Researcher Richard Michod and colleagues at the University of Arizona in 
Tucson have found that high temperatures make the green algae Volvox 
more likely to reproduce sexually.

A news article about the story:

``Is Sex a Cure for Stress? 
  Heat turns normally asexual algae into sexual deviants''

 - http://www.betterhumans.com/News/news.aspx?articleID=2004-06-09-3

Michod interprets the study as evidence for his "sex->gene repair"
theory.

I'm less convinced.  The study is on a creature that reproduces
rather like an aphid - asexually when the going is good and
sexually before dispersal and winter, and - the in the study -
the heat stress activates the sexual behaviour.

However, plenty of other things cause stress - in particular,
pathogens and a changing environment are both stressful - and
other theories about the origin of sex invoke just such
circumstances in the course of their explanation.

So - it's not easy to interpret the observation that stress induces
sexual behaviour as evidence for the gene repair theory of sex -
since the competing theories make similar predictions in this area.

In general, stress might well indicate that your genome is not working out 
- and that some outcrossing might pay off.

However the *reason* for the problem is not necessarily known -
it /might/ be an accumulation of deleterious mutations (Michod's theory) 
- but it might well also be due to a pathogens successfully tuning in to 
it (Hamilton's theory).

This particular organsm probably isn't telling us about that, though:
if late summer naturally signals sexual reproduction for it for 
some reason (perhaps to generate diversity in the face of the high 
selection pressure from the environmental challenges winter produces), 
then heat stress might simply be a means of signalling that - with the
details of the form of the signal being of no consequence.

Organisms - such as aphids - which exhibit facultative parthenogenesis are
indeed useful models with which to examine the short- and long-term 
benefits of sexual reproduction, though.
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