Thursday, May 08, 2008

Platypus Genome Unravels Mysteries of Mammalian Evolution

Genome analysis of the platypus reveals unique signatures of evolution. 2008. Everyone and their dog. Nature 453: 175-183.

Zina Deretsky, National Science Foundation
Platypus DNA reveals its shared history with bird, reptilian and mammalian lineages.
From the press release:

The first genome sequencing project of a mammal that lays eggs, confirms that platypus DNA looks like something of a patchwork.

"Their genomic organization was strange and a little unexpected," says Batzer. "It appeared much more bird- and reptile-like than mammalian, even though it is indeed classified as a mammal."

The platypus occupies the first branch of the mammalian tree of life after the split from "sauropsids" about 315 million years ago. It maintains some long dated features and, as a result, should provide information on how mammals evolved.

One interesting finding for the researchers is that several of the populations seem to have been geographically separated for a long time. Based on an analysis using mobile elements, the population on the island of Tasmania seemed genetically far-distanced compared to other platypus populations from the mainland of Australia.

Platypuses are extremely shy by nature and only a few places like Healesville Sanctuary in Victoria, Australia, have had success breeding them in captivity.

Researchers hope that some of the clues unearthed in the platypus genome might lead to a better understanding of the history of the species and new conservation efforts.

Credit: Healesville Sanctuary, Victoria, Australia
Genome Research is also publishing several related articles including:
Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes. 2008. F. Veyrunes et al. Genome Research.
Therian mammals possess an XY sex-determination system, where females have a pair of X chromosomes, and males harbor one X and one Y chromosome. However, the sex-determination system of platypus stands in stark contrast: the female has five different pairs of X chromosomes, while males have five X chromosomes and five Y chromosomes. This study found that the platypus sex chromosomes are unrelated to the X or Y chromosomes of other mammals, and lack the mammalian sex determining gene SRY and the gene from which it evolved. Instead, their sex chromosomes share extensive homology to those of birds suggesting that our ancient mammal-like reptile ancestor may have had bird-like sex chromosomes and sex determination system.
Conservation of small RNA pathways in platypus. 2008. E.P. Murchinson et al. Genome Research
This paper found that the platypus shares microRNA families uniquely with other mammals, but also uniquely with a representative of birds and reptiles,” explains Hannon. “Thus, the unusual morphology of these animals is also reflected at the genomic level and at the level of its small RNAs.”

See also:

Retroposed SNOfall – A mammalian-wide comparison of platypus snoRNAs. 2008. J. Schmitz et al. Genome Research

Origin of INSL3-mediated testicular descent in therian mammals. 2008. J. Park et al. Genome Research.

Defensins and the convergent evolution of platypus and reptile venom genes