The first Dobzhansky–Muller pair

In the 1930s, Dobzhansky and Muller — two pioneers of fly genetics — proposed that hybrid incompatibilties that contribute to speciation are caused by interactions between genes that have functionally diverged between the hybridizing species. Seventy years later, Daniel Barbash and colleagues have identified the first such pair of genes that cause male lethality in crosses between two Drosophila species. One of the partners encodes a protein that localizes to a heterochromatic region, and the authors suggest that the rapid evolution of sequences in this region has driven the evolution of the incompatibility gene.

Several genes that contribute to speciation are known, but none fulfils the three Dobzhansky–Muller criteria: reduced fitness of the hybrid; functional divergence between the two hybridizing species; and dependence of a pair of genes on each other to cause their incompatibility effects.

No male flies are produced in crosses between Drosophila melanogaster females and Drosophila simulans males. It was previously shown that the Lhr¹ mutation in D. simulans and a mutation in Hmr in D. melanogaster suppress this lethality, and genetic interaction studies hinted that there was something 'Dobzhansky–Mullerian' about this behaviour. Hmr is known to be X-linked and to encode a rapidly evolving DNA-binding protein with an MADF domain. The authors used a combination of mapping and candidate-gene approaches to clone the Lhr gene from D. simulans. Among the rapidly diverging candidates (a hallmark of a Dobzhansky–Muller gene) they found CG18468; it contained a BESS domain, which mediates protein–protein interactions and is often associated with MADF domains. It turns out that the Lhr¹ strain that rescues hybrid incompatibility carries a ∼4 kb insertion in the predicted 5′ UTR of CG18468, and that transcription of CG18468 is reduced in this strain.

Barbash and colleagues introduced D. simulans CG18468 into D. melanogaster on an inducible vector. In a series of crosses, they proved that CG18468 is Lhr — a major-effect hybrid-lethality gene. Using an Hmr hypomorphic mutation, the authors showed that the hypomorph-mediated rescue of hybrid lethality is suppressed by Lhr¹. Moreover, male hybrids that carry a null Hmr mutation are fully viable even when they carry the induced Lhr transgene — in other words, the lethal effects of D. simulans Lhr¹ require functional Hmr. Conversely, the D. melanogaster Hmr requires functional D. simulans Lhr to exert its lethal effects.

Having shown that their pair fulfils all three Dobzhansky–Muller criteria, the authors wondered about the evolutionary forces that drive the divergence of Lhr. Because the Lhr protein localizes to heterochromatin, they propose that it “...may be coevolving with rapidly evolving heterochromatic repetitive DNAs”, consistent with a previously proposed hypothesis on how repetitive DNAs might contribute to hybrid incompatibilities and speciation.