Reproductive Costs to Heterospecific Mating Between Two Hybridizing Katydids (Orthoptera: Tettigoniidae)

Animal taxa meeting in hybrid zones often exhibit partial reproductive isolation. This isolation may result from a variety of causes both before mating (e.g., incompatibilities in courtship behaviors) and subsequent to mating. Understanding the factors effecting reproductive isolation in hybrid zones can offer important insights into the process of speciation and the maintenance of species boundaries. The katydids Orchelimum nigripes Scudder and O. pulchellum Davis (Orthoptera: Tettigoniidae) form 2 hybrid zones in the eastern United States. I carried out breeding studies in the laboratory using animals from pure O. nigripes and O. pulchellum populations in the vicinity of one of these hybrid zones to examine possible modes of postmating reproductive isolation. The number of eggs produced by females mated to heterospecific males was dramatically lower than that of females mated conspecifically, but there was no evidence of any differences in hatch rate or offspring viability between egg clutches from heterospecific and conspecific crosses. Hatch rate, offspring viability, development time, and adult weight of hybrid progenies were all intermediate relative to corresponding values for progenies resulting from the 2 types of conspecific matings, although most of the differences between hybrids and each of the 2 classes of nonhybrid progenies were not statistically significant. The reduced oviposition of heterospecifically mated females suggests that females mated to heterospecific males do not receive the necessary stimulation to trigger oocyte maturation or oviposition behavior. The results reported here show that although some females will mate with heterospecific males, these matings tend to result in substantially reduced reproductive success relative to conspecific matings.