The Sox gene family and its expression during embryogenesis in the teleost fish, medaka (Oryzias latipes)

SRY, the Y chromosomal gene, which determines testis formation during mammalian embryogenesis, encodes a potential transcription factor with a DNA binding motif known as a high mobility group I (HMG) box. We isolated HMG regions of the Sry‐related Sox gene from a medaka (Japanese killifish, Oryzias latipes) genomic library and determined their nucleotide sequences. The medaka Sox HMG box regions could be divided into four regions based on the degree of synonymous substitution and amino acid homology, and form the Sox sequence family consisting of five subgroups (Sox‐1‐1, 1‐2, 2‐1, 2‐2 and 2‐3). The deduced amino acid sequences were similar to human and mouse Sox‐1,2,3 and 14 group, one of which, SOX‐3, is located on the human X chromosome. Interestingly, we found that two marsupial Sry‐like sequences on the Y chromosome are more closely related to the medaka Sox sequences than to other mammalian Srys in the HMG box region. Genomic Southern hybridization revealed that medaka Sox sequences were present on both male and female genomes, but there was no male‐specific Sox sequence. Northern blot hybridization using medaka Sox‐1 and ‐2 PCR probes demonstrated two kinds of transcripts (2.5kb and 1.7kb). The 2.5kb transcripts were found in mature oocytes while the 1.7 kb transcripts first appeared at the gastrulation stage. The 2.5 kb and 1.7kb transcripts were both decreased 3 days after hatching.