Potential selection in native grass populations by exotic invasion

Abstract

Ecological impacts of invasive plant species are well documented, but the genetic response of native species to invasive dominance has been often overlooked. Invasive plants can drastically alter site conditions where they reach dominance, potentially exerting novel selective pressures on persistent native plant populations. Do native plant populations in old exotic invasions show evidence of selection when compared to conspecific populations in adjacent, noninvaded areas? We employ amplified fragment length polymorphism (AFLP) analysis to screen a large number of loci from two native grass species (Hesperostipa comata (Trin. & Rupr.) Barkworth and Sporobolus airoides Torr.) that occur in old infestations of the invasive forb Acroptilon repens. We then compare observed locus by locus F(ST) values with distributions of F(ST) estimated from simulation models under expectation of neutrality. We also compare the proportion of loci possibly linked to selection and those not linked to selection which exhibit parallel trends in divergence between two community types (invaded, noninvaded). Few loci (H. comata, 2.6%; S. airoides, 8.7%) in the two native grasses may be linked to genes under the influence of selection. Also, loci linked to selection showed a greater portion of parallel trends in divergence than neutral loci. Genetic similarities between community types were less than genetic similarity within community types suggesting differentiation in response to community alteration. These results indicate that a small portion of scored AFLP loci may be linked to genes undergoing selection tied to community dominance by an invasive species. We propose that native plants in communities dominated by exotic invasives may be undergoing natural selection.

Impact of Amplified Fragment Length Polymorphism Size Homoplasy on the Estimation of Population Genetic Diversity and the Detection of Selective Loci

“…A high critical probability threshold level of 5% vs. a much lower value of 0.05%: Most authors have considered critical values of 5% (Scotti-Saintagne et al 2004;Acheré et al 2005;Bonin et al 2006;Mealor and Hild 2006;Miller et al 2007;Papa et al 2007;Nosil et al 2008;Smith et al 2008) or 1% (Jump et al 2006;Murray and Hare 2006;Papa et al 2007;Nosil et al 2008). 2.…”

“…2. The mean F ST used to build the null model neutral distribution was a trimmed mean (used by Bonin et al 2006;Miller et al 2007;Nosil et al 2008;Smith et al 2008), the raw (nontrimmed) mean (used by Scotti-Saintagne et al 2004;Mealor and Hild 2006), or the median F ST (used in simulations by Beaumont and Balding 2004).…”

“…The simulated F ST data are used to identify those loci that may not fit the neutral drift model given their unusually low or high F ST values. The software DFDIST is the method most frequently used when using AFLPs (see Scotti-Saintagne et al 2004;Acheré et al 2005;Bonin et al 2006;Jump et al 2006;Mealor and Hild 2006;Murray and Hare 2006;Miller et al 2007;Papa et al 2007;Nosil et al 2008;Smith et al 2008).…”

“…This suggests that homoplasy may produce biases in the estimation of genetic diversity from AFLP markers, but the magnitude and direction of the bias remains to be investigated in detail. In addition, the use of AFLPs to identify selective loci in genome scans is becoming an active area of research in evolutionary studies (Wilding et al 2001;Storz and Nachman 2003;Campbell and Bernatchez 2004;Scotti-Saintagne et al 2004;Storz and Dubach 2004;Acheré et al 2005;Beaumont 2005;Nielsen 2005;Storz 2005;Bonin et al 2006;Jump et al 2006;Mealor and Hild 2006;Murray and Hare 2006;Miller et al 2007;Papa et al 2007;Nosil et al 2008;Smith et al 2008). Genomewide scans of selective loci generally use information on heterozygosity and genetic differentiation (Beaumont 2005;Storz 2005), and the putative biases in these parameters due to homoplasy could produce some misinterpretations.…”

Coevolution between invasive and native plants driven by chemical competition and soil biota

“…However, this dominance likely imposes intense selection pressures on native species, potentially resulting in new coevolutionary relationships that may act to integrate the exotic species into its new community (27). Exotic species can evolve rapidly in their new range (28), and some native species have evolved adaptations to invaders (29), including native plants responding to invasive plant competitors (30)(31)(32)(33). However, no study to date has documented reciprocal coevolution between native and exotic plant species (27).…”