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Richard G. Harrison passed away unexpectedly on April 12th, 2016. In this memoriam we pay tribute to the life and legacy of an extraordinary scientist, mentor, friend, husband, and father.

Two of the most well-supported patterns to have emerged over the past two decades of research in evolutionary biology are the occurrence of divergent natural selection acting on many male and female reproductive tract proteins and the importance of postmating, prezygotic phenotypes in reproductively isolating closely related species. Although these patterns appear to be common across a wide variety of taxa, the link between them remains poorly documented. Here, we utilize comparative proteomic techniques to determine whether or not there is evidence for natural selection acting on the ejaculate proteomes of two cricket species (Allonemobius fasciatus and A. socius) which are reproductively isolated primarily by postmating, prezygotic phenotypes. In addressing this question, we compare the degree of within-species polymorphism and between-species divergence between the ejaculate and thorax proteomes of these two species. We found that the ejaculate proteomes are both less polymorphic and more divergent than the thorax proteomes. Additionally, we assessed patterns of nucleotide variation for two species-specific ejaculate proteins and found evidence for both reduced levels of variation within species and positive selection driving divergence between species. In contrast, non-species-specific proteins exhibited higher levels of within-species nucleotide variation and no signatures of positive selection. Nucleotide and putative functional data for the two species-specific proteins, along with data for a third protein (ejaculate serine protease), suggest that all three of these genes are candidate speciation genes in need of further study. Overall, these patterns of proteome and nucleotide divergence provide support for the hypothesis that there is a causative link between selection-driven divergence of male ejaculate proteins and the evolution of postmating, prezygotic barriers to gene flow within Allonemobius.

Long-term studies of hybrid zones can provide valuable insight into a number of questions that have long attracted the attention of evolutionists. These questions range from the stability and fate of hybrid zones to the relative fitness of hybrids. In this paper we report the results of a 14-year survey of the Allonemobius fasciatus-Allonemobius socius hybrid zone. Populations were collected intensively in 1986 and 1987 and then more sporadically through the end of the 1980s and throughout the 1990s. By documenting changes in the genetic composition of populations near and within the zone during this period of time we assessed: the strength of the reproductive isolation between the two species; the relative growth rates (which can be considered a surrogate of relative fitness) of genotype classes corresponding to hybrids and to pure species individuals; and, the power of single-year and multi-year measurements of relative growth rates to predict changes in the genetic composition of mixed populations through time. In brief, we found very large year-to-year variation in the relative growth rates of pure species and hybrid individuals. This variation may reflect the fact that both species are at the edge of their range and perhaps at the limits of their ability to deal with environmental perturbations. As a consequence of the variation, even multi-year estimates of relative growth rates often provided imprecise predictions regarding the future genotypic composition of mixed populations. Despite our limited ability to predict the dynamics of individual populations, some trends are apparent. A. socius, the southern species, has clearly increased in frequency along a transect through the Appalachian Mountains, indicating that the zone is moving north in this region. In contrast, the zone appeared to be more stable along the East Coast transect. Within mixed populations, character-index profiles are often bimodal and stable through time, indicating relatively strong reproductive isolation between the two species that is not being reinforced, nor is it breaking down.

Polymorphic di-, tri- and tetranucleotide repeats were examined in Allonemobius to determine whether they could serve as useful markers in studies of sperm precedence, population genetics and hybrid zone structure. Ten microsatellite DNA loci were sufficiently polymorphic to be used for paternity tests and showed no evidence of linkage disequilibrium or deviations from Hardy–Weinberg equilibrium in Allonemobius socius. Nine of 10 of these microsatellites can be amplified from three other Allonemobius species, suggesting that these markers will have widespread utility in this ground cricket genus.

