![]() 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. ResultsĬhromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. 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. Placental mammals and marsupials have an XY system, birds have a ZW system. ![]() Sex-determining systems have evolved independently in vertebrates. ![]()
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