THE weird and wonderful duck-billed platypus just got weirder and more wonderful.
These milk-producing mammals lay eggs, have a bird-like bill and a skeleton with some reptilian features. Now it turns out that their sex is determined in a unique way. In most mammals, including humans, sex is decided by the X and Y chromosomes. Two Xs and you are female; XY and you鈥檙e a male. In birds the system is similar: ZW for a female and ZZ for a male.
But in the platypus, X1X1X2X2X3X3X4X4X5X5 makes an animal female and X1Y1X2Y2X3Y3X4Y4X5Y5 makes a male. In other words, rather than a single pair of chromosomes, platypuses have a set of 10 chromosomes determining their sex.
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Frank Gr眉tzner and Jenny Graves of the Australian National University in Canberra and their colleagues, who made the discovery, examined platypus cells under a microscope after staining their chromosomes with fluorescent markers. They also found that the sex chromosomes in the animals鈥� sperm are precisely distributed to give XXXXX-bearing sperm and YYYYY-bearing sperm. When an XXXXX sperm fertilises an egg, which will always be XXXXX, a female platypus is produced. YYYYY sperm produce males Nature, DOI: 10.1038/nature03021).
