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“It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small.” – Neil Armstrong (1930-2012)

Fresh Reads from the Science 'o sphere!

Sunday, March 02, 2008

Sexual Healing

Sex is good.

You are thinking "Duh, and the sky is blue. So what?"

What I mean is: sex isn't merely fun, but also served an important role in the evolution of eukaryotic life.

On Monday, Dr. Adam Wilkins, editor of BioEssays journal gave us a glimpse into the possible origin of sex in his talk entitled "The Evolution of Meiosis".

Prominent evolutionary biologists, such as John Maynard Smith and William Hamilton, have long been fascinated and puzzled by the process of sexual reproduction.

Sex is good for populations in the long run, by increasing the total genetic variation. In fact, this production of genetic variation is one of the main reasons why eukaryotic life has diversified into numerous complex multicellular forms, in contrast to prokaryotic life.

However, sex doesn't seem to enhance the survival of the individual itself, which is rather puzzling. Thus, scientists in the past postulated that sex could have arisen due to group selection.

This isn't an ideal explanation, because group selection is weaker than individual-level natural selection. In addition, our current understanding of how evolution works is that it doesn't have a long-term view.

Could there be a short-term, individual-level advantage that led to the stepwise evolution of sex?

Dr. Wilkins focused on a critical aspect of sex reproduction: meiosis, which is the process that produces four haploid gametes from a single diploid cell. Scientists in this field generally agree that meiosis evolved from the simpler process of mitosis, which produces two genetically identical daughter cells (the method of reproduction for asexual organisms).

Compared to mitosis, meiosis has additional steps - the pairing up of chromosomes, genetic recombination due to crossing over, and the separation of non-sister chromatids during the first cell division.

Dr. Wilkins observed that the process of genetic recombination already exists in prokaryotes, although it is not tied with reproduction. He also noted that the separation of non-sister chromatids is straightforward derivation of sister chromatid separation in mitosis, which means that no new genes were required.

He then argued the chromosome pairing is the main evolutionary novelty in the evolution of meiosis. As the genome size of single-celled organisms became larger and contain a larger number of repeat sequences, DNA repair by recombination becomes less effective due to misalignment.

If the organism had a method of lining up the chromosomes more accurately, DNA repair could be enhanced.

This would provide an immediate advantage to the survival of the individual itself.

Thus, Dr. Wilkins hypothesized that the evolution of meiosis came about through the stepwise process of improving recombinational DNA repair, and identified some genes that could have been involved, such as the cohesins.

He then ended the talk with some experimental proposals to test this hypothesis.

So there you have it!

Sex is good and therapeutic.

I know I am Captain Obvious.


Bayman said...

Sounds like a wicked talk. I gotta track down a paper or something since I missed it.

Lim Leng Hiong said...

I tried but didn't find a paper by Dr. Wilkins himself - from the talk it sounds like it is not yet published. The nearest article I can find is this:

The evolution of meiosis: Recruitment and modification of somatic DNA-repair proteins

Bayman said...

Cool thanks for the paper. From Canada...even better!

I have been spending some time lately trying to get my head around the evolution of BRCA1 so this is right up that alley...