Establishment of F1 hybrid mortality in real time

Ashley Saulsberry, Marisa Pinchas, Aaron Noll, Jeremy A. Lynch, Seth R. Bordenstein, Robert M. Brucker

Measuring the evolutionary rate of reproductive isolation is essential to understanding how new species form. Tempo calculations typically rely on fossil records, geological events, and molecular evolution analyses. The speed at which genetically-based hybrid mortality arises, or the "incompatibility clock", is estimated to be millions of years in various diploid organisms and is poorly understood in general. Owing to these extended timeframes, seldom do biologists observe the evolution of hybrid mortality in real time. Here we report the very recent spread and fixation of complete asymmetric F1 hybrid mortality within eight years of laboratory maintenance in the insect model Nasonia. The asymmetric interspecific hybrid mortality evolved in an isogenic stock line of N. longicornis and occurs in crosses to N. vitripennis males. The resulting diploid hybrids exhibit complete failure in dorsal closure during embryogenesis. These results comprise a unique case whereby a strong asymmetrical isolation barrier evolved in real time. The spread of this reproductive isolation barrier notably occurred in a small laboratory stock subject to recurrent bottlenecks.