Pattern and process in a narrow hybrid zone

Abstract

This paper examines variation in morphology and allozymes in a hybrid zone between two closely related eastern North American species of field cricket (genus Gryllus). I show that patterns of variation across the zone do not conform to a simple model of monotonic clinal variation. In fact, the hybrid zone is a mosaic of populations. Pockets of "pure" parental forms are found within the hybrid zone, and striking reversals in mean character index score occur along transects across the zone. Treating hybrid zones as mosaics has important consequences for thinking about the dynamics of such zones.Patterns of variation in morphology and allozymes are not concordant across the hybrid zone. Rather, there is strong evidence for differential and asymmetric introgression, with morphological integrity maintained despite considerable introgression of alleles at allozyme loci. Species boundaries must be thought of as semipermeable, the permeability varying with he genetic marker used.I also show that there is strong positive assortative mating at one site within the hybrid zone and that assortative mating persists despite introgression at allozyme loci. Habitat isolation and behavioural differences may both affect the extent of assortative mating.

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

“…The Guilford and Ithaca populations have previously been used to characterize barriers to gene exchange between these species (Harrison 1983(Harrison , 1986. Less is known about the Essex and Scranton populations, but based on geography and cricket phenotype, they appear to represent populations that are predominantly G. firmus and G. pennsylvanicus, respectively.…”

“…Field and laboratory studies of the two cricket species (Harrison 1983(Harrison , 1985(Harrison , 1986Harrison & Rand 1989) have identified both pre-and postmating barriers to gene exchange. Premating barriers include ecological (habitat) isolation, temporal isolation and mate choice.…”

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

“…Sharon, CT is SH1 and Guilford, CT is GU2 in Harrison & Arnold (1982). These populations were also sampled in Harrison (1986) and Harrison et al (1987). The Blue Ridge Parkway locality is only a few miles from BR5 of Harrison & Arnold (1982).…”

Allozyme variation in Louisiana irises: a test for introgression and hybrid speciation

“…This expectation must now be qualified by the observation that not all genetic markers demonstrate the same propensity to cross reproductive barriers between hybridizing taxa (e.g., see Harrison, 1986;Arnold et a!., 1987;Marchant et al, 1988). The "semipermeable" nature of the boundary between hybridizing taxa (Harrison, 1986) is best explained Note: the relative area that is shaded, unshaded and/or stippled in each of the individual diagrams does not indicate the proportion, but rather, the presence or absence of diagnostic markers for the three species.…”