Host shifts favor vibrational signal divergence in Enchenopa binotata treehoppers

For specialized herbivorous insects, shifts to novel host plants can have dramatic evolutionary consequences. If mating traits diverge, assortative mating can develop between ancestral and novel host populations and facilitate speciation. Mating signals may diverge under a variety of scenarios. Signal differences may be a consequence of divergence in correlated traits, such as body size. If local communication environments differ, mating signals may also diverge through selection for enhanced transmission. We tested these hypotheses using 2 closely related species in the Enchenopa binotata complex of treehoppers. Each member of this complex specializes on a different host plant species. Their communication modality may make signal divergence likely after a host shift: like many plant-dwelling insects, Enchenopa communicate using substrate-borne vibrations for which the plant itself is the transmission channel. Each species' mating signal is a relatively pure tone, and differences between species in signal frequency are critical for mate recognition. Whereas no support was found for a correlated selection hypothesis, we found support for a signal transmission hypothesis: both species use a signal frequency that transmits well in their contrasting communication environments, suggesting that host shifts may favor signal divergence and ultimately behavioral isolation. Copyright 2008, Oxford University Press.