Comparative phylogeography of Ponto-Caspian amphipods throughout the invaded Baltic and native NW Black Sea donor ranges. Can introduction mode affect genetic diversity?

The Baltic countries harbor a diverse assemblage of alien amphipods of Ponto-Caspian origin. The composition of this fauna was shaped by three invasion waves: 1) pre-20th century dispersals via watershed-connecting canals, 2) deliberate introductions in the 1960s, and 3) new dispersals during the last decade via shipping and pre-existing canals. Given this rich invasion history, we test whether genetic diversity (mitochondrial and nuclear) differs between the native and invaded ranges and between the deliberately introduced species and the ones that dispersed on their own. Our results show a significant decrease in mitochondrial but not nuclear genetic diversity in the invaded Baltic range. We also find that in the invaded range the introduced species exhibit a higher mitochondrial and nuclear genetic diversity than the species that dispersed on their own, while in the native range only the nuclear diversity is higher in introduced species. Mitochondrial diversity was more structured geographically in the native range and the dominant invasive haplotypes were detected in the native populations of all but one species, further highlighting the utility of this marker in tracing invasion sources. Our comparative approach provides important insight into the inter-range genetic diversity of Ponto-Caspian invaders, highlighting the role of introduction mode.