Abstract
Describing long-term evolutionary trajectories of alien species is a fundamental, although rarely possible, step to understand the pivotal drivers of successful invasions. Here, we tackled this task by investigating the genetic structure of the crested porcupine (Hystrix cristata), whose invasion of Italy started about 1500 years ago. Using genome-wide RAD markers, we explored the demographic processes that shaped, and are shaping, the gene pool of the expanding invasive populations and compared their genetic diversity with that of native and invasive populations of both African porcupine species (crested and Cape, H. africaeaustralis). Through coalescence-based demographic reconstructions, we demonstrated that bottleneck at introduction was mild and did not severely affect the reservoir of genetic diversity. Our data also highlighted a marked geographic structure in the invasive populations, indicating that they are likely the results of multiple introduction events. Nevertheless, both the invasive populations and its source show a lower level of diversity relative to other native populations from Sub-Saharan and South Africa, suggesting that demographic history before introduction may have played a role in forging a successful invader. Finally, we showed that the current spatial expansion at the northern boundary of the range is following a leading-edge model characterized by a general reduction of genetic diversity towards the edge of the expanding range. Consistently, random fixation of alleles through gene-surfing seems a more likely explanation than adaptive divergence for the distribution of the few outlier loci with highly divergent frequencies between core and newly colonized areas.
