Empirically simulated spatial sorting points at fast epigenetic changes in dispersal behaviour

During range expansion, the most dispersive individuals make up the range front, and assortative mating between these dispersive phenotypes leads to increased dispersiveness (i.e. spatial sorting). The precise inheritance of dispersal, however, is to date largely unknown in many organisms, thereby hampering any progress in evaluating the adaptive potential of species during range expansion. Using the spider mite Tetranychus urticae, we therefore empirically simulated spatial sorting by means of artificial selection on a unique pre-dispersal behaviour, tightly related to emigration. To separate directionality of the response from potential drift, we mimicked a recurrent low number of founders in replicated selection regimes. Afterwards, we inferred the mode of inheritance of the pre-dispersal behaviour by performing reciprocal crosses between selected (i.e. dispersive) and non-selected (i.e. non-dispersive) mites and by screening for endosymbionts known to be associated with changes in dispersal behaviour. Despite the recurrent low number of founders, the aerial dispersal behaviour responded strongly to the imposed selection pressure. The behaviour furthermore showed a maternal inheritance, though independent of any known dispersal-related endosymbionts. Though cytoplasmic inheritance cannot fully be excluded, we attribute the observed strong and rapid, maternally influenced response in dispersal to transgenerational epigenetic effects. Consequently, we can expect fast evolutionary dynamics during range expansion in the species.