Fitness differences and persistent founder effects determine the clonal composition during population build-up in Daphnia

During the initial stages of population build-up in new habitats, the time at which immigrants arrive may infl uence their contribution to the population and thus determine population geneticstructure. While the numerical advantage associated with founder eff ects may promote dominance of the off spring of early colonizers, fi tness diff erences associated with ecological diff erences among genotypes may potentially overwhelm these founder eff ects. We conducted an outdoor mesocosmexperiment to test whether the sequence of arrival determines the relative contribution of genetic lineages (clones) to populations of the water fl ea Daphnia . A set of D. galeata clones was inoculated pairwise in diff erent time sequences and clonal abundances were monitored over time. In general, we found that, after six weeks (four generations), clones with an initial time advantage of fi ve or fi fteen days (1/3 of a generation or one generation) had higher relative abundances than in the treatments with simultaneous inoculations. Th ese diff erences were in some cases maintained over a longer period (nine months). However, in cases where abundances of genetic lineages varied strongly when inoculated simultaneously, refl ecting strong diff erences in fi tness among clones in our experimental containers, initial numerical advantages could be overwhelmed and the competitively strongest clone became dominant after nine months independently of inoculation sequence. Our results highlight the importance of two diff erent processes that determine the fate of clonal lineages and ultimately determine population geneticstructure. First, the sequence of arrival is crucial, with an advantage of fi ve days being already suffi cient to dominate the population in specifi c cases. Second, if fi tness diff erences among clones are suffi ciently strong, then they may overrule founder eff ects.