Multiple Introductions and Post-Immigration Evolution in Phyla canescens: Phenotypic Differentiation under Divergent Selection Pressures in Invading Populations with High Genetic Diversity.

The evolution of increased competitive ability (EICA) hypothesis suggests that invasive plant species may undergo selection for the loss of defence, and allocation of resources to growth and/or reproduction. More vigorous or competitive populations in invaded ranges have been well-documented in common garden experiments, but due to insufficient population genetic information it is not always clear whether post-immigration evolution has occurred because of founder effects, genetic drift and/or natural selection. Thus, supporting evidence for EICA remains inconclusive. In this study, we implement a hierarchical, multi-continent analysis of the genetics of growth properties and reproductive characteristics in Phyla canescens by combining results from molecular markers and greenhouse common garden experiments. Populations of P. canescens in its invaded regions (Australia and France) have similar levels of genetic diversity as populations in its native range (Argentina). However, the genetic diversity of native populations has been reshuffled and recombined in both invaded regions. This pattern suggests multiple introductions and gene flow among populations during the invasion of P. canescens into both countries. Australian and French populations display phenotypic traits associated with enhanced growth or reproduction compared to Argentine populations. French populations exhibited more vigorous vegetative growth, possibly imparting greater clonal reproduction. On average, French populations possessed faster stem and root growth, and larger ramets with thicker stems and wider leaves. This vegetative growth pattern may enhance the survival and regrowth of plant fragments after disturbance (e.g., prolonged inundation, ploughing, and mowing) in the relatively intensively managed small-plot agricultural systems common in France. In contrast, Australian populations produced more inflorescences (and potentially more seeds) under greenhouse condition, and this trait may be an adaptation to the high climatic variability of Australia. High genetic diversity and variable selection have resulted in divergent patterns of post-immigration evolution that appear to have contributed to the invasion of P. canescens in Australia and France