Although a platypus has a duck-like bill, it is generally not associated with birds, well not until now anyway. Recently, the full set of chromosomes of the platypus was mapped out and findings indicate that the platypus provides the missing link between reptiles and mammals. One major finding was of the platypus’ bird-like sex chromosome, which implies a more recent origin of the mammalian sex chromosome than previously believed.
Monotreme mammals (the platypus and echidna) were the first to diverge from other mammals, including humans, about 200 million years ago and are therefore the closest living mammal to reptiles (which includes birds). This makes them useful specimens to study when researching the evolution of the mammalian sex chromosomes.
For therian mammals, (mammals excluding the platypus and echidna), sex is determined by the female chromosomes XX and XY in a male, in which the Y determines male offspring. The sex chromosomes of birds are not homologous with the mammalian chromosomes, with ZW for females and ZZ for males. These differences indicate that the two systems evolved separately from one another.
The platypus was found to use a multiple sex chromosome system, which has 10 sex chromosomes. In recent studies, the platypus’ X chromosomes were found to share no homology with the ancestral therian X chromosome, yet were found to be homologous to the bird Z chromosome. This similarity implies that the therian chromosomes evolved after the divergence of the monotremes, making the therian sex determination gene at least 145 million years younger than previously believed.
These exciting new findings supply a foundation that inspires further investigations into mammalian evolution and biology.
Primary source:
http://www.genome.org/cgi/doi/10.1101/gr.7101908
Secondary sources:
http://www.fauna.com.au/monotreme/monotreme.php
http://anthro.palomar.edu/animal/animal_5.htm
http://dsc.discovery.com/news/briefs/20041025/platypus.html
http://www.wiresnr.org/images/platypus05.jpg
by Anjie Annear
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