Experimental test of the fitness effects of divergent marine–freshwater chromosomal inversions in stickleback under different salinity conditions

Chromosomal inversions are a type of structural variant that have long interested evolutionary biologists because of their potential role in local adaptation and speciation. However, direct experimental evidence for the fitness consequences of inversions is rare, limiting our ability to dissect the evolutionary forces associated with the spread and maintenance of inversions in natural populations. We tackle this knowledge gap by studying the fitness effects of three chromosomal inversions that consistently differ between marine and freshwater populations of threespine sticklebacks (Gasterosteus aculeatus). Using controlled laboratory crosses, we tested whether inversion genotype influences fitness (measured as survival, standard length, and body condition) across two salinity treatments (freshwater vs saltwater). In both the freshwater and the saltwater treatments, there were no deviations from Mendelian ratios at any of the three inversions. This suggests that there are no intrinsic deleterious effects of these inversions, in contrast to observations from other systems. Overall, there was no effect of inversion genotype on standard length or body size across the two salinity treatments for the chromosome XI and XXI inversions. For the chromosome I inversion, heterozygotes had a slightly lower body condition in the freshwater treatment. Together, these results suggest that the fitness effects of these inversions are not strongly influenced by salinity and that other selective forces might be involved in their evolution. More broadly, these findings highlight the importance of performing empirical tests of fitness effects of chromosomal inversions to better explain their spread and maintenance in nature.