Abstract
BackgroundGeographic differences in floral size sometimes reflect geographic differences in pollinator size. However, we know little about whether this floral size specialization to the regional pollinator size occurred independently at many places or occurred once and then spread across the distribution range of the plant species.ResultsWe investigated the relationship between the local floral size of flowers and local pollinator size in 12 populations of Lamium album var. barbatum on two different mountains in the Japan Alps. Then, using 10 microsatellite markers, we analyzed genetic differentiation among the 12 populations. The results showed that local floral size was correlated with the average size of relevant morphological traits of the local pollinators: floral size was greater in populations visited frequently by the largest flower visitors, Bombus consobrinus queens, than it was in other populations. We also found that the degree of genetic similarity between populations more closely reflected interpopulation geographic proximity than interpopulation similarity in floral size.ConclusionsAlthough genetic similarity of populations was highly associated with geographic proximity, floral size varied independently of geographic proximity and was associated with local pollinator size. These results suggest that in L. album var. barbatum, large floral size evolved independently in populations on different mountains as a convergent adaptation to locally abundant large bumblebee species.
Highlights
Geographic differences in floral size sometimes reflect geographic differences in pollinator size
We investigated the relationship between floral size and pollinator size in 12 populations of L. album var. barbatum in two different mountain areas and confirmed plant–pollinator trait matching in these populations: plants in populations visited by long-tongued pollinators characteristically had long corolla tubes, whereas plants in populations visited by short-tongued pollinators had short corolla tubes
Geographic variation of floral size We found that floral size of L. album var. barbatum and the pollinator assemblage greatly differed among populations (Tukey’s honestly significant difference (HSD), P < 0.05; Table 1)
Summary
Geographic differences in floral size sometimes reflect geographic differences in pollinator size. Floral adaptation to pollinators is thought to be a key mechanism leading to the diversification of flower traits and. Local adaptation of plants to pollinators can lead to plant speciation through the establishment of prezygotic reproductive isolation, because specialization to specific pollinators may preclude pollinator sharing between related plant lineages [4, 22, 33]. According to the Grant–Stebbins model of floral divergence [8, 9, 11, 34], prezygotic reproductive isolation through pollinator-based selection is the main pathway of floral trait diversification. One useful approach to understanding trait diversification and speciation in angiosperms, is to combine an ecological evolutionary analysis of local plant adaptations with an analysis of population genetics to assess the degree of genetic isolation between populations. Knowledge of the patterns of morphological changes associated with intraspecific genetic structures can contribute to our understanding of the early stages of divergence [34]
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