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Sex determination

Platypus: the stranger sexxxxx

Nature Reviews Genetics volume 5, page 885 (2004)Cite this article

The Monotremata subclass of animals, to which the platypus belongs, split off from the mammalian lineage about 210 million years ago, and indeed the karyotype of this species departs in many ways from that of mammals. For one, several of the platypus' 52 chromosomes are known to be unpaired in males. What happens at meiosis is equally unusual, as the chromosomes, like those of some insects and plants but not of mammals, form a chain-like structure. And how sex might be determined with such a seemingly bizarre chromosome arrangement remained to be worked out.

Grützner et al. used a combination of chromosome painting and comparative genomics to shed light on all these questions. By hybridizing eight chromosome paints — which the authors created — to male and female platypus chromosomes, the authors show that males have five copies of an X-like chromosome — which are paired in females — and five Y-like chromosomes. They also establish that the ten sex chromosomes — X1–5 and Y1–5 — link up into a chain at male meiosis. This arrangement would spell disaster without a foolproof means of creating sperm that carry either five Xs or five Ys: as it happens, the chain contains an alternating set of X and Y chromosomes, allowing segregation to occur seamlessly.

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References

ORIGINAL RESEARCH PAPER

  1. Grützner, F. et al. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature 24 October 2004 (doi:10.1038/nature03021)

FURTHER READING

  1. Rens, W. et al. Resolution and evolution of the duck-billed platypus karyotype with an X1Y1X2Y2X3Y3X4Y4X5Y5 male sex chromosome constitution. Proc. Natl Acad. Sci. USA 25 October 2004 (doi_10.1073_pnas.0405702101)

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Authors
  1. Tanita Casci
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Casci, T. Platypus: the stranger sexxxxx. Nat Rev Genet 5, 885 (2004). https://doi.org/10.1038/nrg1514

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