LAUSR

CONTEXT Gene flow in riparian ecosystems is influenced by landscape features such as orography, climate, and salinity. The 'downstream increase in genetic diversity' (DIGD) hypothesis states that the unidirectionality of the watercourse causes an accumulation of genetic diversity towards downstream populations, while upstream populations are more structured and less diverse, especially in water dispersed organisms. METHODS We used chloroplast, and nuclear microsatellites to characterize genetic diversity, structure, and gene flow patterns among populations of Salix humboldtiana across an elevation and salinity gradient in three rivers (Actopan, Antigua, and Blanco) in Mexico. We used optimization of resistance surface methods to determine if genetic distances between populations are correlated with landscape features. RESULTS Positive FIS values evidenced bi-parental inbreeding in some populations, particularly at higher elevations where lower niche availability constrains colonization and persistence. Four genetic groups were distinguished, corresponding to populations in rivers Actopan, Antigua and upstream and downstream Blanco, but with high admixture between Actopan and Antigua. Higher gene flow rates were observed among proximate populations in the same river than among different rivers. Genetic diversity increased towards the river mouths, in support of the DIGD hypothesis, probably due to a higher niche availability and larger population size. Differences among rivers in precipitation patterns and salinity, as well as geographic distance, were significant predictors of gene flow. CONCLUSION Our results depict that the DIGD and gene flow patterns in S. humboldtiana result from the complex interaction among physiography, climate, river salinity and life-history traits of the species. This article is protected by copyright. All rights reserved.