Morphology and genetics of Lythrum salicaria from latitudinal gradients of the Northern Hemisphere grown in cold and hot common gardens.

The aim of this project was to compare the phenotypic responses of global populations of Lythrum salicaria in cold/dry and hot/humid environments to determine if phenotypic plasticity varied between the native and invasive ranges, and secondarily if this variation was linked to genetic diversity. Common garden studies were conducted in Trebon, Czech Republic, and Lafayette, Louisiana, USA (cold/dry vs. hot/humid garden, respectively), using populations from latitudinal gradients in Eurasia and North America. Lythrum salicaria seeds collected from the same maternal plants across these latitudinal gradients were germinated and grown in Trebon and Lafayette. Tissue masses (above-, below-ground, inflorescence and total) of these individuals were assessed at the end of each growing season (2006-2008). Worldwide field measurements of L. salicaria height were made by volunteers from 2004-2016. Biomass and height data were analyzed using the General Linear Model framework and multivariate techniques. Molecular markers (amplified fragment length polymorphisms) of individuals used in the common garden study were analyzed using traditional genetic diversity metrics and Bayesian clustering algorithms in STRUCTURE. Reaction norms were developed from differences in maternal plant responses in Trebon versus Lafayette. In the common garden studies, stem/leaf, root and total biomass generally were highest for individuals grown from seeds collected in the southern part of the range in the cold garden, particularly by the third year of the study. In contrast, inflorescence biomass in the cold garden was higher by the third year in individuals from mid-latitude populations. As measured by volunteers, plants were taller in Eurasia than in North America moving from north to south with the pattern switching southward of 40°N latitude. Genetic diversity was similar between native and non-native invasive populations regardless of geographical origin of the seed and was not significantly different in the GLM Select model (p > 0.05). Reaction norm slopes showed that Eurasia had larger values than North America for reaction norms for above-ground and total biomass. Plants from the seeds of mother plants from Turkey had wide variation in total biomass when grown in Trebon versus Lafayette; this variation in response within certain populations may have contributed to the lack of population-level differences in plasticity. These results indicate no loss of genetic diversity for L. salicaria during its North American invasion, nor reduction in plastic tissue allocation responses to a varying environment, which may help explain some of its invasive qualities and which could be of adaptive value under changing future environments.

Experimental insights on Darwin's cross-promotion hypothesis in tristylous purple loosestrife (Lythrum salicaria).

PREMISE OF THE STUDY: Darwin proposed that the reciprocal arrangement of anthers and stigmas in heterostylous plants promotes cross-pollination through pollen segregation on pollinators' bodies. The floral tube in most heterostylous species constrains the feeding posture of pollinators determining the site of contact with sex organs located at different heights within a flower. Here, we evaluate Darwin's hypothesis in tristylous Lythrum salicaria, a species with a partially tubular corolla, and examine the extent to which the location of sex organs within a flower influence compatible and incompatible pollination. We predicted that the proficiency of cross-pollination would increase for more inserted sex organs due to the restrictions imposed by the floral tube on pollinator positioning. METHODS: We used experimental trimorphic and monomorphic arrays and emasculated flowers to quantify intermorph pollen transfer and capture among all sex-organ heights, and estimated the contribution of intraflower self-pollination, geitonogamous self-pollination, and intramorph outcross pollination to total intramorph pollination. KEY RESULTS: As predicted, disassortative pollination varied significantly with sex-organ height and was highest for short-level organs and lowest for long-level organs. In monomorphic arrays, most intramorph pollination resulted from outcross pollination followed by intraflower and geitonogamous self-pollination. CONCLUSIONS: Our results provide evidence for the Darwinian hypothesis. Reciprocal herkogamy promoted varying degrees of disassortative pollination with the magnitude strongly influenced by sex-organ height within a flower.

Evidence that a herbivore tolerance response affects selection on floral traits and inflorescence architecture in purple loosestrife (Lythrum salicaria).

