Genetic mapping in hybrid zones

Recent advances in genetic mapping methodologies make it feasible to localize quantitative trait loci (QTL) that contribute to adaptation and speciation. However, it has not been possible to employ these methods in many wild species because of difficulties associated with creating and propagating recombinant populations of sufficient size for QTL mapping. Natural hybrid zones contain recombinant individuals resulting from many generations of hybridization and thus offer a potential solution to these problems. For studies of speciation, hybrid zones offer the possibility of mapping QTL simultaneously with assessments of their effects on assortative mating, hybrid fitness, and interspecific gene flow. Here, we explore the problems and prospects associated with genetic map building and QTL analyses in natural hybrid zones by analyzing correlations among markers of known genomic location in four hybrid zones between the wild sunflower species Helianthus annuus and Helianthus petiolaris. Results indicate that mapping in hybrid zones presents many challenges. These include overlap in the strength of marker correlations between linked and unlinked markers, unevenness in marker frequencies along linkages, and heterogeneity in the relationship between marker distances and correlations. All make it difficult to accurately group and order markers or to estimate the distances between them. These problems can be ameliorated by sampling strategies that maximize the difference in linkage disequilibria between linked and unlinked markers and that minimize differences in frequencies among markers or QTL. In addition, studies that employ a previously determined molecular marker map for gene localization have a greater likelihood of success than those that rely on the hybrid zone data for both map construction and QTL analyses.