Differences in pollination success between local and foreign flower color phenotypes: a translocation experiment with Gentiana lutea (Gentianaceae).

BACKGROUND: The adaptive maintenance of flower color variation is frequently attributed to pollinators partly because they preferentially visit certain flower phenotypes. We tested whether Gentiana lutea-which shows a flower color variation (from orange to yellow) in the Cantabrian Mountains range (north of Spain)-is locally adapted to the pollinator community. METHODS: We transplanted orange-flowering individuals to a population with yellow-flowering individuals and vice versa, in order to assess whether there is a pollination advantage in the local morph by comparing its visitation rate with the foreign morph. RESULTS: Our reciprocal transplant experiment did not show clear local morph advantage in overall visitation rate: local orange flowers received more visits than foreign yellow flowers in the orange population, while both local and foreign flowers received the same visits in the yellow population; thus, there is no evidence of local adaptation in Gentiana lutea to the pollinator assemblage. However, some floral visitor groups (such as Bombus pratorum, B. soroensis ancaricus and B. lapidarius decipiens) consistently preferred the local morph to the foreign morph whereas others (such as Bombus terrestris) consistently preferred the foreign morph. DISCUSSION: We concluded that there is no evidence of local adaptation to the pollinator community in each of the two G. lutea populations studied. The consequences for local adaptation to pollinator on G. lutea flower color would depend on the variation along the Cantabrian Mountains range in morph frequency and pollinator community composition.

Red Anthocyanins and Yellow Carotenoids Form the Color of Orange-Flower Gentian (Gentiana lutea L. var. aurantiaca).

Flower color is an important characteristic that determines the commercial value of ornamental plants. Gentian flowers occur in a limited range of colors because this species is not widely cultivated as a cut flower. Gentiana lutea L. var. aurantiaca (abbr, aurantiaca) is characterized by its orange flowers, but the specific pigments responsible for this coloration are unknown. We therefore investigated the carotenoid and flavonoid composition of petals during flower development in the orange-flowered gentian variety of aurantiaca and the yellow-flowered variety of G. lutea L. var. lutea (abbr, lutea). We observed minor varietal differences in the concentration of carotenoids at the early and final stages, but only aurantiaca petals accumulated pelargonidin glycosides, whereas these compounds were not found in lutea petals. We cloned and sequenced the anthocyanin biosynthetic gene fragments from petals, and analyzed the expression of these genes in the petals of both varieties to determine the molecular mechanisms responsible for the differences in petal color. Comparisons of deduced amino acid sequences encoded by the isolated anthocyanin cDNA fragments indicated that chalcone synthase (CHS), chalcone isomerase (CHI), anthocyanidin synthase 1 (ANS1) and ANS2 are identical in both aurantiaca and lutea varieties whereas minor amino acid differences of the deduced flavonone 3-hydroxylase (F3H) and dihydroflavonol 4-reductase (DFR) between both varieties were observed. The aurantiaca petals expressed substantially higher levels of transcripts representing CHS, F3H, DFR, ANS and UDP-glucose:flavonoid-3-O-glucosyltransferase genes, compared to lutea petals. Pelargonidin glycoside synthesis in aurantiaca petals therefore appears to reflect the higher steady-state levels of pelargonidin synthesis transcripts. Moreover, possible changes in the substrate specificity of DFR enzymes may represent additional mechanisms for producing red pelargonidin glycosides in petals of aurantiaca. Our report describing the exclusive accumulation of pelargonidin glycosides in aurantiaca petals may facilitate the modification of gentian flower color by the production of red anthocyanins.

Flower color preferences of insects and livestock: effects on Gentiana lutea reproductive success.

Angiosperms diversification was primarily driven by pollinator agents, but non-pollinator agents also promoted floral evolution. Gentiana lutea shows pollinator driven flower color variation in NW Spain. We test whether insect herbivores and livestock, which frequently feed in G.lutea, play a role in G. lutea flower color variation, by answering the following questions: (i) Do insect herbivores and grazing livestock show flower color preferences when feeding on G. lutea? (ii) Do mutualists (pollinators) and antagonists (seed predators, insect herbivores and livestock) jointly affect G. lutea reproductive success? Insect herbivores fed more often on yellow flowering individuals but they did not affect seed production, whereas livestock affected seed production but did not show clear color preferences. Our data indicate that flower color variation of G. lutea is not affected by insect herbivores or grazing livestock.

Is there a hybridization barrier between Gentiana lutea color morphs?

In Gentiana lutea two varieties are described: G. lutea var. aurantiaca with orange corolla colors and G. lutea var. lutea with yellow corolla colors. Both color varieties co-occur in NW Spain, and pollinators select flower color in this species. It is not known whether a hybridization barrier exists between these G. lutea color varieties. We aim to test the compatibility between flower color varieties in G. lutea and its dependence on pollen vectors. Within a sympatric population containing both flower color morphs, we analyzed differences in reproductive success (number, weight, viability and germinability of seeds) depending on fertilization treatments (autogamy and xenogamy within variety and among varieties). We found a 93% reduction in number of seeds and a 37% reduction in seed weight respectively of autogamy treatments compared to xenogamy crossings. Additionally, reproductive success is higher within color varieties than among varieties, due to a 45% seed viability reduction on hybrids from different varieties. Our results show that G. lutea reproductive success is strongly dependent on pollinators and that a partial hybridization barrier exists between G. lutea varieties.

Selective Pressures Explain Differences in Flower Color among Gentiana lutea Populations.

Flower color variation among plant populations might reflect adaptation to local conditions such as the interacting animal community. In the northwest Iberian Peninsula, flower color of Gentiana lutea varies longitudinally among populations, ranging from orange to yellow. We explored whether flower color is locally adapted and the role of pollinators and seed predators as agents of selection by analyzing the influence of flower color on (i) pollinator visitation rate and (ii) escape from seed predation and (iii) by testing whether differences in pollinator communities correlate with flower color variation across populations. Finally, (iv) we investigated whether variation in selective pressures explains flower color variation among 12 G. lutea populations. Flower color influenced pollinator visits and differences in flower color among populations were related to variation in pollinator communities. Selective pressures on flower color vary among populations and explain part of flower color differences among populations of G. lutea. We conclude that flower color in G. lutea is locally adapted and that pollinators play a role in this adaptation.

Selective pressure along a latitudinal gradient affects subindividual variation in plants.

Individual plants produce repeated structures such as leaves, flowers or fruits, which, although belonging to the same genotype, are not phenotypically identical. Such subindividual variation reflects the potential of individual genotypes to vary with micro-environmental conditions. Furthermore, variation in organ traits imposes costs to foraging animals such as time, energy and increased predation risk. Therefore, animals that interact with plants may respond to this variation and affect plant fitness. Thus, phenotypic variation within an individual plant could be, in part, an adaptive trait. Here we investigated this idea and we found that subindividual variation of fruit size of Crataegus monogyna, in different populations throughout the latitudinal gradient in Europe, was explained at some extent by the selective pressures exerted by seed-dispersing birds. These findings support the hypothesis that within-individual variation in plants is an adaptive trait selected by interacting animals which may have important implications for plant evolution.