More than 99 percent of multicellular eukaryotes reproduce sexually and have evolved elaborate ways to do so, including behavioral, physiological, and biochemical adaptations. So there must be some enduring benefit. But despite years of observing, theorizing, and experimenting, researchers have been unable to pin down exactly what that might be.
But to better understand the pros and cons of sexual reproduction, the best place to look may be the outliers, such as eukaryotic organisms that have weird sex, or those that have no sex at all. Today, research on such outliers is happening in force. Recent studies using freshwater rotifers and genetically engineered yeast, for example, suggest sex is critical in times when environmental stress requires adaptation, while parasite-battling snails make the case that sex evolved as a powerful defense against infection.
There is movement in the field.
When food is scarce, however, they undergo meiosis and form haploid spores that eventually fuse with other spores to form a diploid zygote. But the starvation process is mutagenic in yeast, making it impossible to isolate the effects of sex.
Inwhen Goddard was a postdoc at Imperial College London, he and his colleagues found a way to sidestep the problem: This allowed the researchers to compare starved populations of asexual yeast with starved populations of a sexually reproducing wild-type strain. The team plopped both strains into benign and harsh conditions, measured the fitness of the populations, and got a straightforward "Asexual reproduction quotes about change" By deleting a gene involved in DNA repair, Goddard created strains of both asexual and sexual yeast with increased mutation rates, then compared those strains to normally mutating yeast under benign and stressful conditions.
Researchers have found that a higher rate of sexual reproduction is favored in Brachionus calyciflorus rotifers when the animals are in spatially heterogeneous environments or adapting to new conditions. While sex generates genotypes that are less successful than their asexual counterparts in the short term, sexual populations fare better during periods of environmental change.
The transparent, microscopic animal can be found in ponds and other freshwater communities, and, importantly for the study of sex, it reproduces asexually at low densities but sexually in crowded environments. When bunched together, the rotifers release a chemical cue that stimulates some females to produce haploid eggs that either develop into males or are fertilized to become diploid females. Within a diverse environment, gene combinations may be beneficial in one area but detrimental in another.
If individuals migrate between different regions, sex could be valuable because it breaks apart maladaptive gene combinations while creating new, potentially beneficial ones. Becks designed an experiment to see if a heterogeneous environment would promote sex in rotifers.
After switching rotifers from one environment to another, differing in food type and sodium chloride concentration, Agrawal and Becks counted the number of offspring and assessed the genotype diversity of both sexually derived and asexually derived populations.
They found that sex does generate genetic variation, but in the short term, the genotypes created by sex were on average less successful than their asexual counterparts.
Some genotypes, however, were much more successful, with the result that in the long term, the rotifer lineages Asexual reproduction quotes about change were more likely to make the switch to sex fared better, blossoming during periods of environmental change. The evolutionary dynamics "Asexual reproduction quotes about change" New Zealand snails Potamopyrgus antipodarum lend the strongest support for the idea that escaping parasitic infection could drive the ubiquity of sexual reproduction—the Red Queen hypothesis.
Threatened with infection by more than a dozen species of trematode worms, sexually reproducing P. Asexual clones may start to rise in frequency, but the parasites quickly evolve to infect these increasingly common genotypes, thereby driving them down in frequency once again. The Evolution and Genetics of Sexuality. In it, Lively read about a type of unusual mud snail Potamopyrgus antipodarum that exhibited both sexual and asexual reproduction and was studied by Mike Winterbourn, a Canterbury freshwater ecologist who happened to work just down the hall.
Observing the snails in New Zealand lakes and in the lab, Lively looked for evidence that sex prepares offspring for "Asexual reproduction quotes about change" and competitive environments the tangled bank hypothesis ; that sex yields a range of offspring, which can better adapt to environments that change over time than asexual clones the lottery model ; and that sex is favored for unknown reasons, but abandoned when mates are hard to find the reproductive assurance hypothesis.
Then he looked for evidence to evaluate the Red Queen hypothesis, which posits that interactions with parasites can drive selection for sexual reproduction.
The assumption is that parasites evolve to infect the most common host genotypes, and that sexual Asexual reproduction quotes about change has the advantage of being more likely to produce rare resistant genotypes. Upon infection and reproduction in their hosts, the worms sterilize the snails, putting the snails under strong selection pressure to evade the worms. This ran contrary to the Red Queen, which predicts that parasites would go after the most common genotypes, whether they are produced sexually or asexually.
But as his team continued to genotype the snails, the pattern changed: The researchers replicated the experiment in the lab and saw the same results: We are at a stage where we are pushing the boundaries, both in computer simulations and in experiments. Now at Indiana University in Bloomington, Lively has continued to test the Red Queen hypothesis—looking to see, for example, if clonal genotypes common in the recent past are more susceptible to infection by local populations of parasites.
In work that is currently in press, his team sampled four sites in a lake over five years and determined that asexual individuals averaged across all four sites were more infected than sexual snails, usually by a large amount, in four of those years. Only in the fifth year were sexuals more Asexual reproduction quotes about change, which Lively attributes to the parasites reducing the prevalence of the asexual clones. This April, his team showed that exposure to parasites increases both the rate of mating and the number of different mating partners for both males and females in the sexual snail populations.
Parasites target common genotypes, encouraging host genetic recombination. With so many hypotheses extolling the benefits of sex, one might think asexual creatures are doomed to extinction—unless, that is, there are other ways to achieve those same benefits. Though other animal species, especially insects, occasionally experiment with total asexuality, these attempts are rarely successful.
Bdelloids, on the other hand, have been successfully reproducing for more than 80 million years and have diversified into different species, despite molecular evidence that they lost the ability to have sex tens of millions of years ago.
If we can identify that, then that will give us a strong hint as to why everything else needs to have sex. Bdelloid rotifers are the most successful animals that are completely asexual. By ridding themselves of all their water, for example, desiccated rotifers can escape parasites simply by being blown away in the wind. But there is evidence supporting the Red Queen hypothesis in the unique way that the rotifers evade parasitic infections.
Ridding themselves of all their water, the rotifers become as light as flecks of sand and Asexual reproduction quotes about change away in the wind, leaving their parasites behind.
Christopher Wilson and at Cornell University in New York isolated three bdelloid species from moss, grew them in petri dishes with rainwater, and exposed them to a fungal parasite. When the rotifers were exposed to air, they desiccated within 24 hours, and then a light breeze from several fans blew the dried-up organisms around a wind chamber and onto fresh Petri dishes, where new populations grew—all of them infection free.
The bdelloids rid themselves of six different parasites this way. Last year, researchers found evidence that desiccation also provides a mechanism by which the bdelloids introduce genetic variation.
Desiccation is thought to compromise cellular integrity, which allows the absorption of DNA from other sources—bacteria, fungi, and other nearby organisms. That novel DNA is then put to use, it seems. Thus, while they may not have sex, bdelloid rotifers are diversifying nonetheless, a process that appears critical to survival on this planet.