Rapid loss of self-incompatibility in experimental populations of the perennial outcrossing plant Linaria cavanillesii.

Transitions from self-incompatibility to self-compatibility in angiosperms may be frequently driven by selection for reproductive assurance when mates or pollinators are rare, and are often succeeded by loss of inbreeding depression by purging. Here, we use experimental evolution to investigate the spread of self-compatibility from one such population of the perennial plant Linaria cavanillesii into self-incompatible (SI) populations that still have high inbreeding depression. We introduced self-compatible (SC) individuals at different frequencies into replicate experimental populations of L. cavanillesii that varied in access to pollinators. Our experiment revealed a rapid shift to self-compatibility in all replicates, driven by both greater seed set and greater outcross siring success of SC individuals. We discuss our results in the light of computer simulations that confirm the tendency of self-compatibility to spread into SI populations under the observed conditions. Our study illustrates the ease with which self-compatibility can spread among populations, a requisite for species-wide transitions from self-incompatibility to self-compatibility.

Evolution of nectar spur length in a clade of Linaria reflects changes in cell division rather than in cell expansion.

Background and Aims: Nectar spurs (tubular outgrowths of a floral organ which contain, or give the appearance of containing, nectar) are hypothesized to be a 'key innovation' which can lead to rapid speciation within a lineage, because they are involved in pollinator specificity. Despite the ecological importance of nectar spurs, relatively little is known about their development. We used a comparative approach to investigate variation in nectar spur length in a clade of eight Iberian toadflaxes. Methods: Spur growth was measured at the macroscopic level over time in all eight species, and growth rate and growth duration compared. Evolution of growth rate was reconstructed across the phylogeny. Within the clade we then focused on Linaria becerrae and Linaria clementei, a pair of sister species which have extremely long and short spurs, respectively. Characterization at a micromorphological level was performed across a range of key developmental stages to determine whether the difference in spur length is due to differential cell expansion or cell division. Key Results: We detected a significant difference in the evolved growth rates, while developmental timing of both the initiation and the end of spur growth remained similar. Cell number is three times higher in the long spurred L. becerrae compared with L. clementei, whereas cell length is only 1.3 times greater. In addition, overall anisotropy of mature cells is not significantly different between the two species. Conclusions: We found that changes in cell number and therefore in cell division largely explain evolution of spur length. This contrasts with previous studies in Aquilegia which have found that variation in nectar spur length is due to directed cell expansion (anisotropy) over variable time frames. Our study adds to knowledge about nectar spur development in a comparative context and indicates that different systems may have evolved nectar spurs using disparate mechanisms.

Maintenance of mixed mating after the loss of self-incompatibility in a long-lived perennial herb.

BACKGROUND AND AIMS: Many hermaphroditic plants avoid self-fertilization by means of a molecular self-incompatibility (SI) system, a complex trait that is difficult to evolve but relatively easy to lose. Loss of SI is a prerequisite for an evolutionary transition from obligate outcrossing to self-fertilization, which may bring about rapid changes in the genetic diversity and structure of populations. Loss of SI is also often followed by the evolution of a 'selfing syndrome', with plants having small flowers, little nectar and few pollen grains per ovule. Here, we document the loss of SI in the long-lived Spanish toadflax Linaria cavanillesii, which has led to mixed mating rather than a transition to a high rate of selfing and in which an outcrossing syndrome has been maintained. METHODS: We performed crosses within and among six populations of L. cavanillesii in the glasshouse, measured floral traits in a common-garden experiment, performed a pollen-limitation experiment in the field and conducted population genetic analyses using microsatellites markers. KEY RESULTS: Controlled crosses revealed variation in SI from fully SI through intermediate SI to fully self-compatible (SC). Flowers of SC individuals showed no evidence of a selfing syndrome. Although the SC population of L. cavanillesii had lower within-population genetic diversity than SI populations, as expected, population differentiation among all populations was extreme and represents an FST outlier in the distribution for both selfing and outcrossing species of flowering plants. CONCLUSIONS: Together, our results suggest that the transition to SC in L. cavanillesii has probably been very recent, and may have been aided by selection during or following a colonization bottleneck rather than in the absence of pollinators. We find little indication that the transition to SC has been driven by selection for reproductive assurance under conditions currently prevailing in natural populations.

Bees explain floral variation in a recent radiation of Linaria.

