Population genetic diversity of the clonal self‐incompatible herbaceous plant Linaria vulgaris along an urbanization gradient

How increasing urbanization affects biodiversity is one of the most understudied aspects of global change biology. It is, however, known that it may negatively affect plant population genetic diversity in numerous ways, for example through its negative effects on plant population size, between‐population connectivity, and reproductive success. Therefore, it is important to investigate to what extent different levels of urbanization result in these negative phenomena. Here we used microsatellite markers to investigate urbanization effects on the population genetic structure of 23 populations of the self‐incompatible, partially clonal herb Linaria vulgaris which were sampled across a gradient of urbanization. Clonal diversity as measured by the Pareto‐parameter varied between 1.11 and 2.97 and was negatively correlated to both the degree of urbanization and to population size. Urbanization and population size were not interrelated. The least clonally rich populations also experienced significantly reduced seed set. Irrespective of the degree of urbanization, L. vulgaris populations exhibited strong genetic differentiation (F_ST = 0.33) and there was no significant correlation between genetic and geographic distances, suggesting low gene flow among populations. In conclusion, we showed that urbanization negatively affected fitness of L. vulgaris populations through decreasing their clonal diversity and reproductive success, an effect that may be exacerbated by the low gene flow between populations. Although the effect was modest, the results could probably be extrapolated to bigger cities where it would be considerably more pronounced.     

Host-plant preference of Brachypterolus pulicarius, an inadvertently introduced biological control insect of toadflaxes

Brachypterolus pulicarius (L.) (Coleoptera: Kateridae) is an inadvertently introduced biological control agent that can reduce seed set in two North American invasive species, yellow (Linaria vulgaris P. Mill.) (Scrophulariaceae) and Dalmatian toadflax (Linaria genistifolia (L.) P. Mill. ssp. dalmatica). The beetles are more common on yellow toadflax than on Dalmatian toadflax. To understand their distribution on the two host plants, we investigated whether they prefer one host to the other and whether individuals aggregate toward conspecifics. In field and laboratory experiments where beetles were presented with a choice of both toadflax species, B. pulicarius sampled from both host plants preferred yellow toadflax. However, in the laboratory experiment, beetles collected from Dalmatian toadflax showed a weaker preference for yellow toadflax than beetles collected from yellow toadflax. In the field experiment, all beetle populations sampled showed similar preferences. When given a choice between yellow toadflax plants with and without trapped adult B. pulicarius, beetles preferred plants with conspecifics, suggesting aggregation toward beetle pheromones or host‐plant volatiles induced by beetle activity. These results do not support the current practice of redistributing North American B. pulicarius onto Dalmatian toadflax because of their preference for yellow toadflax.     

Carbon addition interacts with water availability to reduce invasive forb establishment in a semi-arid grassland

Increases in nitrogen (N) availability can favor fast-growing invasive species over slow-growing native species. One way to reduce N availability is to add labile carbon (C) to the soil, which can lead to microbial immobilization of plant available N. This method has been used, with widely varying degrees of success, to both study and control plant invasions. One reason that C addition might not work as expected is that N is not always the limiting resource for plant growth. For example, if plant growth is limited by water, changes in N availability might have little effect on invasion. Here I ask whether effects of C addition on N availability, resident plant biomass, and invasion depend on water availability in semi-arid mixedgrass prairie. Six invasive species were seeded into plots treated with a factorial combination of water (ambient or added) and N (+C, control or +N). Carbon addition reduced capture of mineral N by resin probes (by an average of 73%), and reduced biomass of resident species (from 336 g m⁻² to 203 g m⁻²), both with and without added water. In contrast, because there was little invasion in ambient-water plots, C addition reduced invasion only in added-water plots. Given added water, C addition reduced biomass of Centaurea diffusa by 95%, and prevented invasion by Gypsophila paniculata and Linaria dalmatica. Mechanisms by which C addition reduced invasion varied by species, with added C reducing the growth of individual C. diffusa plants, but reducing numbers of G. paniculata and L. dalmatica individuals.     

Corolla morphology influences diversification rates in bifid toadflaxes (Linaria sect. Versicolores)

Background and Aims

The role of flower specialization in plant speciation and evolution remains controversial. In this study the evolution of flower traits restricting access to pollinators was analysed in the bifid toadflaxes (Linaria sect. Versicolores), a monophyletic group of ∼30 species and subspecies with highly specialized corollas.

Methods

A time-calibrated phylogeny based on both nuclear and plastid DNA sequences was obtained using a coalescent-based method, and flower morphology was characterized by means of morphometric analyses. Directional trends in flower shape evolution and trait-dependent diversification rates were jointly analysed using recently developed methods, and morphological shifts were reconstructed along the phylogeny. Pollinator surveys were conducted for a representative sample of species.

Key Results

A restrictive character state (narrow corolla tube) was reconstructed in the most recent common ancestor of Linaria sect. Versicolores. After its early loss in the most species-rich clade, this character state has been convergently reacquired in multiple lineages of this clade in recent times, yet it seems to have exerted a negative influence on diversification rates. Comparative analyses and pollinator surveys suggest that the narrow- and broad-tubed flowers are evolutionary optima representing divergent strategies of pollen placement on nectar-feeding insects.

Conclusions

The results confirm that different forms of floral specialization can lead to dissimilar evolutionary success in terms of diversification. It is additionally suggested that opposing individual-level and species-level selection pressures may have driven the evolution of pollinator-restrictive traits in bifid toadflaxes.