Thursday, May 08, 2008

Retrogenes and Retrobeasties

Olivia Judson is a popular writer with a background in evolutionary biology. In this week's NYTimes blog where she's featured, she writes the following;
The genomes of most organisms are littered with entities known as retroelements. These are a type of genetic parasite — stretches of DNA that (usually) do nothing useful for the cell, and exist simply to make more copies of themselves. (There are many different kinds of genetic parasite: as much as half of the DNA in the human genome is thought to have originated from them.) The way that retroelements proliferate is complicated, and depends on the element in question, for there are many sorts; but one thing they all have in common is that they, too, depend on the activity of reverse transcriptase.[...]

whether a newly created retrogene will appear and immediately vanish — or whether it will confer some kind of advantage on the organism and thus spread through the population — depends on where in the genome it gets inserted. If it arrives in the “wrong” place, it may not be able to be switched on; or worse, it may destroy the workings of an existing gene and harm the organism in some way. Hence, only a fraction of the retrogenes that are created will succeed in becoming established.

Which brings me to the X-odus. In mammals and in fruit flies, the genetic difference between males and females is that females have two X chromosomes whereas males have an X and a Y. Studies of successful retrogenes in a number of species, including fruit flies, humans, mice and opossums, have all shown the same, striking pattern. The parents of retrogenes are disproportionately likely to be found on the X chromosome. But the retrogenes themselves are typically located elsewhere. In other words, there’s a weird kind of genetic migration going on: successful retrogenes are fugitives from the X.
This is both fascinating to me and very timely in light of new data popularized this week concerning the genome of the duckbilled platypus;
The sex of the platypus is determined by a set of ten chromosomes, an oddity that sets it apart from all other mammals and from birds. These chromosomes link during meiosis to form a chain that ensures every sperm gets a set of all Xs or all Ys. Despite the similar designations, none of the platypus X chromosomes resembles the human, dog or mouse X. “The sex chromosomes are absolutely, completely different from all other mammals. We had not expected that,” says Jennifer Graves of the Australian National University in Canberra, who studies sex differentiation and is an author on the paper. Instead, the platypus Xs better match the avian Z sex chromosome. Another chromosome matches the mouse X, Graves and her colleagues report in Genome Research (F. Veyrunes et al. Genome Res . doi:10.1101/gr.7101908; 2008). This is evidence that placental mammalian sex chromosomes and the sex-determining gene Sry — found on the Y chromosome — evolved after the monotremes diverged from mammals, much later than previously thought. “Our sex chromosomes are a plain old ordinary autosome in the platypus,” Graves says.
Both articles make for some fascinating storytelling, and they complement one another by introducing and amplifying a new insight from the nascent field of genomic mechanics into the popular sphere. Most people don't pay enough attention to this to discern where the science ends and the storytelling begins. As a result, constant vigilance is required to ensure that storytellers with an agenda are never permitted to hijack any aspect of this research for their own political ends while the science endeavoring to understand the phenomenon remains very much an early stage work in progress.