Differential expression of fertility genes boule and dazl in Chinese sturgeon (Acipenser sinensis), a basal fish

The gene family DAZ (deleted in Azoospermia), including boule, dazl and DAZ, performs highly conserved functions in germ cell development and fertility across animal phyla. Differential expression patterns have been demonstrated for the family members in invertebrates and vertebrates including fish. Here, we report the identification of boule and dazl and their expression at both RNA and protein levels in developing and mature gonads of Chinese sturgeon (Acipenser sinensis). Firstly, the isolation of the boule and dazl genes in Chinese sturgeon and the observation of the two genes in coelacanth suggest that dazl originated after the divergence of bony fish from cartilaginous fish but before the emergence of the Actinistia. Quantitative real-time PCR and western blot analyses reveal that boule and dazl RNA and proteins are restricted to the testis and ovary. In situ hybridization and fluorescent immunohistochemistry show that the bisexual mitotic and meiotic germ cell expression of dazl RNA and protein is conserved in vertebrates, while Chinese sturgeon boule RNA and protein exhibit mitotic and meiotic expression in the testis, and also likely display mitotic and meiotic expression in female. Moreover, we directly demonstrate for the first time that sturgeon Balbiani body/mitochondrial cloud disperses in the cytoplasm of early developing oocytes and co-localizes with Dazl to some extent. Finally, urbilaterian boule may also have an ancestral function in oogenesis. Taken together, these results provide useful information on the evolution of DAZ family genes, expression patterns and functions in animal reproduction.