John C. Avise Kim T. Scribner 1993 <p><span>Spatial genetic structure in populations of mosquitofish (</span><i>Gambusia</i><span>) sampled throughout the south-eastern United States was characterized using mitochondrial (mt) DNA and allozyme markers. Both sets of data revealed a pronounced genetic discontinuity (along a broad path extending from south-eastern Mississippi to north-eastern Georgia) that corresponds to a recently recognized distinction between the nominal forms </span><i>G. affinis</i><span> to the west and </span><i>G. holbrooki</i><span>to the east. However, several populations from the general contact region exhibited unusual allelic associations in high frequency, suggestive of evolutionary processes within a zone of introgressive hybridization. These involve: (i) cytonuclear profiles representing combinations of nuclear and mitochondrial genotypes that tended to be more nearly species-specific and concordant elsewhere; and (ii) significant nuclear gametic disequilibria, perhaps attributable to positive assortative mating and/or differential fitnesses of homospecific vs. recombinant genotypes. However, outside this suspected hybrid region, ‘heterospecific’ genetic markers also appeared in low frequency, thus complicating interpretations. These discordant alleles on a broader geographic scale may reflect: (a) the retention of polymorphisms from an ancestral gene pool; (b) occasional evolutionary convergence (especially with respect to electrophoretic mobility of allozyme alleles); (c) the ‘footprints’ of a moving hybrid zone; or (d) differential introgressive penetrance across the current hybrid region.</span></p> application/pdf 10.1111/j.1365-294X.1993.tb00103.x en Wiley Cytonuclear genetic architecture in mosquitofish populations and the possible roles of introgressive hybridization article