Patterns of genetic, morphological and physiological trait variation among native and introduced populations of Bromus rubens

Background/Question/Methods

The evolution of invasiveness hypothesizes that genetic differentiation between introduced and native populations contributes to the success of exotic species. The mechanisms responsible for differentiation are founder effects, adaptation to novel environments, or admixture of previously separated populations. The former mechanisms are paradoxical, in that the few species that successfully invade are expected to have reduced adaptive potential due to genetic bottleneck effects. However, some evidence suggests that although introduced populations lose genetic diversity, quantitative trait diversity may not be significantly altered. Evidence for the latter mechanism suggests that loses of genetic diversity are transient events ameliorated by multiple introductions that increase the adaptive potential of introduced populations. We quantified patterns of genetic, morphological and physiological variation among introduced (Californian) and native (Spanish) populations of the important invasive grass, Bromus rubens, to test these mechanisms. We sequenced chloroplast DNA to assess genetic diversity and structure within and among populations. A common garden approach was used to quantify morphological and physiological traits. 

Results/Conclusions

Californian populations displayed 23% greater genetic diversity and a 17% increase in expected heterozygosity relative to Spanish populations. Of the ten unique haplotypes identified, three were shared among introduced and native populations. Two of these were found in one population in California, but never occurred together within Spanish populations. Analyses of molecular variance reflected this pattern of haplotype isolation among native populations (Φ_PT = 0.84, p < 0.001), but not among introduced populations (Φ_PT = 0.08, p < 0.17). In the common garden experiment plants from introduced populations were significantly larger than from native populations. Shoot height and relative growth rate of the introduced populations were larger than the native populations by 28% and 25%, respectively (shoot height: F = 148.63, p < 0.0001; relative growth rate: F = 82.23, p < 0.001), whereas the native populations showed 58% greater photosynthetic capacity (F = 36.1, p < 0.01). These data indicate that evolutionary change toward more invasive behavior has occurred in the introduced populations. The genetic data suggests that the increased diversity observed in Californian populations resulted from multiple introduction events, followed by admixture of previously isolated haplotypes.