Rapid evolution of hybrid male sterility in the phytophogous insect, Dendroctonus ponderosae

Recently, evidence of reproductive incompatibilities between some Dendroctonus ponderosae populations was discovered. These results were perplexing given a concurrent rangewide phylogeographic analysis that failed to detect pronounced genetic divergence between populations from those same locations. In light of these two findings, we conducted a rangewide crossing study to investigate the evolution of reproductive isolation. Multiple interpopulation crosses were performed between increasingly divergent populations in a common garden experiment and hybrid viability and hybrid fitness were measured. Ample quantities of male and female offspring were produced from all interpopulation crosses, yet hybrid males from geographically distant crosses were effectively sterile. Hybrid male sterility is widely considered the first postmating barrier to develop between divergent taxa (Haldane’s rule) and its occurrence suggests an incipient speciation event within what is currently described as Dendroctonus ponderosae. The geography and host use of populations demonstrating sterility when crossed will be discussed.