The Curious Case Of A Frog’s Extra Chromosomes

Millions of years ago, one frog species diverged into two species and later on became one again. How?

AsianScientist (Nov. 1, 2016) – An international team of scientists has found that the genome of the African clawed frog (Xenopus laevis) is comprised of two different sets of chromosomes from two extinct ancestors. The work was published in Nature and featured on the cover.

Millions of years ago, one species of frog diverged into two species; but millions of years later, the two frogs became one again, only with a few extra chromosomes due to whole genome duplication. Such is the curious case of X. laevis, a frog whose genome contains nearly double the number of chromosomes as the related Western clawed frog, X. tropicalis.

“Because X. laevis is a well-studied model system for cell and developmental biology, it is ideal for studying the effect of polyploidy on evolution,” Simakov explained.

In the evolution of species, different events have occurred over millions of years that have increased the number of chromosomes in some organisms; one such event is polyploidy. While it is relatively rare nowadays to observe a mammal, reptile or bird with an abnormal number of chromosomes, polyploidy is common in fish, amphibians and plants.

Joining forces with scientists across the globe, Professor Daniel Rokhsar from the University of California, Berkeley and head of the Molecular Genetics Unit at the Okinawa Institute of Science and Technology Graduate University examined the genome evolution of X. laevis. Dr. Oleg Simakov, postdoctoral scholar at OIST, developed an algorithm to determine the length of time in millions of years between the divergence and subsequent fusion of the X. laevis ancestral species.

“The most exciting finding from our study is that we can partition the current X. laevis genome into two distinct sets of chromosomes, each descended from a unique ancestral species. While plant studies have been able to show similar results using related species still in existence, this study is the first time this has been done with two extinct progenitor species,” said co-lead author Dr. Adam Session, a former graduate student in Rokhsar’s lab.

This large collaborative project resulted in new knowledge of genome duplication that can be applied to evolutionary studies of other organisms.

The article can be found at: Session et al. (2016) Genome Evolution in the Allotetraploid Frog Xenopus Laevis.


Source: Okinawa Institute of Science and Technology Graduate University.
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