Patterns of genetic diversity reveal multiple introductions and recurrent founder effects during range expansion in invasive populations of Geranium carolinianum (Geraniaceae)

Abstract

Genetic diversity, and thus the adaptive potential of invasive populations, is largely based on three factors: patterns of genetic diversity in the species' native range, the number and location of introductions and the number of founding individuals per introduction. Specifically, reductions in genetic diversity ('founder effects') should be stronger for species with low withinpopulation diversity in their native range and few introductions of few individuals to the invasive range. We test these predictions with Geranium carolinianum, a winter annual herb native to North America and invasive in China. We measure the extent of founder effects using allozymes and microsatellites, and ask whether this is consistent with its colonization history and patterns of diversity in the native range. In the native range, genetic diversity is higher and structure is lower than expected based on life history traits. In China, our results provide evidence for multiple introductions near Nanjing, Jiangsu province, with subsequent range expansion to the west and south. Patterns of genetic diversity across China reveal weak founder effects that are driven largely by low-diversity populations at the expansion front, away from the introduction location. This suggests that reduced diversity in China has resulted from successive founder events during range expansion, and that the loss of genetic diversity in the Nanjing area was mitigated by multiple introductions from diverse source populations. This has implications for the future of G. carolinianum in China, as continued gene flow among populations should eventually increase genetic diversity within the more recently founded populations.

High but variable outcrossing rates in the invasive Geranium carolinianum (Geraniaceae)

“…Lower values for Chinese populations are consistent with results from a broader sampling of 24 Chinese and 16 US populations, where the average inbreeding coeffi cient was −0.09 in China and 0.10 in the US ( Shirk et al, 2014 ). We also estimated F e , the inbreeding coeffi cient calculated under the assumption that the population is at inbreeding equilibrium with a constant outcrossing rate t m .…”

“…The fi rst records of G. carolinianum in China are from Nanjing in 1926( Aedo, 2000Chinese Academy of Sciences, 2012 ), which is in the northeastern part of its current range. Genetic data suggest multiple, independent introductions to the Nanjing area, and levels of genetic diversity in these populations are consistent with more than one founder per founding event ( Shirk et al, 2014 ).…”

“…Higher outcrossing rates in China are consistent with results from an earlier genetic diversity study, which found lower inbreeding coeffi cients in Chinese compared to US populations. Additionally, patterns of genetic diversity in China are consistent with multiple introductions from genetically distinct sources and multiple founders per founder event ( Shirk et al, 2014 ). Thus, initially there may have been little selection for selfi ng in the newly founded populations, as high propagule pressure, which is characteristic of many invasions, reduces the need for selfi ng as reproductive assurance.…”

“…(2) Does mating system vary among invasive and native populations? (3) Are among-population differences in each NAT population in the program GenAlEx v. 6.5 Smouse, 2006 , 2012 ) using a panel of 19 to 23 polymorphic allozyme loci and between 29 and 50 individuals per population ( Shirk et al, 2014 ). This data set is more appropriate for calculating F because maternal genotypes are known rather than inferred by MLTR, and the larger numbers of individuals and loci should improve precision.…”

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.…”