Yes, that’s right. The secret of sex chromosome evolution is hidden in the animal on the Australian 20 cent coin – the platypus.
With a body structure that resembles mammals, reptiles AND birds, the platypus is ironically classified as a mammal due to its ability to produce milk for its young. It is a monotreme, which are mammals that lay eggs instead of giving birth to live young.
The genome of Glennie, a female platypus, was sequenced in Washington University. It was found that the platypus has 52 chromosomes, and, astonishingly, has 10 sex chromosomes, compared with humans, who only have 2. It was found that the platypus sex chromosomes chain up in an X-Y-X-Y alternating pattern, with an X-chromosome (which is actually similar to a sex chromosome of the bird, Z) at one end and a DMRT1 gene, a bird sex determining gene, at the other end.
Also, the genome revealed that the sex-determining gene SRY, which is present in mammals, is absent in the platypus. The sex chromosomes of the platypus are more similar to those of birds than of mammals. The results also imply that ancient animals determined sex just like birds did. Therefore, the human sex chromosomes probably had evolved after the platypuses diverged from the mammalian lineage. This changes our understanding of the evolution of sex chromosomes, in which we previously thought that sex chromosomes were derived from homologous chromosomes that evolved different morphology.
Who would have thought that innocent-looking platypuses would hide so many secrets?!
Written by: Andrea Kam
Primary references:
Platypus genome reveals much about sex
Platypus Genome Shows Beauty Is More than Skin Deep
Platypus Genome Explains Animal's Peculiar Features; Holds Clues To Evolution Of Mammals
Secondary reference:
Evolution Of Sex Chromosomes: The Case Of The White Campion
Further information on news of the platypus genome:
Platypus genome holds key to its testes
Platypus loaded with sex chromosomes
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