Background and Aims: The study of the evolution of floral traits has generally focused on pollination as the primary driver of selection. However, herbivores can also impose selection on floral traits through a variety of mechanisms, including florivory and parasitism. Less well understood is whether floral and inflorescence architecture traits that influence a plant's tolerance to herbivory, such as compensatory regrowth, alter pollinator-mediated selection. Methods: Because herbivore damage to Lythrum salicaria meristems typically leads to an increase in the number of inflorescences and the size of the floral display, an experiment was conducted to test whether simulated herbivory (i.e. clipping the developing meristem) could alter the magnitude or direction of pollinator-mediated selection on a suite of floral and inflorescence architecture traits. Using a pollen supplementation protocol, pollen limitation was compared in the presence and absence of meristem damage in order to quantify any interaction between pollinator and herbivore-mediated selection on floral traits. Key Results: Surprisingly, in spite of an obvious impact on floral display and architecture, with clipped plants producing more inflorescences and more flowers, there was no difference in pollen limitation between clipped and unclipped plants. Correspondingly, there was no evidence that imposing herbivore damage altered pollinator-mediated selection in this system. Rather, the herbivory treatment alone was found to alter direct selection on floral display, with clipped plants experiencing greater selection for earlier flowering and weaker selection for number of inflorescences when compared with unclipped plants. Conclusions: These findings imply that herbivory on its own can drive selection on plant floral traits and inflorescence architecture in this species, even more so than pollinators. Specifically, herbivory can impose selection on floral traits if such traits influence a plant's tolerance to herbivory, such as through the timing of flowering and/or the compensatory regrowth response.

Relationships among three pathways for resource acquisition and their contribution to plant performance in the emergent aquatic Plant Lythrum salicaria (L.).

Three pathways for resource acquisition exist in the emergent aquatic plant, Lythrum salicaria (L.); a subterranean root system, a free-floating adventitious root system, and arbuscular mycorrhiza (AM) fungal hyphae colonizing subterranean roots. This study examined the relationship(s) among these pathways and their contribution to plant performance. If the free-floating adventitious root system and/or AM fungi contribute to plant growth in wetland habitats, we predicted that their absence would result in a significant reduction in plant performance. Furthermore, if a reduction in resource uptake, effected by an absence of free-floating adventitious roots and/or AM fungi, is compensated for by increased allocation to remaining pathway(s) for resource uptake, we predicted altered patterns of resource allocation among shoots and the remaining pathway(s) for resource uptake. Contrary to our predications, plants experiencing adventitious root removal and/or grown in the absence of AM fungi generally had greater biomass and total shoot height than controls. Similarly, while levels of AM colonization and subterranean root biomass displayed a treatment effect, the observed responses did not correspond with our predictions. This was also true for shoot : subterranean root dry weight ratios. Our results indicate that there is interaction among the 3 pathways for resource acquisition in L. salicaria and an effect on plant performance. The adaptive significance of these characteristics is unclear, highlighting the potential difficulties in extrapolating from terrestrial to aquatic plant species and among aquatic plant species with potentially different life history strategies.

Evidence of hybridization between Lythrum salicaria (purple loosestrife) and L. alatum (winged loosestrife) in North America.

BACKGROUND AND AIMS: Although Lythrum salicaria (purple loosestrife) was introduced to North America from Europe in the early 1800s, it did not become invasive until the 1930s. Whether hybridization with L. alatum (winged loosestrife) could have played a role in its ultimate spread was tested. METHODS: Six diagnostic morphological traits (flower number per axil, leaf placement, calyx pubescence, style type, plant height and leaf shape) were surveyed in 30 populations of Lythrum across eastern North America. Patterns of AFLP variation were also evaluated using five primer pairs in a 'global screen' of the same North American populations of L. salicaria and L. alatum described above, in L. salicaria from 11 European populations located in Germany, England, Ireland, Austria and Finland, and in six L. salicaria cultivars. KEY RESULTS: All of the North American L. salicaria populations had individuals with alternate leaf placement and 1-2 flowers per leaf axil, which have not been described in Eurasian L. salicaria but predominate in North American L. alatum. In addition, two L. salicaria populations were intermediate in height and leaf ratio between the typical L. salicaria and L. alatum populations in their native fields and when grown in a common greenhouse. In screens of variation patterns using 279 AFLPs, only two fragments were found that clearly supported introgression from L. alatum to L. salicaria. CONCLUSIONS: The evidence indicates that L. salicaria may have hybridized with L. alatum, but if so, only a small fraction of L. alatum genes have been retained in the genome of L. salicaria. This is unlikely to have led to a dramatic adaptive shift unless the introgression of a few key genes into L. salicaria stimulated a genomic reorganization. It is more likely that crossing among genotypes of L. salicaria from multiple introductions provided the necessary variability for new adaptations to arise.