Temperature-dependent sex determination in the American alligator: AMH precedes SOX9 expression

Gonadal morphogenesis is very similar among mammals, birds, and reptiles, D espite this similarity, each group utilises quite different genetic trigger s for sex determination. In mammals, testis development is initiated by act ion of the Y-chromosome gene SRY, Current evidence suggests that SRY may ac t together with a related gene, SOX9, to activate another gene(s) in the pa thway of testicular differentiation. A downstream candidate for regulation by SRY and SOX9 is AMH, In mouse, Sox9 is expressed in the Sertoli cells of the embryonic testis and it precedes the onset of Amh expression. During m ouse gonadogenesis, Amh is confined to the embryonic testis, although it la ter shows postnatal expression in the ovary. Reptiles such as the American alligator which exhibit temperature-dependent sex determination (TSD) do no t have dimorphic sex chromosomes and apparently no SRY orthologue, SOX9 is expressed during testis differentiation in the alligator; however, it appea rs to be expressed too late to cause testis determination, Here we describe the cloning and expression of the alligator AMH gene and show that AMH exp ression precedes SOX9 expression during testis differentiation. This is the opposite to that observed in the mouse where SOX9 precedes AMH expression, The data presented here, as well as findings from recent expression studie s in the chick, suggest that AMH expression is not regulated by SOX9 in the non-mammalian vertebrates. (C) 1999 Wiley-Liss, Inc.