The Effects of Barriers to Geneflow on the Population Structure and Evolution of Rhinichthys atratulus , Eastern Blacknose Dace (Teleostei: Leuciscidae) in the Mattabesset River Drainage, CT, USA

Abstract

Waterfalls and drainage divides have been found to act as barriers to gene flow in riverine communities, affecting migration and genetic diversity of fish populations. We studied populations of Rhinichthys atratulus , the Eastern Blacknose Dace, in two tributaries of the Mattabesset River, Connecticut, to test hypotheses about genetic diversity in relation to physical barriers to migration and gene flow. We investigated three specific hypotheses: i) That waterfalls or drainage divides should be barriers to gene flow; ii) That the amount of genetic divergence across these barriers should be greater than among populations not separated by physical barriers; iii) That genetic diversity below waterfalls should be greater than the genetic diversity above waterfalls. We tested these hypotheses using mitochondrial and nuclear genetic markers from 191 and 197 individuals, respectively, from seven locations; the populations were separated by three waterfalls and two drainage divides. We found that: i) The waterfalls and drainage divides were associated with genetic divergence among populations, indicating barriers to migration and gene flow; ii) All populations were significantly different from one another for microsatellites and most for nd2 , but that the magnitude of divergence across barriers was not greater than divergence between populations not separated by physical barriers; iii) Genetic diversity was higher above falls for private alleles and private haplotypes but was higher below falls for haplotype and nucleotide diversity. Migration rates among populations ranged from 1 to 3.2 individuals per generation. These migration rates were not sufficient to homogenize the populations over the effects of mutation. Although the mitochondrial data were consistent with an isolation by distance model of evolution, the results were due to the number of private haplotypes in the upper part of the Coginchaug River. We discuss the roles of extinction and geological history, as well as ecological habitat preferences and predation risk in determining genetic divergence among populations.