A comparison of the association between large haplotype blocks under selection and the presence/absence of inversions

Inversions may contribute to ecological adaptation and phenotypic diversity, and with the advent of "second" and "third" generation sequencing technologies, the ability to detect inversion polymorphisms has been enhanced dramatically. A key molecular consequence of an inversion is the suppression of recombination allowing independent accumulation of genetic changes between alleles over time. This may lead to the development of divergent haplotype blocks maintained by balancing selection. Thus, divergent haplotype blocks are often considered as indicating the presence of an inversion. In this paper, we first review the features of a 7.7 Mb inversion causing the Rose-comb phenotype in chicken, as a model for how inversions evolve and directly affect phenotypes. Second, we compare the genetic basis for alternative mating strategies in ruff and timing of reproduction in Atlantic herring, both associated with divergent haplotype blocks. Alternative male mating strategies in ruff are associated with a 4.5 Mb inversion that occurred about 4 million years ago. In fact, the ruff inversion shares some striking features with the Rose-comb inversion such as disruption of a gene at one of the inversion breakpoints and generation of a new allele by recombination between the inverted and noninverted alleles. In contrast, inversions do not appear to be a major reason for the fairly large haplotype blocks (range 10-200 kb) associated with ecological adaptation in the herring. Thus, it is important to note that divergent haplotypes may also be maintained by natural selection in the absence of structural variation.