Charles Darwin was among the first to recognize that –– if a species separates into two geographically distinct niches and undergoes a sufficient number of generations –– the two separate groups will become independent species, no longer able to breed with one another. This is called speciation, and (more than a century later) we now know that chromosomal inversions (as well as different numbers of chromosomes) are major causes of this inability of two species to interbreed. During meiosis (pairing of chromosomes of the egg with the sperm), chromosomal inversions and different numbers of chromosomes almost always lead to early embryonic death or, in lesser cases, an infertile offspring (often called the “hybrid”).
Chromosomal inversions are structural rearrangements, in which the linear gene order (across varying lengths of some DNA segment) is reversed (i.e. inverted). In crosses between two species that differ by one or more inversions, the resulting hybrids can experience meiotic chromosome-pairing problems and may, therefore, become sterile [For example, a mule is the offspring of a male donkey and a female horse; Donkeys and horses are different species –– having different numbers of chromosomes: the mule has 63 chromosomes, which reflects a mixture of the donkey’s 62 and the the horse’s 64. The different structures and numbers usually prevent chromosomes from pairing up properly to create successful embryos, rendering mules almost always infertile].
Authors [see attached article] reconstructed the evolutionary histories of chromosomal inversions in fruit flies (Drosophila persimilis and Drosophila pseudoobscura) to show that –– contrary to widely accepted ideas –– these inversions already had existed as polymorphisms in the ancestor of both species before their initial split into separate species. These findings will force evolutionary geneticists to reconsider the role of chromosomal inversions during the process of speciation.
These data raise the possibility that –– the higher genetic divergence of sequences, spanning across these chromosomal inversions, and an association with “hybrid-incompatibility genes” –– may be the most important factors during long-term segregation of these inversions. In other words, the role of chromosomal inversions during speciation should not be seen as protectors of existing hybrid incompatibilities, but rather as a solid foundation for their formation to occur later.
PLoS Genet July 2o18; 14: e1007526