1. We examined the ecological genetics of the invasive cladoceran Daphnia lumholtzi in a reservoir (Lake Texoma) in the southern USA. This species originates from the Old World subtropics and has spread across North America since the late 1980s after its inadvertent introduction to a reservoir in northeastern Texas.
2. The population genetic structure of D. lumholtzi was examined seasonally on 22 dates over a 3-year period along a natural temperature and salinity gradient.
3. A two-allele polymorphism at the phosphoglucose isomerase (PGI) locus was detected, while other loci tested showed no variation. Significant temporal heterogeneity existed in genotype frequencies with a major shift between summer and autumn.
4. Results of a multiple linear regression analysis revealed that a significant amount of variation was explained by day length, temperature and conductivity. Additionally, significant spatial heterogeneity of genotype frequencies between lake stations was observed, but was restricted to the summer.
5. Clonal isolates were used in controlled laboratory temperature and salinity tolerance experiments. Results suggest that salinity and temperature tolerance differed between PGI genotypes.
6. Genotype × environment interactions may play a significant role in the micro-evolutionary dynamics of this invasive species and may have facilitated its rapid expansion across the North American continent.