The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z
1 Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 OES, UK
2 School of Molecular and Biomedical Science, The University of Adelaide, Adelaide, 5005 SA, Australia
3 Institute of Cytology and Genetics, Russian Academy of Sciences, Siberian Department, 630090, Novosobirsk, Russia
4 School of Animal Sciences, Currumbin Sanctuary, Queensland University, Brisbane, QLD 4072, Australia
5 Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia
Genome Biology 2007, 8:R243 doi:10.1186/gb-2007-8-11-r243Published: 16 November 2007
Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.
Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X1, X2, X3, X5, and in chromosome Y1.
Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.