The ground crickets, Allonemobius fasciatus and A. socius are sister species and form a mosaic hybrid zone in the eastern United States. In this study, we developed RAPD markers and constructed genetic linkage maps for the two species, as a first step toward mapping quantitative trait loci that control conspecific sperm precedence. The A. fasciatus map consists of 25 markers mapped to 8 linkage groups, spanning 400.1 contiguous centiMorgans (cM). The average distance between two framework markers is 23.5 cM. The A. socius map consists of 20 markers mapped to 6 linkage groups, spanning 400.5 contiguous cM. The average distance between two framework markers is 30.4 cM. A sex chromosome was identified in the A. socius map. These are the first reported genetic linkage maps for Orthoptera. Resume : Les grillons Allonemobius fasciatus et A. socius sont des especes soeurs et forment une zone d'hybrides en mosaique dans l'Est des Etats-Unis. Dans cette etude, les auteurs ont developpe des marqueurs RAPD et assemble des cartes de liaison genetique chez ces deux especes. Ce travail constitue la premiere etape en vue de la cartographie de caracteres quantitatifs qui controlent la dominance seminale intraspecifique. La carte du A. fasciatus comprend 25 marqueurs assembles en huit linkats qui totalisent 400,1 cM. La distance moyenne entre marqueurs de reference adjacents est de 23,5 cM. Chez le A. socius, la carte est composee de 20 marqueurs formant six linkats qui s'etendent sur 400,5 cM. La distance moyenne entre deux marqueurs de reference est de 30,4 cM. Un chromosome sexuel a ete identifie chez le A. socius. Il s'agit des premieres cartes de liaison genetique rapportees chez les orthopteres. Mots cles : Allonemobius, allozyme, grillon, zone d'hybrides, cartes genetiques, RAPD. (Traduit par la Redaction) Chu and Howard 847

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.

The use of molecular genetic techniques is becoming increasingly widespread in analyses of hybrid zones. Yet, exactly how many molecular markers are required to provide a given degree of resolution remains an open question. We present statistical models that relate the number of markers examined to their power to discriminate between pure species, F1’s, and backcross individuals. Our models indicate that only about four or five markers are required to provide a coarse classification of individuals in hybrid zones, whereas upwards of 70 markers are required to discriminate between pure species and advanced backcrosses. The models provide hybrid zone researchers a basis upon which to balance the time and expense of examining large numbers of markers against the magnitude of classification errors when smaller numbers of markers are examined.

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.

This is a special issue of Genetica that has its origins in a symposium held in honor of Richard G. Harrison at Ithaca, New York on July 22–23. Former students of Rick Harrison organized the symposium and most of the speakers were former students, as well. The quality and breadth of the talks were a testament to Rick’s influence as a thinker, synthesizer, and mentor and it is only appropriate to reflect on Rick’s contributions to the fields of evolutionary ecology, systematics, and genetics in this preface to the symposium articles.

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.

An intriguing aspect of the current renaissance in investigations of the genetics of reproductive isolation is that it has been dominated by studies that resemble work done in the 1930s, 1940s, and 1950s. The dominant model organism (Drosophila), research approaches, and traits of interest (sterility and inviability of hybrids) all harken back to this earlier era. Herein, we explore the factors that led to a rebirth of interest in the genetics of reproductive isolation and to the adoption of the approaches of an earlier generation of biologists. At the same time, we appeal for more intensive investigations of traits that reproductively isolate closely related species, inclusion of a greater range of organisms in studies of reproductive isolation, and focus on a broader range of questions surrounding speciation. We end with a description of ongoing quantitative trait loci (QTL) studies of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and Allonemobius socius. We have found several QTL with large effects on variance in patterns of sperm utilization in backcross females. Moreover, some QTL have an antagonistic effect on conspecific sperm, a finding that lends support to the hypothesis that rapid evolution of conspecific sperm precedence is a by-product of sexual conflict.

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.

Starch gel electrophoresis was used to study patterns of genetic variation within and among seven species of North American Chauliognathus . All species displayed levels of genetic variation lower than are usually found in insects. Geographic variation in allele frequencies within a species appeared to be limited, whereas differentiation among species was substantial. The differences among species often involved loci monomorphic within populations. Despite the abrupt genetic discontinuities separating species, the sharing by five species of a polymorphism at the PGI locus suggests that variation at PGI might be maintained by some form of diversifying selection. Finally, phylogenetic trees constructed from electrophoretic evidence for the seven species are consistent with phylogenies based on morphology and ecology.

The evolution of barriers to gene exchange is centrally important to speciation. We used the crickets Allonemobius fasciatus and A. socius to investigate the genetic architecture of conspecific sperm precedence (CSP), a postinsemination prezygotic reproductive barrier. With amplified fragment-length polymorphism (AFLP) markers and controlled crosses we constructed linkage maps and estimated positions of QTL associated with CSP. The majority of QTL have low to moderate effects, although a few QTL exist in A. socius with large effects, and the numbers of QTL are comparable to numbers of genes accounting for species differences in other studies. The QTL are spread across many unlinked markers, yet QTL placed with linked markers are on a small number of linkage groups that could reflect the role of the large Allonemobius sex chromosome in prezygotic isolation. Although many QTL had positive effects on conspecific sperm utilization several QTL also exerted negative effects, which could be explained by intraspecific sexual conflict, sperm competition, or epistasis of introgressed genes on novel backgrounds. One unexpected outcome was that A. socius CSP alleles have a stronger effect than those from A. fasciatus in hybrid females, causing hybrids to behave like A. socius with regard to sperm utilization. Implications of this asymmetry in the Allonemobius hybrid zone are discussed.

