Patterns of Variation and Linkage Disequilibrium in a Field Cricket Hybrid Zone

Abstract

s-«The distribution of multilocus genotypes found within a natural hybrid zone is determined by the sample of genotypes present when the hybrid zone first formed, by subsequent patterns of genetic exchange between the hybridizing taxa, and by drift and selection within each of the hybrid zone populations. We have used anonymous nuclear DNA restriction fragment polymorphisms (RFLPs) to characterize the array of multilocus genotypes present within a well-studied hybrid zone between two eastern North American field crickets, Gryllus pennsylvanicus and Gryllusfirmus. These crickets hybridize along a zone of contact that extends from New England to Virginia. Previous studies have shown that both premating and postmating barriers exist between the two cricket species, but the absence of diagnostic morphological and allozyme markers has made it difficult to assess the consequences of these barriers for genetic exchange. Analyses based on four diagnostic anonymous nuclear markers indicate that hybrid zone populations in Connecticut contain few FI hybrids, and that nonrandom associations persist among nuclear gene markers, between nuclear and cytoplasmic markers, and between molecular markers and morphology. Field cricket populations within the hybrid zone are not "hybrid swarms" but consist primarily of crickets that are very much like one or the other of the parental species. Despite ample opportunity for genetic exchange and evidence for introgression at some loci, the two species remain quite distinct. Such a pattern appears to be characteristic of many natural hybrid zones.

Partial island submergence and speciation in an adaptive radiation: a multilocus analysis of the Cuban green anoles

“…Meanwhile, introgression of A. allisoni haplotypes into A. porcatus occurs primarily in eastern Cuba, where two populations of A. porcatus contain only A. allisoni haplotypes (figure 4). This type of geographically structured discordance is not expected when the retention of ancestral polymorphisms is the underlying cause (Harrison & Bogdanowicz 1997;García-París et al 2003). Moreover, ancestral polymorphisms are expected to persist for longer in nDNA than mtDNA (Moore 1995), which is the reverse of the pattern observed in Cuban green anoles.…”

“…Gyllensten & Wilson 1987;Tegelström 1987;Good et al 2003). Introgression is also distinguished from incomplete lineage sorting by its characteristic geographical signal, with introgressed haplotypes being most common where the ranges of distinct populations abut one another (Harrison & Bogdanowicz 1997;Goodman et al 1999;García-París et al 2003). …”

Searching for candidate speciation genes using a proteomic approach: seminal proteins in field crickets

“…Because the cricket species G. pennsylvanicus and G. firmus have diverged very recently, we expect that most regions of the genome will continue to reveal shared ancestral polymorphisms (Broughton & Harrison 2003). Indeed, previous efforts to identify diagnostic differences between the two species have generally been unsuccessful and most genomic regions that have been surveyed exhibited evidence of shared polymorphisms (Harrison & Bogdanowicz 1997;Broughton & Harrison 2003). Because field cricket females are highly promiscuous, and because fertilization barriers are partly responsible for reproductive isolation between G. firmus and G. pennsylvanicus (Harrison 1983), genes encoding seminal proteins are likely to be under strong diversifying selection and may be involved in barriers to gene exchange.…”

Inferences about the origin of a field cricket hybrid zone from a mitochondrial DNA phylogeny

“…A study of mtDNA restriction fragment length polymorphisms (RFLPs) in Connecticut populations revealed four speciesspecific polymorphisms (Harrison et a!., 1987). DiagThe Genetical Society of Great Britain, Heredity, 79, 484-494. nostic anonymous nuclear RFLPs have also been described, and together with mtDNA, suggest that the two cricket species remain distinct despite ample opportunity for genetic exchange (Harrison & Bogdanowicz, 1997). In this paper we analyse sequence data from the cytochrome oxidase I and II (COl-COIl) region of the mtDNA molecule to examine relationships among crickets sampled from populations along the length of the hybrid zone.…”