Genetic mixing facilitates adaptation to a novel environmental constraint

Anthropogenic activities, including climate change, pose a major challenge to insect populations. Climate change can affect the length and timing of seasons, which can alter the time available for insects to complete their life cycles and successfully reproduce. Intraspecific hybridization between individuals from genetically distinct populations, or admixture, can boost fitness in populations facing novel selection pressures. Admixture can particularly benefit small and isolated populations with high levels of genetic load by masking deleterious alleles, thereby increasing fitness, and also by increasing the genetic variation available for adaptive evolution. To evaluate the effects of admixture on populations exposed to a novel life cycle constraint, we used the red flour beetle, Tribolium castaneum, as a model system. Distinct laboratory populations were kept isolated or admixed to create populations containing 1 to 4 lineages. We then compared the growth rate of admixed (2-4 lineage) to non-admixed (1 lineage) populations while rearing them under a shortened generation time with a restricted reproduction period for three generations. No difference in growth rate was observed after two generations. After three generations, admixture of two or more lineages significantly increased growth rate, with no difference between admixture amounts (comparing 2 vs. 3 vs. 4 lineage crosses). This lag in the increase in growth rate for the admixed populations suggests fitness increased via adaptation to the novel constraint rather than by masking of deleterious alleles. Our study highlights the importance of admixture for facilitating rapid adaptation to environmental change.