The role of pollinators in floral divergence has long attracted the attention of evolutionary biologists. Although abundant studies have reported the effect of pollinators on flower-shape variation and plant speciation, the influence of pollinators on plant species differentiation during rapid radiations and the specific consequences of shifts among similar pollinators are not well understood. Here, we evaluate the association between pollinators and floral morphology in a closely related and recently diversifying clade of Linaria species (sect. Supinae subsect. Supinae). Our approach combined pollinator observations, functional floral morphometric measures and phylogenetic comparative analyses. The fauna visiting Linaria species was determined by extensive surveys and categorized by a modularity algorithm, and the size and shape of flowers were analysed by means of standard and geometric morphometric measures. Standard measures failed to find relationships between the sizes of representative pollinators and flowers. However, discriminant function analyses of geometric morphometric data revealed that pollination niches are finer predictors of flower morphologies in Linaria if compared with phylogenetic relationships. Species with the most restrictive flowers displayed the most slender spurs and were pollinated by bees with larger proboscides. These restrictive flower shapes likely appeared more than once during the evolutionary history of the study group. We show that floral variation can be driven by shifts between pollinators that have been traditionally included in a single functional group, and discuss the consequences of such transitions for plant species differentiation during rapid radiations.

Invasive forb benefits from water savings by native plants and carbon fertilization under elevated CO2 and warming.

As global changes reorganize plant communities, invasive plants may benefit. We hypothesized that elevated CO2 and warming would strongly influence invasive species success in a semi-arid grassland, as a result of both direct and water-mediated indirect effects. To test this hypothesis, we transplanted the invasive forb Linaria dalmatica into mixed-grass prairie treated with free-air CO2 enrichment and infrared warming, and followed survival, growth, and reproduction over 4 yr. We also measured leaf gas exchange and carbon isotopic composition in L. dalmatica and the dominant native C3 grass Pascopyrum smithii. CO2 enrichment increased L. dalmatica biomass 13-fold, seed production 32-fold, and clonal expansion seven-fold, while warming had little effect on L. dalmatica biomass or reproduction. Elevated CO2 decreased stomatal conductance in P. smithii, contributing to higher soil water, but not in L. dalmatica. Elevated CO2 also strongly increased L. dalmatica photosynthesis (87% versus 23% in P. smithii), as a result of both enhanced carbon supply and increased soil water. More broadly, rapid growth and less conservative water use may allow invasive species to take advantage of both carbon fertilization and water savings under elevated CO2 . Water-limited ecosystems may therefore be particularly vulnerable to invasion as CO2 increases.

Congruence between distribution modelling and phylogeographical analyses reveals Quaternary survival of a toadflax species (Linaria elegans) in oceanic climate areas of a mountain ring range.

· The role of Quaternary climatic shifts in shaping the distribution of Linaria elegans, an Iberian annual plant, was investigated using species distribution modelling and molecular phylogeographical analyses. Three hypotheses are proposed to explain the Quaternary history of its mountain ring range. · The distribution of L. elegans was modelled using the maximum entropy method and projected to the last interglacial and to the last glacial maximum (LGM) using two different paleoclimatic models: the Community Climate System Model (CCSM) and the Model for Interdisciplinary Research on Climate (MIROC). Two nuclear and three plastid DNA regions were sequenced for 24 populations (119 individuals sampled). Bayesian phylogenetic, phylogeographical, dating and coalescent-based population genetic analyses were conducted. · Molecular analyses indicated the existence of northern and southern glacial refugia and supported two routes of post-glacial recolonization. These results were consistent with the LGM distribution as inferred under the CCSM paleoclimatic model (but not under the MIROC model). Isolation between two major refugia was dated back to the Riss or Mindel glaciations, > 100 kyr before present (bp). · The Atlantic distribution of inferred refugia suggests that the oceanic (buffered)-continental (harsh) gradient may have played a key and previously unrecognized role in determining Quaternary distribution shifts of Mediterranean plants.

Interaction frequency and per-interaction effects as predictors of total effects in plant-pollinator mutualisms: a case study with the self-incompatible herb Linaria lilacina.

