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.