Accessory reproductive gland proteins (Acps) in Drosophila evolve quickly and appear to play an important role in ensuring the fertilization success of males. Moreover, Acps are thought to be involved in establishing barriers to fertilization between closely related species. While accessory glands are known to occur in the males of many insect groups, the proteins that are passed on to females by males during mating have not been well characterized outside of Drosophila. To gain a better understanding of these proteins, we characterized ESTs from the accessory glands of two cricket species, Allonemobius fasciatus and Gryllus firmus. Using an expressed sequence tag (EST) approach, followed by bioinformatic and evolutionary analyses, we found that many proteins are secreted and, therefore, available for transfer to the female during mating. Further, we found that most ESTs are novel, showing little sequence similarity between taxa. Evolutionary analyses suggest that cricket proteins are subject to diversifying selection and indicate that Allonemobius is much less polymorphic than Gryllus. Despite rapid nucleotide sequence divergence, there appears to be functional conser- vation of protein classes among Drosophila and cricket taxa. Resume´ : Les proteines des glandes reproductives accessoires chez le genre Drosophila evoluent rapidement et semblent jouer un role important dans le succes des males en matiere de fecondation. De plus, ces proteines sont soupconnees d'une implication dans l'etablissement de barrieres ala fecondation entre especes proches. Tandis que des glandes accessoires sont presentes chez les males de nombreux groupes d'insectes, les proteines transmises des males aux femelles lors de l'accouplement n'ont pas etebien caracterisees a l'exterieur du genre Drosophila. Afin de mieux connaoˆtre ces proteines, les auteurs ont caracterisedes EST des glandes accessoires chez deux especes de grillons, Allonemobius fasciatus et Gryl- lus firmus .Al'aide d'une approche EST, suivie d'analyses bioinformatiques et evolutives, les auteurs ont trouveque plu- sieurs proteines etaient secretees et ainsi disponibles pour transfert a la femelle lors de l'accouplement. De plus, la plupart des EST etaient inedits puisqu'ils montraient peu de similariteentre taxons. Des analyses evolutives suggerent que les pro- teines de grillons sont sujettes aune selection divergente et indiquent que l'Allonemobius est beaucoup moins polymorphe que le Gryllus .E n depit d'une rapide divergence de la sequence nucleotidique, il semble y avoir une conservation fonc- tionnelle des classes de proteines chez le genre Drosophila et les grillons. Mots cles : Allonemobius, Gryllus ,evolution rapide, isolement reproductif, fluide seminal. (Traduit par la Redaction)

Patterns of reproductive isolation between two sympatric species of oaks, Quercus gambelii and Q. grisea, that exhibit strong ecological differentiation were examined. A full diallel cross using four trees of each species (i.e. all possible pollinations among eight trees) was performed. This design was repeated at two sites that represent different outcomes of sympatry: (1) a xeric mountain ridge where many hybrids are established (HZ); and (2) a mesic valley bottom where virtually no hybrids are established (MOCYN). By measuring fruit survival at several developmental stages, both the timing and strength of reproductive barriers within and between sites, species, cross types, and pollen dosage levels were examined. In three of four cases, heterospecific fruit set was significantly reduced compared to conspecific fruit set. This reduction occurred after the time of fertilization, but before the onset of embryo growth. Increasing the dose of pollen from an average of 9–194 grains/stigma did not affect this result. Thus, early postfertilization processes play a strong role in species fidelity in these oaks. Quercus gambelii experienced a five-fold decrease in conspecific fruit set at HZ relative to MOCYN. In contrast, heterospecific fruit set of Q. gambelii was the same at both sites. Poor Q. gambelii pollen performance is implicated as playing the major role in this result. One Q. gambelii individual at HZ was highly fecund, and had higher heterospecific than conspecific fruit set; slight introgression in this tree was detected uisng RAPD markers. The Environmental Emasculation Hypothesis that posits that environmental stress can increase the probability of hybrid formation by reducing the competitive ability of male gametes of one species is proposed.

