Thinking about baker's yeast, we tend to focus on its useful side, notably as an invaluable aid in producing fluffier bread. It's easy to forget that it also has a sex life. In the haploid stage, two mating types are produced, which can be viewed as essentially analogous to the two sexes in vertebrates like ourselves. As soon as two sexes are present, the way is open for sexual selection, where organisms can out-reproduce others if they are better at securing more or superior mates. Sexual selection is known to drive the evolution of a multitude of reproductive traits, but it has also been argued that this evolutionary force could be involved in both increasing and decreasing diversity. By driving isolation, new species may be formed but it may also push populations towards the edge. The dangerous context of flirting with extinction could be exacerbated further when environmental conditions change, especially if change occurs too rapidly. In research published in Biology Letters, Luke Reding and colleagues addressed these themes experimentally using the technique of experimental evolution.
The team exposed populations of Saccharomyces cerevisiae to two different intensities of sexual selection by manipulating sex ratio. They then contrasted these sexually selected lines with asexual populations. Weak and strong sexual selection were generated by setting up populations in which the ratio between mating types was either balanced or skewed. Skewing the ratio increased competition among the spores of the more common mating type. To examine adaptation rates, all populations were allowed to evolve for some 250 generations exposed to a stressful high-salt environment in a glucose-containing medium. Population responses were also monitored under conditions where glucose was replaced with sucrose (representing an additional challenge, as digestion requires production of the enzyme invertase). The authors performed an ambitious series of assays testing population fitness, mating propensity between populations (subjected to the same sexual selection treatment) and extinction risk.
The team found that populations evolving under strong sexual selection were less able to adapt to the harsh salty conditions in comparison to their counterparts exposed to other treatments. This reduced performance under duress is possibly a consequence of shifting resources towards greater investment in mating pheromones in order to increase attractiveness when competition is magnified. This finding is particularly noteworthy because we know that being sexual rather than asexual usually allows faster adaptation. Strikingly, this benefit seems to disappear if sexual selection is too strong. Similar results emerged regardless of whether the medium fed to populations contained glucose or sucrose. Interestingly, the study also revealed a trend suggestive of emerging reproductive isolation between populations exposed to strong sexual selection. Crosses between individual lines subjected to strong sexual selection tended to have a lower success rate. So the action of sexual selection might also increase divergence between populations, thus promoting diversity.
Stepping back to view the bigger picture, these promising results confirm that sexual selection can be an agent of evolutionary change and may shape biodiversity in various biological systems. Environmental change can clearly act in conjunction with population-level characteristics such as sex ratio. This, in turn, shows that we urgently need a greater understanding of such interactions, especially in the context of fast-moving climate change.
