A single episode of sexual reproduction can produce large variation in population growth rates under dual stressors

Yawako W. Kawaguchi, Masato Yamamichi

Journal of Evolutionary Biology (2025) DOI:10.1093/jeb/voaf041

Press release (In Japanese only)

Sexual reproduction has been a central topic in evolutionary biology because of its many costs: why have organisms evolved sexual reproduction despite the many costs of sex? To answer the question, researchers have conducted laboratory experiments to measure population growth rates with and without sexual reproduction under a stressor. Here we show that a single episode of sexual reproduction can produce a large amount of variation in population growth rates under dual stressors by laboratory experiments of a green alga, Closterium peracerosum–strigosum–littorale complex. We observed population dynamics of the alga under dual stressors and confirmed that high salinity and low pH decreased growth rates. By comparing parental and their hybrid F₁ populations, we observed larger variation in growth rates of F₁ populations (i.e., transgressive segregation) when pH was low. Interestingly, even when parental populations had negative growth rates, some F₁ populations showed positive growth rates in severe environmental conditions due to the large variation in population growth. By utilizing the recently obtained genomic information of the alga, we conducted a gene ontology (GO) enrichment analysis and found that genes with copy number variations between parental strains were more frequently associated with pH stress-related terms than salt stress-related terms. Our results suggest that recombination and variation in the number of gene copies might produce large genetic variation in the F₁ generation. This will be an important step toward a better understanding of evolution of sex and evolutionary rescue where rapid contemporary evolution prevents population extinction in changing environments.