title: Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes.
abstract: In eutherian ('placental') mammals, sex is determined by the presence or absence of the Y chromosome-borne gene SRY, which triggers testis determination. Marsupials also have a Y-borne SRY gene, implying that this mechanism is ancestral to therians, the SRY gene having diverged from its X-borne homologue SOX3 at least 180 million years ago. The rare exceptions have clearly lost and replaced the SRY mechanism recently. Other vertebrate classes have a variety of sex-determining mechanisms, but none shares the therian SRY-driven XX female:XY male system. In monotreme mammals (platypus and echidna), which branched from the therian lineage 210 million years ago, no orthologue of SRY has been found. In this study we show that its partner SOX3 is autosomal in platypus and echidna, mapping among human X chromosome orthologues to platypus chromosome 6, and to the homologous chromosome 16 in echidna. The autosomal localization of SOX3 in monotreme mammals, as well as non-mammal vertebrates, implies that SRY is absent in Prototheria and evolved later in the therian lineage 210-180 million years ago. Sex determination in platypus and echidna must therefore depend on another male-determining gene(s) on the Y chromosomes, or on the different dosage of a gene(s) on the X chromosomes.
authors.affiliation: Comparative Genomics Group, Research School of Biological Sciences, the Australian National University, Canberra, ACT 2601, Australia. mary.wallis@anu.edu.au
authors.forename: M C
authors.initials: MC
authors.lastname: Wallis
authors.forename: P D
authors.initials: PD
authors.lastname: Waters
authors.forename: M L
authors.initials: ML
authors.lastname: Delbridge
authors.forename: P J
authors.initials: PJ
authors.lastname: Kirby
authors.forename: A J
authors.initials: AJ
authors.lastname: Pask
authors.forename: F
authors.initials: F
authors.lastname: Grützner
authors.forename: W
authors.initials: W
authors.lastname: Rens
authors.forename: M A
authors.initials: MA
authors.lastname: Ferguson-Smith
authors.forename: J A M
authors.initials: JA
authors.lastname: Graves
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D020223:Chromosome Painting; D004268:DNA-Binding Proteins / Q000235:genetics*; D006609:High Mobility Group Proteins / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D008969:Molecular Sequence Data; D010987:Platypus / Q000235:genetics*; D055748:SOXB1 Transcription Factors; D017386:Sequence Homology, Amino Acid; D019849:Sex Determination Processes; D051876:Sex-Determining Region Y Protein / Q000235:genetics* / Q000378:metabolism; D004442:Tachyglossidae / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics*; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*
chemical_list: D004268:DNA-Binding Proteins; D006609:High Mobility Group Proteins; C527785:SOX3 protein, human; D055748:SOXB1 Transcription Factors; D051876:Sex-Determining Region Y Protein; D014157:Transcription Factors