Print PageDetailed ecological understanding of invasive species is expected to facilitate the mitigation of financial costs and losses in ecosystem services resulting from species invasions. Although invasive plant species have been well-studied in terms of ecological phenomena associated with establishment and spread, we still know little regarding the potential role of genetic variability in invasion. Population genetic approaches can detect historical events such as the number and timing of introductions, genetic bottlenecks, and the degree of genetic mixing among populations. Analyses such as these can also highlight genetic and geographic contributors to establishment success and spread. Cogongrass (Imperata cylindrica (L.) P. Beauv.) is a persistent and costly invasive grass species that is found throughout the tropical and subtropical regions of the world. Since its introduction into the United States in the early 20th century, cogongrass has been confirmed in seven southeastern states where its invasion negatively affects the ecology of highly diverse native systems. We focused this study on populations in Mississippi and Alabama because each has been subjected to direct introductions of cogongrass from outside the United States. We employed dominant, amplified fragment length polymorphism (AFLP) markers to assess (1) how much extant genetic diversity resides within populations in the U.S., and (2) how geographic distance correlates with genetic distance. We expect that genetic variation will be greatest near the sites of known introductions and decrease with increasing geographic distance.
Results/Conclusions
The percentage of polymorphic loci per population ranges from 5.5% to 30.2% in 19 populations examined thus far. One measure of population subdivision (φPT, an FST analog) indicates significant genetic differentiation between cogongrass populations in Mississippi and Alabama, where φPT = 0.323 (P < 0.001). Data suggest that the majority of genetic variation occurs within populations (68%), with only 28% of the genetic variation among populations. A previous study found 56% of genetic variation within populations and 44% among, whereas without regional partitioning we found 69% of the variation within and 31% among populations, with φPT = 0.309 (P<0.001). Nei’s genetic distance among populations ranges from 0.001-0.038, with the greatest genetic distances among populations in Mississippi. These results imply some structuring of genetic variation is present. The occurrences of unique polymorphisms (loci present in only one population) in a few of the sampled populations may support historical introductions into McNeil, MS, and Mobile Bay, AL, and lend support for intraspecific hybridization between introduced populations that were previously isolated in Asia.
