| Summary: | Hybrid zones act as natural experiments that can provide insights into the factors governing species formation and maintenance. In order to investigate these factors, I examined a hybrid zone between two subspecies of Atlantic killifish, Fundulus heteroclitus. Previous research has shown that these subspecies differ both genetically and phenotypically, but very little work had examined the hybrid zone between them. I used a suite of genetic markers to describe the genetic pattern within this hybrid zone and laboratory breeding experiments to
investigate the forces responsible for its maintenance.
Based on hybrid indices calculated using microsatellite and SNP (single nucleotide polymorphism) loci, a trimodal hybrid zone located in Beaverdam Creek (in the Metedeconk river system in New Jersey) separates the two subspecies of killifish, suggesting that while some F1 hybrids are produced, backcross types are rare. This pattern persisted across several sampling sites and across two years, suggesting that this pattern was not a sampling artifact. By investigating the geographical patterns of genetic variation in 30 SNPs along the Atlantic coast, I found that clines in mitochondrial DNA markers and in SNPs in several nuclear genes with mitochondrially-associated functions were coincident, concordant and exceptionally steep compared to those of other loci. I used tension zone analyses to conclude that these clines are likely being maintained either by selection or by assortative mating. The observed cytonuclear disequilibria also suggested a role for cytonuclear epistasis in maintaining this hybrid zone.
Within Beaverdam Creek, there was no genetic differentiation between samples taken at locations differing in temperature and salinity, suggesting that habitat specialization on these abiotic variables is not involved in the maintenance of the hybrid zone. However, my results from a "choice" breeding experiment among individuals originating from the extremes of the species' distributions suggested a possible role for positive assortative mating.
Taken together, my research provides evidence that differentiation in mitochondrial properties resulting in selection or assortative mating could be involved in the maintenance of distinct subspecies of F. heteroclitus, and points to a potential general role for divergence in energy metabolism as a mechanism in promoting or maintaining species differences.
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