Geographical differentiation in floral traits along the distribution range of the Patagonian oil-secreting Calceolaria polyrhiza: do pollinators matter?

COSACOV, A.; COCUCCI A. A.; SÉRSIC, A. N.

ANNALS OF BOTANY

OXFORD UNIV PRESS

Lugar: Oxford; Año: 2013 vol. 113 p. 251 - 266

0305-7364

Background and Aims: The underlying evolutionary processes of pollinator-driven floral iversification are still poorly understood. According to the Grant-Stebbins model speciation begins with adaptive local differentiation in response to spatial heterogeneity in pollinators. Although this crucial process links the micro- and macroevolution of floral adaptation, it has received little attention. We studied geographical phenotypic variation of the Patagonian Calceolaria polyrhiza and its pollinators, two oil-collecting bee species that differ in body size and geographical distribution. Methods: We examined patterns of phenotypic variation and its relationship with pollinators and abiotic factors. Six floral and seven vegetative traits were measured in 45 populations distributed across the entire species range. We selected 25 sites and retrieved their climatic and edaphic parameters; we captured 2-16 bees/site of the most frequent pollinator species and measured a critical flower-bee fitting trait involved in effective pollination. Geographical patterns of phenotypic and environmental variation were examined using uni- and multivariate analyses. We explored decoupled geographic variation between corolla area and fit-related floral traits using Mantel test. Key Results: Body length of pollinators and floral fit-related trait were strongly correlated with each other. Geographic variation of mechanical fit-related traits was decoupled from variation in corolla size. The latter floral trait had a geographical pattern consistent with that of vegetative traits and was mainly affected by climatic gradients. Conclusions: The results are consistent with pollinators playing a key role in shaping floral phenotype at a geographical scale, promoting the differentiation of two floral ecotypes. The relationship between the critical floral fit-related trait and bee length remained significant even in models that included various environmental variables and an allometric predictor (corolla area). The abiotic environment also has an important role, mainly affecting floral size. Decoupled geographical variation between floral fit-related traits and floral size would represent a strategy to maintain plant-pollinator phenotypic matching in this environmentally heterogeneous area.