Why are there different species on Earth? Part of the reason is because hybrids between species are often inviable or sterile. Because of such "hybrid incompatibility", separate species' gene pools can't blend together, and each species ends up following its own evolutionary path. But why does hybrid incompatibility exist? It's always better to pass on your genes than to have inviable offspring, so natural selection should not favor hybrid incompatibility. In the early 20th century, an answer came from a genetic model developed by Bateson, Dobzhansky, and Muller, showing how interacting genes can evolve incompatibilities as a side-effect of divergence between species, resulting in hybrid inviability or sterility. Now, we are beginning to discover some genes that fit their model, and we can investigate how such unfavorable genetic interactions arise. Is it because different, incompatible, mutations spread by chance through separate populations? Or is natural selection driving the evolution of genes involved in hybrid incompatibilities?
Papilio glaucus (the Eastern Tiger Swallowtail) and Papilio canadensis (the Canadian Tiger Swallowtail) are two species of North American butterflies. Although similar in appearance, they are adapted to different climates, and they show mild hybrid incompatibility. Sasha Tulchinsky, a Ph.D candidate in OEB, is studying interactions between mitochondrial and nuclear genes in these species. Mitochondria have their own genome, and their genes must work together with genes from the cell's nucleus in order to make the mitochondrial protein complexes that produce energy for the cell. Often, hybrids between species have malfunctioning mitochondria, because the mitochondrial genes from one species don't work well with the nuclear genes from the other. How this so-called "cytonuclear incompatibility" evolves is an area of active research. One possibility is that, because the mitochondrial genome has a high mutation rate, slightly disadvantageous mutations in mitochondrial genes manage to spread through a population by chance. Mutations in nuclear genes that correct for these mitochondrial mutants are then favored by natural selection, so each species ends up with its own set of genes that are adapted to work together. When these genes mix in a hybrid, they meet genes they're not co-adapted with, and incompatibility results. This is called the "compensatory coevolution" model, but it's not the only possible explanation. Adaptation to different climates may involve changes in interacting genes (and therefore hybrid incompatibility) as natural selection works to optimize mitochondrial function at a certain temperature. Sasha sequences mitochondrial and nuclear genes from butterflies across a naturally occurring hybrid zone in order to learn about the genes' evolutionary history, and understand how hybrid incompatibilities evolve in general.