Genetic variation in mutualistic and antagonistic interactions in an invasive legume

How ecological and evolutionary theory expanded the 'ideal weed' concept

Since Baker's attempt to characterize the 'ideal weed' over 50 years ago, ecologists have sought to identify features of species that predict invasiveness. Several of Baker's 'ideal weed' traits are well studied, and we now understand that many traits can facilitate different components of the invasion process, such as dispersal traits promoting transport or selfing enabling establishment. However, the effects of traits on invasion are context dependent. The traits promoting invasion in one community or at one invasion stage may inhibit invasion of other communities or success at other invasion stages, and the benefits of any given trait may depend on the other traits possessed by the species. Furthermore, variation in traits among populations or species is the result of evolution. Accordingly, evolution both prior to and after invasion may determine invasion outcomes. Here, we review how our understanding of the ecology and evolution of traits in invasive plants has developed since Baker's original efforts, resulting from empirical studies and the emergence of new frameworks and ideas such as community assembly theory, functional ecology, and rapid adaptation. Looking forward, we consider how trait-based approaches might inform our understanding of less-explored aspects of invasion biology ranging from invasive species responses to climate change to coevolution of invaded communities.

Stay in shape: Assessing the adaptive potential of shell morphology and its sensitivity to temperature in the invasive New Zealand mud snail Potamopyrgus antipodarum through phenotypic plasticity and natural selection in Europe

Climate change may force organisms to adapt genetically or plastically to new environmental conditions. Invasive species show remarkable potential for rapid adaptation. The ovoviviparous New Zealand mud snail (NZMS), Potamopyrgus antipodarum, has successfully established across Europe with two clonally reproducing mitochondrial lineages since its arrival in the first half of the 19th century. Its remarkable variation in shell morphology was shown to be fitness relevant. We investigated the effects of temperature on shell morphology across 11 populations from Germany and the Iberian Peninsula in a common garden across three temperatures. We analyzed size and shape using geometric morphometrics. For both, we compared reaction norms and estimated heritabilities. For size, the interaction of temperature and haplotype explained about 50% of the total variance. We also observed more genotype by environment interactions indicating a higher degree of population differentiation than in shape. Across the three temperatures, size followed the expectations of the temperature-size rule, with individuals growing larger in cold environments. Changes in shape may have compensated for changes in size affecting space for brooding embryos. Heritability estimates were relatively high. As indicated by the very low coefficients of variation for clonal repeatability (CV A ), they can probably not be compared in absolute terms. However, they showed some sensitivity to temperature, in haplotype t more so than in z, which was only found in Portugal. The low CV A values indicate that genetic variation among European populations is still restricted with a low potential to react to selection. A considerable fraction of the genetic variation was due to differences between the clonal lineages. The NZMS has apparently not been long enough in Europe to accumulate significant genetic variation relevant for morphological adaptation. As temperature is obviously not the sole factor influencing shell morphology, their interaction will probably not be a factor limiting population persistence under a warming climate in Europe.

A private channel of nitrogen alleviates interspecific competition for an annual legume

The way resource availability predictably alters interspecific interactions and may favor one resource-acquisition strategy over another is critical for understanding context dependency. The ubiquity of nitrogen (N) limitation across terrestrial environments is a driver of plant competition and the association of some plants with N-fixing bacteria (rhizobia) may alleviate competition with nonfixing plants. Conversely, when available soil N is elevated, competitive advantages imparted by rhizobia are hypothesized to decline because nonfixing species are able to acquire those nutrients readily. We manipulated competition, soil N, and soil microbial inoculation, employing the ground bean Amphicarpaea bracteata, a native annual N-fixing legume, and jewelweed Impatiens capensis, a native co-occurring nonfixing annual. We found that legume performance was negatively impacted by interspecific competition, but less so under lower soil N in both the greenhouse and field. The legume invested a greater proportion of resources in rhizobia when competing, but only under low N. Also consistent with predictions, a competition-by-microbial-inoculation interaction demonstrated that negative effects of competition were alleviated by rhizobia. Finally, we detected an interaction between inoculation and fertilization, whereby N addition resulted in increased performance for uninoculated legumes, but a small decline in performance for inoculated plants, the latter likely representing a cost of mutualism. Thus, several lines of evidence point to the legume-rhizobia mutualism being more beneficial under competition and limited soil N. Competing I. capensis, in contrast, benefited from N addition regardless of the addition of soil microbes. In a survey of natural populations, legume and rhizobia growth were positively correlated at population edges (where interspecific competition is expected to be higher, the mutualism is stronger), whereas at population centers we found no association. Isotopic evidence confirmed a higher degree of rhizobial N-fixation at population edges compared to centers. Taken together, our results demonstrate an important role for the largely private channel of nitrogen in legume competitive performance, but with the benefits imparted by rhizobia being predictably weaker at higher soil fertility. We speculate that alleviation of competitive impacts through resource partitioning is an important and yet largely overlooked aspect of the evolutionary ecology of legume-rhizobia interactions.

Decreased coevolutionary potential and increased symbiont fecundity during the biological invasion of a legume-rhizobium mutualism

Although most invasive species engage in mutualism, we know little about how mutualism evolves as partners colonize novel environments. Selection on cooperation and standing genetic variation for mutualism traits may differ between a mutualism's invaded and native ranges, which could alter cooperation and coevolutionary dynamics. To test for such differences, we compare mutualism traits between invaded- and native-range host-symbiont genotype combinations of the weedy legume, Medicago polymorpha, and its nitrogen-fixing rhizobium symbiont, Ensifer medicae, which have coinvaded North America. We find that mutualism benefits for plants are indistinguishable between invaded- and native-range symbioses. However, rhizobia gain greater fitness from invaded-range mutualisms than from native-range mutualisms, and this enhancement of symbiont fecundity could increase the mutualism's spread by increasing symbiont availability during plant colonization. Furthermore, mutualism traits in invaded-range symbioses show lower genetic variance and a simpler partitioning of genetic variance between host and symbiont sources, compared to native-range symbioses. This suggests that biological invasion has reduced mutualists' potential to respond to coevolutionary selection. Additionally, rhizobia bearing a locus (hrrP) that can enhance symbiotic fitness have more exploitative phenotypes in invaded-range than in native-range symbioses. These findings highlight the impacts of biological invasion on the evolution of mutualistic interactions.

Environmental variation impacts trait expression and selection in the legume-rhizobium symbiosis

PREMISE

The ecological outcomes of mutualism are well known to shift across abiotic or biotic environments, but few studies have addressed how different environments impact evolutionary responses, including the intensity of selection on and the expression of genetic variance in key mutualism-related traits.

METHODS

We planted 30 maternal lines of the legume Medicago lupulina in four field common gardens and compared our measures of selection on and genetic variance in nodulation, a key trait reflecting legume investment in the symbiosis, with those from a previous greenhouse experiment using the same 30 M. lupulina lines.

RESULTS

We found that both the mean and genetic variance for nodulation were much greater in the greenhouse than in the field and that the form of selection on nodulation significantly differed across environments. We also found significant genotype-by-environment (G × E) effects for fitness-related traits that were generated by differences in the rank order of plant lines among environments.

CONCLUSIONS

Overall, our results suggest that the expression of genotypic variation and selection on nodulation differ across environments. In the field, significant rank-order changes for plant fitness potentially help maintain genetic variation in natural populations, even in the face of directional or stabilizing selection.