Founding fish : gene duplication contributes to immunological diversity in bottlenecked populations of introduced rainwater killifish

Population bottlenecks, and subsequent loss of genetic diversity, are a common occurrence for introduced species. The MHC Class IIDB locus must constantly adapt to detect shifting pathogen communities and is used in this study to assess functional diversity in 2 regions with introduced populations of Rainwater killifish, Lucania parva. L. parva was likely introduced to San Francisco Bay from the Pecos River, New Mexico through a series of sport fish stockings to southern then northern California during the 1940’s and 1950’s. Demographic patterns of San Francisco and New Mexico populations were assessed using microsatellite, D-loop and MHCIIDB diversity. A single D-loop allele was found in all populations and three of four microsatellite loci were fixed at shared alleles, suggesting a strong bottleneck and common ancestry between all populations sampled. MHC diversity was lowest in New Mexico and one very similar allele was shared between the regions sampled. Inferred functional diversity was also assessed using physiochemical properties of MHC codons under positive selection and alleles were grouped into supertypes. Supertype usage and allelic diversity was relatively uniform between San Francisco and New Mexico. Despite low allelic diversity, functional diversity was spread across duplicated MHCIIDB loci. Two supertypes (out of four) were found to dominate in all populations. This reduced MHC diversity may be a result of relaxed parasite mediated selection as decreased immune response, assortative mating or new parasite pressures.