Genetic diversity and population structure in Polygonum cespitosum: insights to an ongoing plant invasion

Post-introduction evolution of a rapid life-history strategy in a newly invasive plant

A central question in invasion biology is whether adaptive trait evolution following species introduction promotes invasiveness. A growing number of common-garden experiments document phenotypic differences between native- and introduced-range plants, suggesting that adaptive evolution in the new range may indeed contribute to the success of invasive plants. Yet these studies are often subject to methodological pitfalls, resulting in weak evidence for post-introduction adaptive trait evolution and leaving uncertain its role in the invasion process. In a common-garden glasshouse study, we compared the growth, life-history, and reproductive traits of 35 native- and introduced-range Polygonum cespitosum populations. We used complementary approaches including climate-matching, standardizing parental conditions, selection analysis, and testing for trait-environment relationships to determine whether traits that increase invasiveness adaptively evolved in the species' new range. We found that the majority of introduced-range populations exhibited a novel trait syndrome consisting of a fast-paced life history and concomitant sparse, reduced growth form. Selection analysis confirmed that this trait syndrome led to markedly higher fitness (propagule production) over a limited growing season characteristic of regions within the introduced range. Additionally, several growth and reproductive traits showed temperature-based clines consistent with adaptive evolution in the new range. Combined, these results indicate that, subsequent to its introduction to North America over 100 generations ago, P. cespitosum has evolved key traits that maximize propagule production. These changes may in part explain the species' recent transition to invasiveness, illustrating how post-introduction evolution may contribute to the invasion process.

Plasticity to drought and ecotypic differentiation in populations of a crop wild relative

Populations of widely distributed species often exhibit geographic variation in functional traits in response to environmental heterogeneity. Such trait variation may be the result of different adaptive mechanisms, including genetically based differentiation, phenotypic plasticity or a combination of both. Disentangling the genetic and environmental components of trait variation may be particularly interesting in crop wild relatives, since they may provide unique reservoirs of genetic diversity for crop improvement. In this study, we assessed ecotypic differentiation and patterns of plasticity to drought in populations of Lupinus angustifolius, a Mediterranean crop wild relative, from two climatically distinct regions in the Iberian Peninsula. Using an outdoor common garden, we compared phenotypic responses of inbred maternal families to two ecologically meaningful water availability treatments (drought and high-moisture). We measured 18 different functional traits related to growth, morphology, phenology and reproduction. Plants in the drought treatment grew less, had lower leaf chlorophyll content and photochemical efficiency, but also reproduced faster, produced larger seeds and altered leaflet morphology through increased leaflet thickness, higher leaflet dry matter content and lower specific leaf area. We also found significant differences between regions that likely reflect adaptation to climatically distinct environments, with populations from the south showing a faster onset of reproduction, higher leaf thickness and higher seed size, consistent with the drier conditions experienced in southern sites. Plasticity to drought was in most cases in the same direction as quantitative genetic differentiation (i.e. cogradient variation), providing evidence of the adaptive value of the plastic change. Our results show that both genetic differentiation and plasticity can generate adaptive phenotypic variation in L. angustifolius, and help to identify potentially valuable genetic resources to incorporate into breeding programmes.

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

BACKGROUND AND AIMS

Many invasive species severely threaten native biodiversity and ecosystem functioning. One of the most prominent questions in invasion genetics is how invasive populations can overcome genetic founder effects to establish stable populations after colonization of new habitats. High native genetic diversity and multiple introductions are expected to increase genetic diversity and adaptive potential in the invasive range. Our aim was to identify the European source populations of Frangula alnus (glossy buckthorn), an ornamental and highly invasive woody species that was deliberately introduced into North America at the end of the 18th century. A second aim of this study was to assess the adaptive potential as an explanation for the invasion success of this species.

METHODS

Using a set of annotated single-nucleotide polymorphisms (SNPs) that were assigned a putative function based on sequence comparison with model species, a total of 38 native European and 21 invasive North American populations were subjected to distance-based structure and assignment analyses combined with population genomic tools. Genetic diversity at SNPs with ecologically relevant functions was considered as a proxy for adaptive potential.

KEY RESULTS

Patterns of invasion coincided with early modern transatlantic trading routes. Multiple introductions through transatlantic trade from a limited number of European port regions to American urban areas led to the establishment of bridgehead populations with high allelic richness and expected heterozygosity, allowing continuous secondary migration to natural areas.

CONCLUSIONS

Targeted eradication of the urban populations, where the highest genetic diversity and adaptive potential were observed, offers a promising strategy to arrest further invasion of native American prairies and forests.

A Survey of Genetic Variation and Genome Evolution within the Invasive Fallopia Complex

The knotweed taxa Fallopia japonica, F. sachalinensis and their interspecific hybrid F. × bohemica are some of the most aggressive invaders in Europe and North America and they are serious threats to native biodiversity. At the same time, they constitute a unique model system for the creation of hybrids and studies of the initiation of evolutionary processes. In the presented study, we focused on (i) examining genetic diversity in selected populations of three Fallopia taxa in the invaded (Poland) and native ranges (Japan), (ii) establishing genome size and ploidy levels and (iii) identifying ribosomal DNA (rDNA)-bearing chromosomes in all of the taxa from the invaded range. We found that the genetic diversity within particular taxa was generally low regardless of their geographical origin. A higher level of clonality was observed for the Polish populations compared to the Japanese populations. Our study suggests that the co-occurrence of F. sachalinensis together with the other two taxa in the same stand may be the source of the higher genetic variation within the F. × bohemica hybrid. Some shift towards the contribution of F. japonica alleles was also observed for selected F. × bohemica individuals, which indicates the possibility of producing more advanced generations of F. × bohemica hybrids. All of the F. sachalinensis individuals were hexaploid (2n = 6x = 66; 2C = 6.01 pg), while those of F. japonica were mostly octoploid (2n = 8x = 88; 2C = 8.87 pg) and all of the F. × bohemica plants except one were hexaploid (2n = 6x = 66; 2C = 6.46 pg). Within the chromosome complement of F. japonica, F. sachalinensis and F. × bohemica, the physical mapping of the rDNA loci provided markers for 16, 13 and 10 chromosomes, respectively. In F. × bohemica, a loss of some of rDNA loci was observed, which indicates the occurrence of genome changes in the hybrid.

An ideal weed: plasticity and invasiveness in Polygonum cespitosum

The introduced Asian plant Polygonum cespitosum has only recently become invasive in northeastern North America, spreading into sunny as well as shaded habitats. We present findings from a multiyear case study of this ongoing species invasion, drawing on field environmental measurements, glasshouse plasticity and resurrection experiments, and molecular genetic (microsatellite) data. We focus in particular on patterns of individual phenotypic plasticity (norms of reaction), their diversity within and among populations in the species' introduced range, and their contribution to its potential to evolve even greater invasiveness. Genotypes from introduced-range P. cespitosum populations have recently evolved to express greater adaptive plasticity to full sun and/or dry conditions without any loss of fitness in shade. Evidently, this species may evolve the sort of "general-purpose genotypes" hypothesized by Herbert Baker to characterize an "ideal weed." Indeed, we identified certain genotypes capable of extremely high reproductive output across contrasting conditions, including sunny, shaded, moist, and dry. Populations containing these high-performance genotypes had consistently higher fitness in all glasshouse habitats; there was no evidence for local adaptive differentiation among populations from sunny, shaded, moist, or dry sites. Norm of reaction data may provide valuable insights to invasion biology: the presence of broadly adaptive, high-performance genotypes can promote a species' ecological spread while providing the fuel for increased invasiveness to evolve.