Isolation and expression of RlYB2, a germ cell-specific Y-box gene in Rana

Y-box proteins are a highly conserved family of gene expression regulatory factors. During gametogenesis they may play a dual role, as both transcriptional activators of germ cell-specific genes and as translational repressors of stored maternal transcripts. We report the identification of RlYB2, a Y-box homolog gene specifically expressed in the germ cells of green frogs belonging to Rana esculenta complex, a model system characterized by a hybridogenetic gametogenesis. In developing germ cells of the hybrid R. esculenta, arisen by natural cross of the parental species R. ridibunda and R. lessonae, one set of the parental genomes is excluded and the remaining one, first endoreduplicates and then is transmitted to gametes. In situ hybridizations performed on gonadal tissues showed that the RlYB2 transcript was widely expressed in the ooplasm at early stages of oogenesis in both the parental species and hybrids. Interestingly, a hybridization signal, presumably related to RlYB2-like nascent transcripts, was observed in nuclei of stage II oocytes. The presence of RlYB2 mRNA during early oogenesis suggests that this gene may be involved in regulating the transcription and/or translation of maternal mRNAs in this special vertebrate model system.