Hybrid incompatibility is a phenomenon in plants and animals, wherein offspring produced by the mating of two different species or populations have reduced viability and/or are less able to reproduce. Examples of hybrids include mules and ligers from the animal world, and subspecies of the Asian rice crop Oryza sativa from the plant world. Multiple models have been developed to explain this phenomenon. Recent research suggests that the source of this incompatibility is largely genetic, as combinations of genes and alleles prove lethal to the hybrid organism. Incompatibility is not solely influenced by genetics, however, and can be affected by environmental factors such as temperature. The genetic underpinnings of hybrid incompatibility may provide insight into factors responsible for evolutionary divergence between species.
Background
Hybrid incompatibility occurs when the offspring of two closely related species are not viable or suffer from infertility. Charles Darwin posited that hybrid incompatibility is not a product of natural selection, stating that the phenomenon is an outcome of the hybridizing species diverging, rather than something that is directly acted upon by selective pressures. The underlying causes of the incompatibility can be varied: earlier research focused on things like changes in ploidy in plants. More recent research has taken advantage of improved molecular techniques and has focused on the effects of genes and alleles in the hybrid and its parents.
Dobzhansky-Muller model
The first major breakthrough in the genetic basis of hybrid incompatibility is the Dobzhansky-Muller model, a combination of findings by Theodosius Dobzhansky and Joseph Muller between 1937 and 1942. The model provides an explanation as to why a negative fitness effect like hybrid incompatibility is not selected against. By hypothesizing that the incompatibility arose from alterations at two or more loci, rather than one, the incompatible alleles are in one hybrid individual for the first time rather than throughout the population - thus, hybrids that are infertile can develop while the parent populations remain viable. The negative fitness effects of infertility are not present in the original population. In this way, hybrid infertility contributes in some part to speciation by ensuring that gene flow between diverging species remains limited. Further analysis of the issue has supported this model, although it does not include conspecific genic interactions, a potential factor that more recent research has begun to look in to.
Gene identification
Decades after the research of Dobzhansky and Muller, the specifics of hybrid incompatibility were explored by Jerry Coyne and H. Allen Orr. Using introgression techniques to analyze the fertility in Drosophila hybrid and non-hybrid offspring, specific genes that contribute to sterility were identified; a study by Chung-I Wu which expanded on Coyne and Orr's work found that the hybrids of two Drosophila species were made sterile by the interaction of around 100 genes. These studies widened the scope of the Dobzhansky-Muller model, who thought it likely that more than two genes would be responsible. The ubiquity of Drosophila as a model organism has allowed many of the sterility genes to be sequenced in the years since Wu's study.
Modern directions
With modern molecular techniques, researchers have been able to more accurately identify the underlying genetic causes of hybrid incompatibility. This has led to both the development of expansions to the Dobzhansky-Muller model. Recent research has also explored the possibility of external influences on sterility as well.
The "snowball effect"