool for studying hematopoiesis in oyster.

An expressed sequence tag program was undertaken to isolate genes involved in defense mechanisms of the Pacific oyster, Crassostrea gigas. Putative function could be assigned to 54% of the 1142 sequenced cDNAs. We built a public database where all EST information are accessible through numerous search profiles (http://www.ifremer.fr/GigasBase). Based on sequence similarities we identified 20 genes that may be implicated in immune function. We investigated the expression of four of these genes during bacterial challenge of oysters. Three of them were induced in response to challenge lending support to their involvement in oyster immunity. Moreover, four other genes were highly homologous to components of the NF-kappa B signaling pathway which is involved in innate immune response in Drosophila and mammals. Altogether, our results open a new way to investigate the immune response in mollusks.