It is widely recognized that pollinators vary in their effectiveness in pollination mutualisms, due both to differences in flower-pollinator morphological fit as well as pollinator behaviour. However, pollination webs typically treat all interactions as equal, and we contend that this method may provide misleading results. Using empirical and theoretical data, we present the case study of a self-incompatible herb in which the number of flowers visited by a pollinator cannot be used as a surrogate for the total effect of a pollinator on a plant due to differences in per-visit effectiveness at producing seeds. In self-incompatible species, the relationship between interaction frequency and per-interaction effect may become increasingly negative as more flowers per plant are visited due to geitonogamous pollen transfer. We found that pollinators making longer bouts (i.e. visiting more flowers per plant visit) had an overall higher pollination success per bout. However, per-interaction effects tended to decrease as the bout progressed, particularly for pollinators that cause higher pollen deposition. Since the same interaction frequency may result from different combinations of number of bouts (plant visits) and bout length (flowers visited/bout), pollinators making repeatedly shorter bouts may contribute more to plant reproduction for the same number of flowers visited. Consequently, the magnitude of the differences in number of interactions of different insect types may be overridden by the magnitude of the differences in effectiveness as pollinators, even if the same pollinators consistently interact more frequently. We discuss two predictions regarding the validity of using interaction frequency as a surrogate for plant seed production (as a measure of total effect), depending on the degree of self-compatibility, plant size and floral display. We suggest that the role of interaction frequency must be tested for different species, environments, and across wider scales to validate its use as a surrogate for total effect in plant-pollinator networks.

Assessing environmental risks for established invasive weeds: Dalmatian (Linaria dalmatica) and yellow (L. vulgaris) toadflax in North America.

Environmental risk assessments characterizing potential environmental impacts of exotic weeds are more abundant and comprehensive for potential or new invaders than for widespread and well-established species such as Dalmatian (Linaria dalmatica [L.] Mill.) and yellow (L. vulgaris Mill.) toadflax. Specific effects evaluated in our assessment of environmental risks posed by yellow and Dalmatian toadflax included competitive displacement of other plant species, reservoirs of plant disease, animal and insect use, animal toxicity, human toxicity and allergenicity, erosion, and wildfire. Effect and exposure uncertainties for potential impacts of toadflax on human and ecological receptors were rated. Using publicly available information we were able to characterize ecological and human health impacts associated with toadflax, and to identify specific data gaps contributing to a high uncertainty of risk. Evidence supporting perceived negative environmental impacts of invasive toadflax was scarce.

A comparison of antirrhinoside distribution in the organs of two related Plantaginaceae species with different reproductive strategies.

A study of two related plants (Antirrhinum majus L. and Linaria vulgaris Mill.) containing the same defensive compound (the iridoid glucoside, antirrhinoside) but with reproductive strategies that differ during ontogeny was undertaken. Young leaves are important to plants due to their higher photosynthetic rates and, therefore, should be better protected with higher concentrations of defensive compounds such as antirrhinoside. Declining concentrations of antirrhinoside as leaves aged was found for A. majus but this was generally not the case for L. vulgaris. Concentrations of antirrhinoside in root tissue were low and constant throughout ontogeny for A. majus whereas for L. vulgaris root levels of antirrhinoside were high during the period when vegetative growth is its sole means of reproduction. Antirrhinoside in L. vulgaris roots declined relative to A. majus roots during budding and flowering. During flowering, significantly less antirrhinoside and relative biomass are devoted to L. vulgaris flowers than in A. majus. While these findings are consistent with Optimal Defense Theory (ODT) further work on the distribution of antirrhinoside and the effect of insect herbivory on plant fitness in other related species is needed.

Corolla herbivory, pollination success and fruit predation in complex flowers: an experimental study with Linaria lilacina (Scrophulariaceae).

BACKGROUND AND AIMS: Herbivory on floral structures has been postulated to influence the evolution of floral traits in some plant species, and may also be an important factor influencing the occurrence and outcome of subsequent biotic interactions related to floral display. In particular, corolla herbivory may affect structures differentially involved in flower selection by pollinators and fruit predators (specifically, those ovopositing in ovaries prior to fruit development); hence floral herbivores may influence the relationships between these mutualistic and antagonistic agents. METHODS: The effects of corolla herbivory in Linaria lilacina (Scrophulariaceae), a plant species with complex flowers, were considered in relation to plant interactions with pollinators and fruit predators. Tests were made as to whether experimentally created differences in flower structure (resembling those occurring naturally) may translate into differences in reproductive output in terms of fruit or seed production. KEY RESULTS: Flowers with modified corollas, particularly those with lower lips removed, were less likely to be selected by pollinators than control flowers, and were less likely to be successfully visited and pollinated. As a consequence, fruit production was also less likely in these modified flowers. However, none of the experimental treatments affected the likelihood of visitation by fruit predators. CONCLUSIONS: Since floral herbivory may affect pollinator visitation rates and reduce seed production, differences among plants in the proportion of flowers affected by herbivory and in the intensity of the damage inflicted on affected flowers may result in different opportunities for reproduction for plants in different seasons.