East Lansing, MI, USA. Evolutionary biologists have observed the mechanism whereby an organism can correct for intersex mutations to ensure a balanced sex ratio and continuing propagation of the species.

When genomic and genetic mechanisms for sex determination are disrupted, the nematode, a model organism with reproductive genetics that are highly predictive for humans, can quickly evolve ways to bypass the problem, finding ways to compensate and create a balanced sex ratio.


There are important evolutionary and ecological consequences of sex determination and while it is known that a large variety of sex determination mechanisms exist, there has been limited information available on how they actually evolve. Ian Dworkin and Christopher Chandler are members of the BEACON Center for the Study of Evolution in Action at Michigan State University. They led a team, including researchers from Iowa State University and the University of Oregon, that used an experimental evolution approach to study adaptations in sexual determination of nematodes, more commonly known as worms.

The new findings appear in the journal Evolution. The scientists showed that even mutations that induce intersex will eventually weaken with the passage of generations, as the organism recovers toward its wild-type phenotypes. This is interesting from an evolutionary perspective because it shows that protection of species propagation overrides other considerations.

Sex determination is, at minimum, genomic. It involves the precise packaging of heritability information into genes, which are then organized on chromosomes. But the genetic information would be useless without the other resources provided by the human genome (such as error correction and regulation, and more). Breakdowns, such as processing errors or the introduction of mutations, can lead to unexpected variations from statistical norms. Sometimes error-correction processes are overwhelmed and non-standard outcomes occur.

What then? "Our findings show the nematodes evolved quickly to diminish any negative effects caused by mutations in the sex-determining mechanisms," said Christopher Chandler, a post-doctoral researcher who led the study.

The proportion of males to females in a population is called the sex ratio, specifically defined in biology as the proportion of males in the population. The ratio can change somewhat from conception (the primary ratio) throughout an orgaism's existence. The secondary ratio is recorded at the time of birth, followed by the ratio of mature organisms (tertiary).

Sex determination is a critical developmental decision with major ecological and evolutionary consequences, yet a large variety of sex determination mechanisms exist and we have a poor understanding of how they evolve. Substantial theoretical and empirical work has suggested that compensatory adaptations to genetic mutations may play a role in the orderly evolution of sex determination.

Focused on the genomics, and controlling for external influences, Chandler studied 50 generations of nematodes after introducing a pair of mutations in two key sex-determining genes that normally help worms develop into males or females.

The effects of these mutations also depend on the environmental temperatures, which cause intersexual phenotypes at intermediate temperatures (but still weakly to moderately fertile). So the team tested whether worms adapted to the mutations at just one temperature or across a range of temperatures.

"Unless we grew them in pretty warm environments, it didn't seem to matter much — the worms evolved to do better across a range of temperatures," Chandler said. At the genetic level, worms bypassed the problem rather than fixing it. Ian Dworkin, assistant professor of zoology, said "There was little or no change in the genes involved, and instead they made the changes elsewhere. As they evolved, they swiftly compensated to create a balance with respect to their sex."

The findings have big implications for how sex determination evolves. Sex determination is important for reproduction in all organisms and it is tightly controlled at the gene level.

"Our findings show the mechanisms themselves are flexible and adaptable from an evolutionary viewpoint," Chandler said. "If something goes wrong with the control mechanisms, a work-around can quickly be found to restore the balance."

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