STRESS may not be much of a turn-on for us, but it seems that, at least in algae, sex may have evolved as a response to stressful conditions.

Aurora Nedelcu at the University of New Brunswick in Fredericton, Canada, showed that sex in the alga Volvox carteri (pictured) is a response to oxidative stress - damage caused by reactive oxygen-containing compounds (New Scientist, 19 June 2004, p 14). Her latest study compared the relevant genes in V. carteri with those in its single-celled relative Chlamydomonas reinhardtii, revealing the evolutionary connections between the two.

Sex in V. carteri is induced by heat stress, but in C. reinhardtii it is nitrogen deprivation that triggers sex. Nedelcu's comparison showed that the sex gene in V. carteri has a similar sequence to a gene family in C. reinhardtii that gets switched on when the plant is nitrogen stressed. This, she argues, means the genes are evolutionarily related and implies that sex in V. carteri evolved from general stress response genes (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2005.3151). Sex is a useful stress response because producing new gene combinations could allow the algae to adapt to a changing environment.