Wednesday, May 31, 2006

Sex and Recombination - Part I: A Question

Now that my PhD proposal is off to Zurich, I should return to the project I was working on in the interregnum between my M.Sc. defence and Zurich-Crunch-Time. In March, I presented a 35-minute talk to the Biological Sciences department here that constituted an attempt to answer a question arising from my M.Sc. work.

"Why do we see differences in recombination rates between the sexes within a species?"

I've presented my M.Sc. twice; once at my defence, and a second time, with virtually the identical presentation, a couple of weeks later to the Zoological Museum at the University of Zurich (you might think that I travel a great deal, but you would be mistaken, my trip to Zurich was an abberation in a general pattern of couch-potato-ness). After each presentation, I was asked from the audience some variant of the above question.

Also, most of my M.Sc. work went into a paper I co-authored (lead author) with my thesis committee and submitted to Zebrafish in December. When we got the reviews back (a story in itself), the only question that took longer than 2 minutes to answer was, again, a variation of the above. I had assumed that this question had already been answered, at least in a theoretical or modelling way, and all I had to do was find the appropriate review paper and cite it. Not true.

So, in my insanity, I decided to write that review paper myself. 'cause hey, it's not like I have anything better to do!

I'll post some of my work on this question here, but I'm going to try to restrict what shows up on this blog to just the barest outline, since I don't want to get "scooped". There's actually a pretty lively discussion about scientists posting their work on blogs before and / or after official publication out there in the blogosphere. Coturnix, for example, has brought this up once or twice. That blog is too huge, and too often focussed on American politics (see title of that blog) for me to spend much time there.

First, defining the question. Most of my readers (2 out of 3 counts as "most") are probably wondering what the hell I'm babbling about, if they've read this far.

OK, very short version: recombination is the process of shuffling gene and allele combinations during meiosis. Chromosomes can exchange parts in a process called "crossing-over". This process is controlled by enzymes, but I'm not going to get into that. Recombination can be detected by examining the progeny of a genetic cross; the rate at which it happens depends on several factors, like the distance between two loci on a chromosome (lots of hidden detail there) and the characteristics of the enzymes involved. Interestingly, the average recombination rate across a genome is a phenotypic trait; it varies and evolves just like any other trait. Once someone starts digging into the literature on this subject, something I think is counter-intuitive becomes apparent: males and females do recombination differently in most species. OK, males and females have different gametes (that's what defines the sexes), and gametes are made by meiosis, and meiosis has as one of its most prominent features the process of recombination. So it's not really that surprising that recombination rates can be different in the sexes.

There's lots of initial confusion around this subject that comes up whenever I start talking about it. Maybe it's just the way I talk and write. When I talk about recombination rate differences by sex, I am talking about males and females making gametes, not about different types of sperm, or differences in male vs. female offspring. Fathers have both sons and daughters, mothers have both sons and daughters - I'm talking about the processes that go into making gametes, not the processes that go into making sons vs. daughters. This is the single most common misunderstanding that shows up when I talk about this.

OK, clear? Next time, I'll define the work that has produced these observations.

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