Why sex evolved

Despite the obvious benefits of sex, it's an activity that's biologically disadvantageous under most conditions. Now, a new study published online today (13 October) in Nature helps explain why sex may have evolved, despite its downside.

Single asexual female
monogonont rotifer.
Image: Kuiper and Becks

Specifically, the paper tracked a eukaryote for nearly 100 generations and found that the species was more likely to switch from asexual to sexual reproduction if it encountered varying physical settings, suggesting sex may help species adapt to diverse environments."The paper is an outstanding breakthrough," evolutionary biologist linkurl:Sally Otto;https://www.zoology.ubc.ca/%7Eotto/ of the University of British Columbia told The Scientist. "It's the first study to track -- in real time -- the evolution of sex in a multicellular eukaryote, finding that higher rates of sex evolve in a spatially complex environment," said Otto, who was not involved in the research.The evolution of sex has long puzzled biologists, as its disadvantages seem to outweigh its benefits in most situations. When an organism reproduces asexually, for example, it passes on 100 percent of its genetic information to the next generation. A sexually reproducing organism, on the other hand, only passes on half its genes -- a huge evolutionary cost. But sex does have its advantages, such as much more genetically variable offspring, and has evolved in many species. But under which conditions? To answer this question, many researchers have turned to theoretical models of populations, manipulating various environmental parameters to see when sex might be favored, and have generated many promising hypotheses. For example, these theoretical models have predicted that sex is more likely to evolve in heterogeneous environments because some individuals will find themselves in different surroundings that they must quickly adapt to in order to survive and reproduce. Sex allows them to do this by creating new gene combinations that may have an advantage over existing, potentially maladaptive genotypes. In spatially homogeneous environments, on the other hand, there is less need for adaptation to new surroundings, and sex is less likely to evolve.Experimental evidence for this and other theories, however, has been hard to come by.To address this deficiency, evolutionary ecologists linkurl:Lutz Becks;https://www.eeb.utoronto.ca/people/research+and+postdocs/becks and linkurl:Aneil Agrawal;https://labs.eeb.utoronto.ca/agrawal/ of the University of Toronto studied populations of monogonont rotifers, which can reproduce both sexually and asexually. Specifically, they reproduce asexually until populations reach a certain density, at which point females begin signaling to one another to produce males -- the first step in sexual reproduction. By monitoring these rotifers cultured under different conditions, Becks and Agrawal looked for differences in rates of sexual reproduction among the different populations.The team cultured the rotifers in pairs of subpopulations in one of three conditions: both with high-quality food (homogeneous), both with low-quality food availability (homogeneous), or one with high-quality and one with low-quality food (heterogeneous). They then simulated migration by transferring a proportion of each subpopulation to the other environment, which only differed in the heterogeneous treatment. After 6 and 12 weeks, or approximately 45 and 90 generations, the researchers exposed all rotifers to the high-density, sex-initiating signal, and measured the percentage of individuals that switched to sexual reproduction. As predicted, rotifers in the heterogeneous environment were more likely to make the switch than those in either homogeneous treatment. "The experimental data seem pretty clear to me," said evolutionary geneticist linkurl:Bill Birky;https://eebweb.arizona.edu/faculty/birky/BirkyLab.html of the University of Arizona, who was not involved in the research. "They do seem to have demonstrated a short-term advantage for sex in heterogeneous environments.""This study provides the first empirical demonstration that sex may be critically important in bringing together genes from near and far into more fit combinations," Otto said in an email.

Sexual females carrying resting eggs
(the darker eggs) and asexual females
(with the lighter shaded amictic eggs).
Image: Kuiper and Becks

But "there are two parts to sex," said evolutionary ecologist linkurl:Terry Snell;https://www.biology.gatech.edu/people/terry-snell/?id=terry-snell of the Georgia Institute of Technology, who did not participate in the study -- producing the signal that triggers some individuals to initiate sexual reproduction, and responding to that signal. Because the researchers provided the signal artificially, it was unclear if the rotifers in the heterogeneous environment would be more likely to secrete that signal themselves, and thus convert to sexual reproduction on their own. "By experimental design, they were simply testing the ability of the females to respond to the signal."To test the other component of sexual reproduction, Becks and Agrawal also tracked how many eggs were produced sexually versus asexually, without providing an artificial signal to initiate sexual reproduction. Again, they found that rotifers in heterogeneous environments produced more eggs sexually than those in homogeneous environments, but the difference was not nearly as great as the differences in propensity for sex. Still, the trend is there, suggesting that "real differences [are] occurring among these populations," Becks said. And as always, there is more work to be done. "I think it will move us further down the road in understanding the evolution of sex," Snell said, "but it's not the last word."Indeed, Becks plans to use the rotifer system to study other predictions regarding the evolution of sex, such as whether the existence of parasites gives sexual organisms a significant advantage, allowing them to evolve mechanisms of resistance. "[These rotifers] allow us to test the evolution of sex within populations," he said. "And we have the means to measure and manipulate it."L. Becks and A.F. Agrawal, "Higher rates of sex evolve in spatially heterogeneous environments," Nature, DOI: 10.1038/nature09449, 2010.
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[29th July 2009]*linkurl:Sexual conflict opposes sexual selection;https://www.the-scientist.com/news/display/25282/
[24th October 2006]