Comparative genomics approach to the expression of figα, one of the earliest marker genes of oocyte differentiation in medaka (Oryzias latipes)

We analyzed molecular cascades of sex differentiation in medaka gonads by examining the transcriptional regulation of the oocyte-expressed gene, figα. We first confirmed that figα is one of the earliest marker genes of oocyte differentiation by quantitative RT-PCR and in situ hybridization. Expression of putative figα target genes, zpc4 and zpb, followed that of figα. A meiosis-specific gene, scp3, showed expression temporally and spatially similar to figα. To characterize the cis-regulatory sequences of figα, we compared genomic organizations of vertebrate figα genes. Both number and sequence homology of the C-terminal exons showed divergence, suggesting their less important roles. In the frog, Xenopus tropicalis, and in many teleosts, figα is located between hexokinase 2 and β-adducin. We compared this genomic region for potential cis-regulatory elements and found no DNA stretches with high homology. In spite of this lack of sequence similarities, fluorescent protein transgenes surrounded with figα flanking sequences from the compact genomes of fugu or Tetraodon faithfully reproduced the endogenous expression of figα in the medaka oocytes, indicating conserved regulatory mechanisms.