Predictions using quantitative genetic models generally assume that the variance-covariance matrices remain constant over time. This assumption is based on the supposition that selection is generally weak and hence variation lost through selection can be replaced by new mutations. Whether this is generally true can only be ascertained from empirical studies. Ideally for such a study we should be able to make a prediction concerning the relative strength of selection versus genetic drift. If the latter force is prevalent then the variance-covariances matrices should be proportional to each other. Previous studies have indicated that females in the two sibling cricket species Allonemobius socius and A. fasciatus do not discriminate between males of the two species by their calling song. Therefore, differences between the calling song of the two males most likely result from drift rather than sexual selection. We test this hypothesis by comparing the genetic architecture of calling song of three populations of A. fasciatus with two populations of A. socius. We found no differences among populations within species, but significant differences in the G (genetic) and P (phenotypic) matrices between species, with the matrices being proportional as predicted under the hypothesis of genetic drift. Because of the proportional change in the (co)variances no differences between species are evident in the heritabilities or genetic correlations. Comparison of the two species with a hybrid population from a zone of overlap showed highly significant nonproportional variation in genetic architecture. This variation is consistent with a general mixture of two separate genomes or selection. Qualitative conclusions reached using the phenotypic matrices are the same as those reached using the genetic matrices supporting the hypothesis that the former may be used as surrogate measures of the latter.

Conspecific sperm precedence is widespread in animals, appears to evolve rapidly, and is thought to have the potential to prevent hybridization between closely related species. However, to date no study has tested the isolating potential of such a barrier in mixed populations of two taxa under conditions in which other potential barriers to gene flow are controlled for or are prevented from operating. We tested the isolating potential of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and A. socius in population cage experiments in which the frequency of the two species was varied. Despite the observation of abundant interspecific matings, the proportions of hybrid progeny were low and differed statistically from the proportions expected in the absence of conspecific sperm pre- cedence. The results demonstrate that conspecific sperm precedence can severely limit gene flow between closely related species, even when one species is less abundant than the other.

The white oaks Quercus gambelii and Q. grisea overlap in distribution in New Mexico and Arizona. Within the region of overlap, there are numerous instances of contact between the two taxa. In some areas of contact morphologically, intermediate trees are common, whereas in others, morphologically intermediate trees are rare or absent. We describe a set of RAPD markers that distinguish between the two species and use these markers to examine patterns of gene exchange in an area of contact in the San Mateo Mountains of New Mexico. The markers are highly coincident with morphology and confirm that hybridization between the two species takes place. Despite the occurrence of hybrids, both species remain distinct, even in areas of sympatry, and marker exchange appears to be limited.

Thorough examinations of purported cases of reproductive character displacement are critical for reaching an understanding of the role of reinforcement in the evolution of reproductive barriers between closely related species. In this paper, we report the results of an extensive investigation of male calling song variation in the ground crickets, Allonemobius fasciatus and A. socius. Contrary to the results of an earlier study, we uncovered little evidence of displacement of songs in areas of overlap. We discuss explanations for the lack of displacement as well as for the discrepancies between the results of the current study and those of the earlier study.

The ground crickets Allonemobius fasciatus and A. socius meet in a mosaic zone of overlap and hybridization stretching from the East Coast to at least Illinois. To test whether male calling song differences were enhanced in sympatry, we analyzed the songs of crickets from inside and outside the zone of overlap along two transects. No evidence of calling song displacement was found in A. socius males from populations within the zone of overlap. On the other hand, A. fasciatus displayed calling song displacement in three populations. Our results are consistent with the hypothesis that the selective pressure exerted by the challenge from a related species is frequency dependent. While not a conclusive demonstration, the observed shifts in calling song are strongly suggestive of reproductive character displacement.

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.

Six biotype bulk populations and two geographic populations of the Hessian fly, Mayetiola destructor (Say), were analyzed at 15 presumptive genetic loci using starch gel electrophoresis. Fourteen of the 15 loci appeared monomorphic. One locus, Pgi, varied in biotypes B, E, and a population from Mason, Mich. The level of genetic variation among Hessian fly populations is among the lowest reported for insects. Genetic differentiation among Hessian fly biotypes and populations is also low, ranging from a Nei's D of 0.000 to 0.002. Protein electrophoresis does not appear to provide a quick procedure to identify Hessian fly biotypes.

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.

Hexokinase variation in insects appears to be under the control of a single locus in some species and under multiple-locus control in others. It is often difficult to distinguish the number of loci controlling hexokinase expression. Analysis of hexokinase electrophoretic patterns in six species of mosquitoes and five species of crickets, as well as a review of hexokinase variation in other insect species, is used to emphasize the importance of interspecific comparisons when making genetic inferences. Evidence is provided which adds support for multiple hexokinase loci in dipterans. Hexokinase control by multiple loci may be difficult to determine in some species because of tight linkage, disequilibrium, and/or posttranslational modification.