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Invasion success of crustacean zooplankton: adaptive mechanisms vs. broad physiological tolerance

Final Activity Report Summary - EVOLEXOTIC (Invasion success of crustacean zooplankton: adaptive mechanisms vs. broad physiological tolerance)

Biological invasions of exotic species raise concerns because of their detrimental effects on native species, biodiversity, ecosystems and related high economic costs. However, evolutionary mechanisms involved in the success of invasive species remain poorly understood in the vast majority of organisms. Evidence is accumulating that invasiveness is influenced by microevolutionary processes such as adaptive capacity rather than by broad physiological tolerance to the environment, enabling colonisation of new habitats by invasive-s that differ greatly from their habitat of origin.

We studied these contrasting mechanisms using experimental approaches as well as evidence from the field. As model invasive organisms, two crustaceans were chosen which have each invaded new continents across large geographical scales: the water flea Daphnia lumholtzi (Cladocera, Crustacea) native to Africa, Asia and Australia and invasive in North America, and the brine shrimp Artemia franciscana (Branchiopoda, Crustacea) native to the American continent, and invasive in Europe.

Genotype by environment (G x E) experiments were conducted, which for Daphnia lumholtzi indicated differential survival of allozyme genotypes related to temperature. These findings were supported in a detailed spatio-temporal field study of a North American D. lumholtzi population (Lake Texoma, Oklahoma), which suggested the presence of significant G x E interactions that produce population responses to shifts in temperature. In a further step we applied microsatellite markers to improve the resolution of the population genetic structure within Lake Texoma. For Lake Texoma, results indicate that genotype diversity follows a seasonal pattern possibly related to temperature, thus paralleling our findings from the allozyme study. The results obtained may explain why the species native to subtropical regions has been able to invade areas outside its normal temperature range, such as in the Laurentian Great Lakes of North America.

In addition to studying a local population of D. lumholtzi within the invaded range we conducted a macrogeographic study along a latitudinal gradient in North America and also included populations from the native range (Africa, Asia, Australia). Results suggest that this species was introduced several times to the North American continent and that the source populations are located on more than one continent. Further, the analysis of population genetic structure suggests that the North American populations are structured along a latitudinal gradient, possibly as a result of differing climatic conditions in these regions.

To complement the results obtained for Daphnia lumholtzi, we studied the population genetic structure of Artemia franciscana in a solar saltern near Cadiz, Southern Spain. We chose populations from basins with different salinities for DNA extraction and for a G x E experiment. The experimental conditions were chosen to match the ranges within which populations were sampled (salinity) or which populations naturally encounter in the course of the year (temperature). The results from this second part of the study are currently being analysed and are expected to add important insight into how and to what extent microevolutionary processes play a role in shaping invasive success of exotic species.