Effects of water pollution and river fragmentation on population genetic structure of invasive mosquitofish

Highlights: • We examined genetic variation of Gambusia holbrooki in a polluted river reservoir. • Mosquitofish at the most polluted sites showed genetic distinction from other studied sites along the river basin. • Changes at the GPI-2 locus agree with predicted response to acute exposure to mercury. • Microsatellite loci also distinguished fish at the vicinity of most polluted sites. • Immigration, even from sites beneath the dam, mitigates genetic losses at polluted sites. We analyzed variation at the GPI-2 locus and eleven microsatellite loci of eastern mosquitofish Gambusia holbrooki populations introduced to the Ebro River (Spain), sampling above and below a dam (Flix Reservoir) where severe chronic pollution has been well documented. Allele frequency changes at the GPI-2 locus in the sites nearest to the polluted sediments agree with previous results from studies in mercury-exposed populations of this highly invasive fish. Genetic distinction of the mosquitofish collected close to the polluted sediments was detected at the GPI locus but also at the presumptive neutral microsatellite loci. Recent migration rates estimated from microsatellites indicated that around 30% of fish collected in a specific location were immigrants from upstream and downstream sources. Such high migration rates probably contribute to the mosquitofish's invasive success and suggest that the consequences on the mosquitofish regional genetic structured of high levels of water toxicants could be mediated by immigration from other sites, but the effect of pollutants on local diversity might be higher than observed here.