Abstract
<jats:p> <jats:italic>Doublesex</jats:italic> and <jats:italic>mab-3</jats:italic> related ( <jats:italic>DMRT</jats:italic> ) genes encode a family of transcription factors central to sexual development across metazoa. <jats:italic>DMRT</jats:italic> genes are characterised by a highly conserved DNA binding domain (DM) while flanking regions may vary between species. Gene duplication and loss has shaped the diversity of the DMRT genes with several unresolved questions about their evolution. The most well characterised and conserved <jats:italic>DMRT</jats:italic> gene, <jats:italic>DMRT1</jats:italic> , functions as a sexual regulator universally in metazoans. In chicken, <jats:italic>DMRT1</jats:italic> is located on the Z chromosome and acts as a dosage dependent primary sex determination gene. In therian mammals <jats:italic>DMRT1</jats:italic> is autosomal, however, two copies are required for male development. Interestingly in the basal lineage of egg-laying mammals (monotremes), <jats:italic>DMRT1</jats:italic> is localised on the X specific part of one of the X chromosomes. This provided the first evidence of a sex chromosome system with homology to the avian Z chromosome and raises questions about the function and evolution of <jats:italic>DMRT1</jats:italic> in egg-laying mammals. To gain insight into the evolution of mammalian <jats:italic>DMRT</jats:italic> genes we performed sequence and expression analysis of monotreme <jats:italic>DMRT</jats:italic> genes and comparative analysis with other vertebrates. In monotremes, we identified <jats:italic>DMRT</jats:italic> genes 1-7, and show that <jats:italic>DMRT8</jats:italic> is absent, suggesting that <jats:italic>DMRT8</jats:italic> evolved in therian mammals after the divergence of monotremes. Sequence and expression analysis revealed multiple monotreme specific <jats:italic>DMRT1</jats:italic> isoforms with additional protein-coding exons. The independent evolution of monotreme specific changes in <jats:italic>DMRT1</jats:italic> may be the first indication of functional or regulatory differences in monotreme <jats:italic>DMRT1</jats:italic> . </jats:p>