Wednesday, March 2, 2011

Of sex and death

Why does sex exist? And why having sex with another organism instead of oneself? These are two of the questions that Patrick Phillips, from the Unviersity of Oregon, tried to answer in his talk at the PRBB last week.

The biologist uses the nematode C.elegans for his research, which consists primarily in recreating evolution in the lab. How does that work? It basically consists of two steps: artificial selection+adaptation to the laboratory conditions. That is, he creates a novel environment, generates mutations in the worms, and sees which ones adapt and which ones die. Cruel? Not more than reality…

The advantages of these evolutionary experiments in the lab is that they are controlled and can be replicated. The problems are that they are limited in time (while real evolution takes 1000s of years) and there’s also a limited population size, which means that rare events (as in rare mutations) won’t be seen. Regardless of these inconveniences, Phillips managed to convince the audience that these experiments can prove that sex is good – to get rid of deleterious mutations and to increase genetic variation, which provides a better adaptation to the changing environment.

According to the scientist, one reason C. elegans is good for studying evolution is that they can be frozen. When you are doing the kind of experiments he does, if you freeze worms from different generations along the experiment, you have the equivalent to a ‘fossil register’ that allows you to compare the organisms at the ‘beginning’ and the ‘end’ of the evolution phase you are studying. Isn´t that cool?

Apart from sex, Phillips also talked about death – or why we age. Again, the elegans nematode plays an important role in ageing research: actually some mutants can live up to 10 times their usual lifespan. That is the equivalent of a human living 1000 years!

His experiment consisted in breading the worms for 323 generations by selecting only one worm in each generation to spread the population. As a consequence, the population size went dooooown and the worms were very sick (ah, the lack of genetic variation!). He then, playing God, saved the species by letting a higher number of worms reproduce for another 60 generations. When he compared the genome of the sick (thanks to his ‘fossil record’) and the recovered worms he found very few changes: only about 10 nucleotides! He went back in history to check when each change had taken place – again, thanks to the frozen worms (amazing, eh?). What he found is that the ‘recovery’ mutations were not the result of ‘mutating back’ to the original sequence, but rather they were compensatory mutations.

All in all, he showed us what he called an ‘emerging paradigm in evolutionary biology’, a new way of studying evolution: create a perturbation (mutation); propagate the species for 50-100 generations to let them recover fitness; sequence the genome to find out which changes have occurred; use genetics to confirm the results.

Voilà! Now you can try it at home :)

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