Hitchhiking of deleterious mutations within chromosomal inversions.

The suppression of recombination can have the benefit of associating multiple adaptive alleles but at the cost of increasing linkage between beneficial and deleterious alleles. Chromosomal inversions provide a natural genomic context to study such genetic hitchhiking. Inversions suppress recombination in heterokaryotypes, effectively creating large linkage blocks that behave like partially asexual regions. Synthesizing theory and empirical studies on variation in inversion frequencies among well-studied ecotypes highlights an emerging pattern of two evolutionary trajectories in nature. An excess of inversion homokaryotypes, due to their association with an ecologically adaptive trait, results in the purging of deleterious mutations facilitated by recombination. In contrast, without strong ecological selection, deleterious mutations can accumulate in inversion heterokaryotypes, which then rise in frequency due to overdominance.