SURVIVAL OF HYBRIDS IN A MOSAIC HYBRID ZONE

The ground crickets Allonemobius fasciatus and A. socius meet in a mos aic hybrid zone that stretches from New Jersey at least as far west as Illinois. Within mixed populations from the contact zone, ''pure'' sp ecies individuals predominate. To determine whether hybrids are less v iable than pure-species individuals, and to assess whether the high pr oportion of pure-species individuals in mixed populations results from hybrid inviability, we performed a cohort analysis. In this study, fi ve mixed populations from the hybrid zone were each sampled three to f ive times from the fall of 1986 to the spring of 1988. Individuals wer e assigned to one of three categories based on their genotypes: A. soc ius (individuals harboring only alleles unique to A. socius), hybrid ( individuals with alleles unique to both species), and A. fasciatus (in dividuals harboring only alleles unique to A. fasciatus). This samplin g and measurement scheme permitted monitoring of the proportion of hyb rid individuals in a population over time. The results were fairly con sistent. The relative survival of A. socius was greater than the relat ive survival of members of the other two groups in four of the five po pulations. The relative viability of A. fasciatus was greater than tha t of hybrids in one population, approximately equal to that of hybrids in two populations, and less than that of hybrids in two populations. These results not only shed light on an important component of fitnes s, viability from hatching to adulthood, but they demonstrate that los s of hybrid individuals during the course of the field season will not explain deviations from random mating expectations present in mixed p opulations in late summer. The deviations must be due to assortative m ating or to a reproductive barrier operating prior to egg hatch.