Systematic identification of genes expressed during early oogenesis in medaka

Subtractive hybridization was used to identify differences in gene expression between medaka (Oryzias latipes) males and females during sex differentiation. Fifty female-specific cDNA fragments were cloned. They can be classified into three groups by virtue of whether their earliest expression is at 1, 5, or 30 days after hatching. All 15 near full-length cDNAs belonging to the first two groups were cloned. Many of these female-specific genes are coordinately expressed in oocytes at the earliest stages of oogenesis. Some of the genes that were identified by their sequences include egg envelope proteins, oocyte-specific RNA binding proteins, and a transcription factor containing a basic helix-loop-helix motif.