title: Discordant introgression in a rapidly expanding hybrid swarm.
abstract: The erosion of species boundaries can involve rapid evolutionary change. Consequently, many aspects of the process remain poorly understood, including the formation, expansion, and evolution of hybrid swarms. Biological invasions involving hybridization present exceptional opportunities to study the erosion of species boundaries because timelines of interactions and outcomes are frequently well known. Here, we examined clinal variation across codominant and maternally inherited genetic markers as well as phenotypic traits to characterize the expansion and evolution of a hybrid swarm between native Cyprinella venusta and invasive Cyprinella lutrensis minnows. Discordant introgression of phenotype, microsatellite multilocus genotype, and mtDNA haplotype indicates that the observable expansion of the C. venusta × C. lutrensis hybrid swarm is a false invasion front. Both parental and hybrid individuals closely resembling C. lutrensis are numerically dominant in the expansion wake, indicating that the non-native parental phenotype may be selectively favored. These findings show that cryptic introgression can extend beyond the phenotypic boundaries of hybrid swarms and that hybrid swarms likely expand more rapidly than can be documented from phenotypic variation alone. Similarly, dominance of a single parental phenotype following an introduction event may lead to instances of species erosion being mistaken for species displacement without hybridization.
authors.affiliation: Department of Ecology, Evolution and Behavior, University of Minnesota St. Paul, MN, USA.
authors.forename: Jessica L
authors.initials: JL
authors.lastname: Ward
authors.affiliation: Department of Ecology and Evolutionary Biology, Tulane University New Orleans, LA, USA.
authors.forename: Mike J
authors.initials: MJ
authors.lastname: Blum
authors.affiliation: U.S. Geological Survey, Fort Collins Science Center Fort Collins, CO, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Walters
authors.affiliation: Department of Biological Sciences, Duquesne University Pittsburgh, PA, USA.
authors.forename: Brady A
authors.initials: BA
authors.lastname: Porter
authors.affiliation: U.S. Geological Survey, Southeast Ecological Science Center Gainesville, FL, USA.
authors.forename: Noel
authors.initials: N
authors.lastname: Burkhead
authors.affiliation: Georgia Museum of Natural History and Odum School of Ecology, University of Georgia Athens, GA, USA.
authors.forename: Byron
authors.initials: B
authors.lastname: Freeman
keywords: Cyprinella; competition; hybrid zone; introgression; invasive species; selection