Local adaptation and maladaptation during the worldwide range expansion of a self-fertilizing plant
Species having experienced rapid range expansion represent unique opportunities to evaluate the dynamics of adaptation during colonization of new environments. We investigated the consequences of range expansion on local adaptation of a successful worldwide colonizer, the shepherds purse Capsella bursa-pastoris. This species is an annual weed that originated recently in Eurasia and has now broadly colonized both temperate and subtropical areas. We assessed the performance, genetic diversity, and phenology of field-collected accessions belonging to three distinct genetic clusters of decreasing age (Middle East, Europe and Asia) in three common gardens in Europe, Asia and North America. To understand the genetic basis of local adaptation in this species, we also tested for correlation between SNP allele frequencies and environmental factors in Europe and Asia. Overall, we showed that patterns of local adaptation depended on population history: some older populations were weakly adapted to local conditions while those closer to the front of the colonization wave, far from the origin of the species, were maladapted whatever the common gardens. Altogether, our results have important consequences for the understanding of the evolution and adaptation of self-fertilizing plant during range expansion.