Colonization of new habitats is a demanding task for introduced species. We are exploring the ecology and genetic structure of populations of the amphipod Gammarus tigrinus – a species native to estuaries of the Atlantic seaboard of North America – in the Great Lakes and Europe. The species was introduced to the United Kingdom more than 60 years ago, and it has spread to mainland Europe, including the Baltic Sea, in more recent decades. It was first reported in the Great Lakes in 2001, and is now present in most of the lakes. The transition from native to introduced populations suggests the possibility of founder effects in the latter populations, whereas moving from brackish to freshwater is a major challenge for most species.
We used the molecular marker COI, a mitochondrial gene called cytochrome c oxidase 1, to assay genetic structure in native populations along the Atlantic seaboard. Interestingly, distinct clades were noted, which were separated from one another near Cape Cod, MA. We reason, in a paper published in Evolution (2006), that these clades may represent northern and southern species. Other species are also reported to have discontinuities in this region, suggesting that a biogeographic barrier may prevent effective dispersal between northern and southern populations.
In a second study, we contrasted COI genetic structure of the same native populations with introduced populations in the Great Lakes and in Europe to identify invasion ‘pathways’. Populations in the Great Lakes were genetically identical to that in the Hudson River, NY, which was also very similar to that in the St. Lawrence River. Some populations in Europe had higher diversity than any of the native populations from which they may have been drawn, and the presence of specific haplotypes indicates that at least two separate source populations were required to achieve the diversity observed in these populations. The findings provide additional support to the concept that invasive populations need not experience genetic bottlenecks, or that if they do this effect is counteracted by influx of immigrants from genetically distinct sources. This study will be published in 2006 in the journal Molecular Ecology.
Our next step in this project will utilize these spatial patterns of genetic diversity to test how important sampling effort is to the interpretation of tests designed to identify invasion pathway. Often investigators (including us) will use only a small number of individuals in analyses of genetic diversity at each of a number of different sites, and from these data determine which population(s) sourced an invasive population. Clearly, as sampling size declines for each population assayed, the likelihood of incorrectly determining a source population increases. In addition, we test whether dropping individual source populations will affect the outcome of source identification. These studies are important because unless investigators utilize a sufficient number of source populations, and a sufficient number of individuals from each source, the likelihood of obtaining an erroneous result increases.