Wednesday, September 1, 2010

Accessible Research: Why hasn't Y disappeared?

Of course, here, I'll be biased towards science that I am more familiar with, but you'll just have to put up with that for now.

In the next installment of tasty tid-bits for your brain, I'll look at a paper published in GBE, that you can view here, open access. (Gotta love open access!)

Can Intra-Y Gene Conversion Oppose the Degeneration of the Human Y Chromosome? A Simulation Study

Gabriel A. B. Marais*,1Paulo R. A. Campos2 and Isabel Gordo3

Genome Biology and Evolution Vol. 2:347; doi:10.1093/gbe/evq026 

The title is pretty informative to someone in the field, but you might be wondering: Why is the Y degenerating? What do I care if it is lost? And what is gene conversion?

Well, to understand why the Y is degenerating, we need to know where it came from. In humans the X and Y chromosomes are called the sex chromosomes. Females have two X chromosomes (XX) and males have one of each (XY). In mammals sex, and specifically maleness, is determined genetically by a gene on the Y chromosome. The X and Y were originally identical to each other, but after the Y gained this male-determining gene, it started to become a male-specific chromosome, losing most (97%) of its genes! Whereas the X and Y used to recombine (swap bits of DNA) in order to fix mutations and deletions, the Y now is mostly non-recombining. This means that deletions and errors on the Y cannot be fixed by partnering with the X - it is a lonely chromosome. 

Some people have wondered: What mechanisms help the degenerated Y stick around? Because it cannot recombine (to fix errors) why doesn't it just lose all of its content? The authors here run computer simulations to try and answer this very question.

We need to bring one up more bit of background - even with all of the degeneration and gene loss on the Y chromosome, there has been substantial gene gain in one particular region: the ampliconic region. This region is made of palindrome (yes, exactly like Madam I'm Adam - the same forward and backwards) of long repeated gene regions. Think:

Palindrome = Copy1.Copy2.Copy3.-.3ypoC.2ypoC.1ypoC

These researchers wondered whether pairing and sharing between the identical regions of each palindrome might serve as a mechanism to retain gene content on the Y chromosome. And, indeed, they found that this was true in their simulations!

So, it seems like, even though the Y has degraded quite a bit, it may have figured out a loophole, a way to keep from being lost forever. Good news for all those worried guys out there - looks like you'll be keeping your male-determining chromosome for a little while longer. :)

No comments: