Sex determination and sex chromosome evolution in the Platyfish #Xiphophorus maculatus#

In contrast to the situation observed in mammals and birds, fish display an amazing diversity of sex determination systems. Almost nothing is known about the molecular and evolutionary basis of this phenomenon. Bacterial artificial chromosome (BAC) contigs including the sex-determining region of the platyfish Xiphophorus maculatus and covering 2.5-3.5 megabases of the X and Y chromosomes have been constructed and sequenced. Thirty-nine BAC clones have been sequenced so far to completion, leading to the identification of 63 sex chromosomal genes. Interestingly, eleven of these genes are also located on human sex chromosomes, but no synteny was found with other fish gonosomes. Through comparison between X and Y sequences, the sex-determining region has been delimited. Gene candidates have been identified for all loci involved in pigmentation, cancer formation and sexual development that are linked to the master sex-determining gene of the platyfish. Numerous genes, including the melanocortin hormone receptor gene mc4r , are amplified in the sexdetermining region of X. maculatus. Gonad-specific genes were identified, including two genes with so far unknown function organized in tandem and exclusively expressed in oocytes. One gene, called swimy , is present on the Y but not on the X chromosome and represents an excellent candidate for the master sex-determining gene. This gene encodes a ubiquitously expressed protein, with different predicted domains involved in nucleic acid binding and protein-protein interactions. Interestingly, through the use of an alternative exon in spermatogonia, a testis-specific longer isoform is produced, with different domains involved in protein modification. (Texte integral)