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The sibling ground crickets Allonemobius fasciatus and A. socius meet along a mosaic hybrid zone at ≈ 40°N latitude in eastern North America. In this paper we report the findings of a genetic analysis of calling-song variation within and among six cricket populations sampled along a transect through the hybrid zone in southern New Jersey. We compared aspects of the calling song of both wild-caught and laboratory-reared crickets to test the hypothesis that population differences in song observed in the wild were genetically based. We found significant, species-level differences in all aspects of the calling song, and these differences persisted even after a generation of common-garden rearing in the laboratory, supporting the hypothesis that interspecific variation observed in the wild largely reflects genetic differentiation between the two taxa. A discriminant function analysis indicated that individual crickets could be assigned to the proper taxon with less than 10% error, supporting the premise that calling song could be used by female crickets as a mechanism for species recognition. One population, collected from within the hybrid zone and containing significant numbers of hybrid individuals, was intermediate in its calling song, presumably reflecting this population's mixed genetic makeup. In this hybrid zone population, song phenotype was highly correlated to a hybrid index score generated using species-specific alleles at four diagnostic allozyme markers, suggesting a multigenic basis to calling-song variation in these crickets as well as linkage disequilibrium between markers and song. Based on an analysis of laboratory-reared full-sib families, broad-sense heritabilities for calling-song characteristics were generally significant in the two A. socius populations, whereas many components of song showed no significant family effects in the three A. fasciatus populations. The genotypically mixed, hybrid zone population showed very high heritabilities for most calling-song components, which likely reflect the influence of interspecific gene flow on genetic variation for quantitative traits.

Postinsemination barriers to fertilization generally have been ignored by biologists interested in the origin and nature of reproductive isolation among closely related terrestrial animals. Yet evidence presented in this paper indicates that such a barrier bears primary responsibility for the reproductive isolation between the ground crickets Allonemobius fasciatus and Allonemobius socius. Postinsemination barriers to fertilization may isolate many other terrestrial animals as well, but the design of most laboratory hybridization experiments precludes the detection of these barriers.

The ground crickets Allonemobius fasciatus and A. socius meet in a mosaic hybrid zone that stretches from New Jersey at least as far west as Illinois. Within mixed populations from the contact zone, "pure" species individuals predominate. To determine whether hybrids are less viable than pure-species individuals, and to assess whether the high proportion of pure-species individuals in mixed populations results from hybrid inviability, we performed a cohort analysis. In this study, five mixed populations from the hybrid zone were each sampled three to five times from the fall of 1986 to the spring of 1988. Individuals were assigned to one of three categories based on their genotypes: A. socius (individuals harboring only alleles unique to A. socius), hybrid (individuals with alleles unique to both species), and A. fasciatus (individuals harboring only alleles unique to A. fasciatus). This sampling and measurement scheme permitted monitoring of the proportion of hybrid individuals in a population over time. The results were fairly consistent. The relative survival of A. socius was greater than the relative survival of members of the other two groups in four of the five populations. The relative viability of A. fasciatus was greater than that 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 fitness, viability from hatching to adulthood, but they demonstrate that loss of hybrid individuals during the course of the field season will not explain deviations from random mating expectations present in mixed populations in late summer. The deviations must be due to assortative mating or to a reproductive barrier operating prior to egg hatch.

Two closely related species of ground crickets, Allonemobius fasciatus and A. socius, overlap and hybridize in a contact zone in the eastern United States. In earlier work, Howard (1986) described geographic variation in the width of the zone and in the strength of reproductive isolation between the two ground crickets. The zone was wider in the hills and mountains of southeastern Ohio and West Virginia than along the eastern coastal plain, and reproductive isolation appeared to be stronger where the zone was wider. Howard attributed the greater width in the mountains to the wide intermingling and patchy distribution of habitats appropriate for a species adapted to a northern climate and for a species adapted to a southern climate. He also pointed out that the mosaicism and the increased breadth of the zone in the mountains enhanced the probability of occurrence of reinforcement. We tested three predictions that emerged from Howard's hypothesized links among topographic diversity, zone width, and the strength of reproductive isolation. The first two predictions were fulfilled. The northern cricket, A. fasciatus, occurred in the high mountains south of its previously known distributional limit; and the zone narrowed considerably in Illinois, an area of low topographic diversity. These results provide further evidence for the importance of the environment in determining the structure of the zone. The third prediction was falsified. Contrary to the prediction, the strength of reproductive isolation between the two species was as strong in Illinois as in the Appalachian Mountains. This result suggests that if reinforcement has occurred in the zone, the width of the zone has not been a major factor in the process.

Eurosta solidaginis, a gallmaking tephritid (Diptera), infests at least two species of Solidago in the eastern United States. We used horizontal starch gel electrophoresis of enzymes to examine whether populations on the two principal host plant species, Solidago altissima and S. gigantea, have diverged genetically. At the d-β-hydroxybutyrate dehydrogenase (HBDH) locus, the predominant allele in all nine populations from S. altissima (HBDH1.00 ) was absent or uncommon in all but 1 of the 12 populations from S. gigantea. At the phosphoglucomutase locus (PGM), the most common allele in all populations was PGM1.00 , but the frequency of this allele was, with one exception, higher in populations from S. gigantea than in populations from S. altissima. Genetic heterozygosity was usually greater in populations from S. altissima (H = 0.028) than in populations from S. gigantea (H = 0.009). A phylogenetic tree derived from a genetic distance matrix clustered gallmaker populations from the same host plant together.

