The evolution of flower-pollinator trait matching, and why do some alpine gingers appear to be mismatched?

BACKGROUND AND AIMS: Morphological matching between flower and pollinator traits has been documented in diverse plant lineages. Indeed, the matching of corolla tube length and pollinator tongue length has been cited repeatedly as a classic case of coevolution. However, there are many possible evolutionary routes to trait matching. Our aim here is both to review the evolutionary mechanisms of plant-pollinator trait matching and to investigate a specific case of trait matching/mismatching in a genus of alpine gingers. METHODS: Roscoea gingers with long corolla tubes in the western Himalayas have pollinators with correspondingly long tongues, but the match between corolla tube and pollinator tongue lengths is not seen in the eastern Himalayas. Six floral traits were measured, including corolla tube depth, an internal trait controlling pollinator access to nectar. We calculated coefficients of variation and phylogenetically controlled correlation patterns of these traits in six Roscoea species in order to gain possible insights into stabilizing selection and modularization of these traits. KEY RESULTS: The distal (nectar-containing) portion of the corolla tube exhibited lower coefficients of variations than did the basal portion. This is consistent with the hypothesis that pollinators mediate stabilizing selection on the distal, but not basal, portion of the corolla tube. This result, combined with phylogenetic data, suggests that the elevated liquid level of nectar in the distal tube evolved subsequent to dispersal into the eastern Himalayan region and loss of long-tongue pollinators. After accounting for phylogeny, corolla tube length, anther length, style length and labellum width were all intercorrelated. Corolla-tube depth was not part of this covariational module, however, suggesting separate adaptation to short-tongued pollinators. CONCLUSIONS: The reduction in functional corolla tube depth in the Roscoea appears to be related to the loss of long-tongued pollinators associated with dispersal to the eastern Himalayas and pollination by short-tongued pollinators. The apparent mismatch between floral tubes and pollinator tongues is a case of cryptic trait matching between flowers and pollinators, underscoring the importance of combining floral anatomy with pollination ecology in assessing plant-pollinator trait matching.

Modularity increases rate of floral evolution and adaptive success for functionally specialized pollination systems.

Angiosperm flowers have diversified in adaptation to pollinators, but are also shaped by developmental and genetic histories. The relative importance of these factors in structuring floral diversity remains unknown. We assess the effects of development, function and evolutionary history by testing competing hypotheses on floral modularity and shape evolution in Merianieae (Melastomataceae). Merianieae are characterized by different pollinator selection regimes and a developmental constraint: tubular anthers adapted to specialized buzz-pollination. Our analyses of tomography-based 3-dimensional flower models show that pollinators selected for functional modules across developmental units and that patterns of floral modularity changed during pollinator shifts. Further, we show that modularity was crucial for Merianieae to overcome the constraint of their tubular anthers through increased rates of evolution in other flower parts. We conclude that modularity may be key to the adaptive success of functionally specialized pollination systems by making flowers flexible (evolvable) for adaptation to changing selection regimes.

Heterostyly promotes compatible pollination in buckwheats: Comparisons of intraflower, intraplant, and interplant pollen flow in distylous and homostylous Fagopyrum.

PREMISE OF THE STUDY: Heterostyly, the reciprocal positioning of stigmas and anthers in different floral morphs, has long been thought to promote intermorph pollination. However, extensive intramorph pollination occurs commonly in heterostylous species, leading to recurrent questions about the functional and evolutionary significance of heterostyly. METHODS: To identify the sources of stigmatic pollen (autogamous [intraflower], geitonogamous [intraplant], vs. interplant), we emasculated either one flower or entire plants in experimental populations of the two closely related buckwheat species, distylous Fagopyrum esculentum and homostylous F. tataricum. Differences in pollen size allowed unambiguous identification of pollen on stigmas. RESULTS: Only 2.4% of F. tataricum pollen and 1.5% of F. esculentum pollen arrived successfully on compatible stigmas of other plants. In the former (homostylous) species, 71.3% of the pollen load on stigmas was autogamous, 10.8% was geitonogamous, and 17.9% was interplant. In the latter (distylous) species, 37.45% of the pollen on stigmas was autogamous, 13.8% was geitonogamous, 17.0% was intramorph, and 31.75% was intermorph. The amount of incompatible pollen arriving on stigmas was greatly decreased by both one-flower and whole-plant emasculations, and thus, the proportion of compatible pollen deposited increased with one-flower emasculation and increased even more with whole-plant emasculation. CONCLUSIONS: Our quantification of pollen-donor sources in these two species indicated that heterostyly in Fagopyrum esculentum provided a nearly 2-fold fitness advantage (in terms of compatible pollination) over expected (random) pollen transfers between morphs. Because of reduced herkogamy, the homostylous F. tataricum was highly autogamous.

Sistemas de polonização de duas espécies simpátricas de Dalechampia (Euphorbiaceae) no Amazonas, Brasil

Summary During September, 1978 observation were made near Cacau Pirêra, in the vicinity of Manaus, Amazonas, Brazil, on the pollination systems of two sympatric species of Dalechampia (Euphorbiaceae). The flowers of Dalechampia are unisexual; three pistillate and usually 9-10 staminate flowers are aggregated into a functionally bisexual inflorescence which is subtended by two large showy bracts. The two sympatric species which occur near Manaus, D. affinis and D. scandens differ from each other in time of day that the inflorescences are open for pollination, size of bracts, the distances from the flowers to the gland which secretes resin (the pollinator attractant), and the amount of resin secreted. D. affinis secretes a larger amount of resin than D. scandens, attracting Euglossa sp. and Hypanthidium nr. melanopterum. Because the distance between the gland and stigma is relatively great, only the larger bee, Euglossa, pollinates D. affinis. D. scandens secretes a smaller amount of resin and attracts only small bees, Hypanthidium nr. melanopterum, which do pollinate this small-flowered species. The attraction of different bees by such similar plant species is probably the result of the tendency for the larger bees i.e. Euglossa) to forage resin only from sources with large amounts of resin, "ignoring" inflorescences with small amounts of resin. Although both D. affinis and D. scandens are visited by Hypanthidium, the pollen flow between the two species is probably fairly low. Hypanthidium pollinates D. scandens early in the day before the inflorescences of D. affinis are open. It is too small a bee to transfer any pollen to the stigmas of D. affinis when inflorescences of that species are open in the late afternoon.

Resumo Dalechampia affinis (Euphorbiaceae) e D. scandens crescem simpaticamente nos arredores de Manaus, Amazonas, Brasil. Durante uma pesquisa, realizada em setembro de 1978, observou-se que as duas espécies diferiam uma da outra no tamanho da glândula floral que secreta resina (o atrativo do polinizador), na disposição das flores estaminadas e pistiladas na inflorescência, e no periodo do dia que as brácteas da inflorescência se abrem. Tais diferenças resultam de ser D. affinis polinizada principalmente por Euglossa (Apidae) e D. scandens principalmente por Hypanthidium (Megachilidae). Como uma conseqüência dessas diferenças há. provavelmente, pouco fluxo de pólen entre as duas espécies observadas.