AMHY and sex determination in egg-laying mammals

Abstract

The sex chromosomes of egg-laying mammals (monotremes), which lack the sex determining gene SRY, evolved independently to those of all therian mammals. Here we characterise the candidate monotreme sex determining gene, the Y-localised anti-Müllerian hormone gene (AMHY) and trace its expression during the period of sexual differentiation. Monotreme AMHX and AMHY gametologues have significant sequence divergence at the promoter, gene and protein level, likely following an original allele inversion in the common monotreme ancestor but retain conserved features of TGF-β molecules. Expression of sexual differentiation genes in the echidna fetal gonad were significantly different from that of therian mammals. AMHY expression was seen exclusively in the male gonad during sexual differentiation, whereas AMHX was expressed in both sexes. Experimental ectopic expression of platypus AMHX or AMHY in the chicken embryo did not masculinise the female urogenital system, a possible result of mammalian specific changes to AMH proteins preventing function in the chicken. Our results provide fundamental insight into the first steps of monotreme sex chromosome evolution and sex determination with developmental expression data strongly supporting AMHY as the primary male sex determination gene.