Genetic Diversity and Population Structure in Polygonum cespitosum: Insights to an Ongoing Plant Invasion

Abstract

Molecular markers can help elucidate how neutral evolutionary forces and introduction history contribute to genetic variation in invaders. We examined genetic diversity, population structure and colonization patterns in the invasive Polygonum cespitosum, a highly selfing, tetraploid Asian annual introduced to North America. We used nine diploidized polymorphic microsatellite markers to study 16 populations in the introduced range (northeastern North America), via the analyses of 516 individuals, and asked the following questions: 1) Do populations have differing levels of within-population genetic diversity? 2) Do populations form distinct genetic clusters? 3) Does population structure reflect either geographic distances or habitat similarities? We found low heterozygosity in all populations, consistent with the selfing mating system of P. cespitosum. Despite the high selfing levels, we found substantial genetic variation within and among P. cespitosum populations, based on the percentage of polymorphic loci, allelic richness, and expected heterozygosity. Inferences from individual assignment tests (Bayesian clustering) and pairwise F ST values indicated high among-population differentiation, which indicates that the effects of gene flow are limited relative to those of genetic drift, probably due to the high selfing rates and the limited seed dispersal ability of P. cespitosum. Population structure did not reflect a pattern of isolation by distance nor was it related to habitat similarities. Rather, population structure appears to be the result of the random movement of propagules across the introduced range, possibly associated with human dispersal. Furthermore, the high population differentiation, genetic diversity, and fine-scale genetic structure (populations founded by individuals from different genetic sources) in the introduced range suggest that multiple introductions to this region may have occurred. High genetic diversity may further contribute to the invasive success of P. cespitosum in its introduced range.

An ideal weed: plasticity and invasiveness inPolygonum cespitosum

“…8), and there was no association between genetic similarity and habitat similarity along either light or moisture axes. 91 This result is concordant with the lack of local adaptation found in the quantitative-genetic studies of population diversity discussed above (Refs. 74 and 84; in highly selfing species such as P. cespitosum, the entire genome is inherited, so variation in neutral markers can be expected to correspond generally with variation in quantitative traits).…”

“…15 Indeed, four Asian populations of P. cespitosum had lower within-population allelic richness than the New England populations (see Appendix S4 in Ref. 91). A pattern of increased rather than reduced molecular-genetic diversity in introduced-range populations has been found in several invasive plants and animals.…”

“…The experimentally identified highperformance group was as genetically diverse as the control genotypes that were randomly sampled from across populations: both groups included substantial quantitative genetic variation, as well as similar amounts of variation for neutral microsatellite markers. 66,91 Thus, there is no evidence that a few specific "super-genotypes" have spread across the introduced range, as has occurred in certain wellstudied plant invasions (e.g., single super-genotype of the invasive grass Pennisetum setaceum; 92 aggressive lineages of Phragmites australis 93,94 ). Instead, P. cespitosum genotypes with accelerated life histories and consistently high reproductive output may have evolved independently in a number of different introduced-range populations.…”

“…91 In this collaborative study, we examined nine polymorphic microsatellite markers in a sample of 35 individuals from each of 16 New England populations (total N = 516). As expected for a largely self-fertilizing species, observed heterozygosity was low at all nine loci.…”

Transatlantic invasion routes and adaptive potential in North American populations of the invasive glossy buckthorn,Frangula alnus

“…Although both phenotypic plasticity (Davidson et al, 2011;Higgins and Richardson, 2014) and epigenetic variation (Richards et al, 2012) have also been proposed to compensate for this lack of population genetic variation, many invasive species have apparently benefited from repeated introductions, increasing propagule pressure and genetic variation in the new range (Kolbe et al, 2004;Dlugosch and Parker, 2008;Matesanz et al, 2014;Shirk et al, 2014;Barriball et al, 2015). Repeated introductions from multiple source populations can be expected to be especially beneficial for adaptive genetic diversity in the invasive range when the source populations in the native range cover distinct environmental conditions (e.g.…”