Dynamic Cohort Analysis Reveals Fluctuating Patterns of Selection Within a Hybrid Zone Between the Killifish Fundulus heteroclitus and F. grandis

Hybrid zones provide excellent opportunities to study speciation processes and ecological interactions between recently diverged taxa. Historically, hybrid zones have been divided into those in which fitness of hybrids is independent of the environment, and those in which environmental factors influence the fitness of different genotypes. The present study investigated the temporal genetic patterns at a location within a hybrid zone between the killifish Fundulus heteroclitus and F. grandis, in an effort to determine the extent and directionality of hybridization and the fitness of different genotypes. Fishes collected over the course of three years were placed into two age classes and genotyped at three nuclear loci and one mitochondrial locus that are highly differentiated between the species, allowing for comparison of genetic patterns between different age classes of the same cohorts. Individuals of hybrid descent were prevalent at the study site, the majority of which were likely advanced generation hybrids or backcrosses to one of the parental taxa. The cohort analyses revealed decreased abundance of both single and dilocus hybrid genotypes, and directional changes in allele frequency with increased age in some, but not all cohorts. These fluctuating patterns of selection across the course of the study suggest that fitness is likely strongly influenced by environmental factors.