Remington (1968) argued that 13 suture zones exist in North America. Remington defined a suture zone as, ''a band of geographic overlap between major biotic assemblages, including some pairs of species or semispecies which hybridize in the zone'' (p. 322). Although initially controversial, the idea that suture zones exist has picked up momentum over the past decade, due largely to the phylogeographic work of Hewitt, Avise, and their colleagues. Nevertheless, the reality of suture zones has not yet been subjected to rigorous analysis using statistical and geographic information system (GIS) approaches. To test for the existence of Remington's suture zones, we first identified 117 terrestrial hybrid zones in Canada and the United States through a literature search for the key words ''cline,'' ''contact zone,'' ''hybrid zone,'' and ''hybridization'' in articles published between 1970 and 2002. The 117 hybrid zones were mapped using a GIS approach and compared with a digitized version of Remington's original suture zone map. Overall, there does appear to be an association between hybrid zones and suture zones, but this association is largely attributable to clustering of hybrid zones in only two of the 13 suture zones recognized by Remington. The results suggest that evolutionary biologists should retain some skepticism toward Remington's suture zones.

Many hybrid zones have a mosaic structure, yet we know of no theoretical work that examines the impact of mosaicism on the outcome of evolution. We developed a computer simulation model designed to test whether the outcome of reinforcing selection differs in a mosaic and a clinal hybrid zone. Our model was a one-dimensional stepping-stone model. The mosaic and clinal hybrid zones that we modeled were, respectively, a mosaic maintained by differential fitness of the interacting taxa in patchy habitats and a tension zone. We modeled changes in gene frequency at two biallelic loci, A and B. Hybrids at the A locus were selected against. An allele at the B locus caused assortative mating at the A locus, which promoted reinforcement; there was a selective cost to this allele. In a mosaic hybrid zone, spatial variation in the fitness of A-locus homozygotes in different patches caused gene and genotype frequencies at the A and B loci to differ greatly from those in a tension zone. Compared to a tension zone, a mosaic hybrid zone had a broader region in which hybrids could be formed and, thus, a broader region in which the assortative- mating allele provided a net selective advantage (via decreased production of the less fit A-locus hybrids). This caused the assortative-mating allele to be favored under a broader set of conditions in a mosaic hybrid zone than in a tension zone. In mosaic and tension hybrid zones, both low and high levels of migration could prevent the establishment of the allele that promoted reinforcement, but the allele could establish under a wider range of migration rates in a mosaic than in a tension zone. In a tension zone, both low and high levels of selection against A-locus hybrids could prevent the establishment of the assortative-mating allele. In a mosaic hybrid zone, the assortative-mating allele established under lower levels of selection against hybrids than in a tension zone, and high levels of selection did not impede the establishment of this allele. Overall, our work illustrates how the structure of a hybrid zone can alter the outcome of an important evolutionary process, in this case, reinforcement.

In this paper I describe a zone of overlap and hybridization between the ground crickets, Allonemobius fasciatus and A. socius (Orthoptera: Gryllidae). The former occurs in the northeastern United States, the latter is found in the southeastern United States. They meet in a contact zone of variable width that extends from the East Coast to at least Ohio. In the Allegheny Mountains, where appropriate habitats for a species adapted to a northern climate and a species adapted to a southern climate are patchily distributed and widely intermingled, the zone is broad-at least 200 km. Along the coastal plain of New Jersey, where appropriate habitats are not as widely intermingled, the zone is comparatively narrow (5-100 km). Levels of hybridization were assessed by analyzing electrophoretic data, using population genetic models and a character index approach. Both methods of data analysis agreed-where the zone is broader, levels of hybridization appear reduced.

All Rhagoletis reportedly establish associations with one or more bacterial species, but the bases for these interactions and their implications for host race formation and speciation are poorly understood. Here we present the results of four studies designed to increase our understanding of these relationships. In the first study, we identify the bacteria associated with seven Rhagoletis taxa by surveying the inhabitants of the esophageal bulb, an organ whose major function appears to be the housing of microorganisms. The results suggest that no bacterium has entered into an obligate symbiotic relationship with any of the Rhagoletis taxa surveyed, although one bacterium, Klebsiella oxytoca, is a very common associate of six of the seven. In the second study we use horizontal starch gel electrophoresis to determine the genetic similarity of K. oxytoca clones isolated from different Rhagoletis populations. This analysis provides a rare look into the genetic structure of natural populations of an enteric bacterium and permits the construction of a dendrogram for the clones-a dendrogram which indicates that there is no clear-cut pattern to the distribution of K. oxytoca genotypes among Rhagoletis. Taken together, the above studies provide indirect evidence that the bacteria associated with Rhagoletis are not important determinants of host plant specificity. The third and fourth studies assess two possible functions associated bacteria may perform for Rhagoletis: pectic substances degradation and nitrogen fixation. Our results do not lend support to either function.