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Abstract
The genetic structure of a hybrid zone can provide insights into the relative roles of the various factors that maintain the zone. Here, we use a multilocus approach to characterize a hybrid zone between two subspecies of killifish (Fundulus heteroclitus, Walbaum 1792) found along the Atlantic coast of North America. We first analysed clinal variation along the Atlantic coast using a single-nucleotide polymorphism in the mitochondrial DNA (mtDNA) displacement loop (D-loop) and a panel of nine nuclear microsatellite markers. A model constraining all clines to the same width and centre was not significantly different from a model in which the clines were allowed to vary independently. Locus-by-locus analysis indicated that the majority of nuclear clines shared the same centre as the mtDNA cline, and the widths of these clines were also narrower than that predicted by a neutral model, suggesting that selection is operating to maintain the hybrid zone. However, two of the nuclear clines had widths greater than the neutral prediction and had centres that were displaced relative to the mtDNA cline centre. We also found that a marsh located near the centre of the mtDNA cline demonstrated a bimodal distribution of nuclear hybrid index values, suggesting a deficit of first-generation hybrids and backcrossed genotypes. Thus, selection against hybrid genotypes may be playing a role in maintaining this hybrid zone and the associated steep nuclear and mtDNA clines.
Highlights
When two recently diverged taxa meet and reproductive isolation is not complete, hybridization can occur, which causes a transition between the morphological, physiological2015 The Authors
Observations of Hardy–Weinberg equilibrium (HWE) and linkage equilibrium at multiple loci are typical in unimodal zones, whereas the presence of heterozygote deficit and linkage disequilibrium suggests that reproductive isolation and/or local adaptation of either parental type to divergent habitats are acting to reduce the success of hybrid offspring [7]
Among the 13 locations sampled, a total of 61 pairs of microsatellite loci were found to be in significant linkage disequilibrium prior to correction for multiple testing, with 18 pairs of linkage disequilibria remaining significant after FDR-adjustment for multiple comparisons
Summary
When two recently diverged taxa meet and reproductive isolation is not complete, hybridization can occur, which causes a transition between the morphological, physiological2015 The Authors. Analysis of the shape of clines can be used to infer the strength of selection on various regions of the genome [9], because steep clines with cline widths narrower than predicted by a neutral model suggest that the differentiated alleles at these loci (or closely linked loci) are being maintained by selection [10,11] Another approach to the analysis of hybrid zones involves explicit consideration of multilocus genetic variation through the calculation of the hybrid index, which is an estimate of the similarity of each hybrid individual to the parental types. Observations of Hardy–Weinberg equilibrium (HWE) and linkage equilibrium at multiple loci are typical in unimodal zones, whereas the presence of heterozygote deficit and linkage disequilibrium (and cytonuclear disequilibrium) suggests that reproductive isolation and/or local adaptation of either parental type to divergent habitats are acting to reduce the success of hybrid offspring [7]
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