A single episode of sexual reproduction can prevent population extinction under multiple stressors

Recent studies have shown that organisms can adapt to changing environments through rapid contemporary evolution. Although intraspecific genetic variation is necessary for rapid evolution to occur, little is known how genetic variation has been produced and maintained before rapid evolution. Here we show that a single episode of sexual reproduction can produce a large amount of intraspecific trait variation that allows population growth in degraded environments by laboratory experiments of a green alga, Closterium peracerosum-strigosum-littorale complex. We observed population dynamics of the alga under multiple stressors and confirmed that high salinity and low pH decreased population growth rates. By comparing parental and their hybrid F1 populations, we observed larger variation in population 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 found greater enrichment of genes with copy number variations in terms related to pH stress than those to salt stress in a gene ontology (GO) enrichment analysis. 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 evolutionary rescue, where rapid evolution prevents population extinction in changing environments.