Abstract
Floral traits and the relative contribution of autonomous selfing to total seed set varies geographically and is often driven by the availability and abundance of suitable pollinators and/or the presence of co-flowering relatives. In the latter case, competition for pollinator services and costs of hybridization can select for floral traits that reduce interspecific gene flow and contribute to prezygotic isolation, potentially leading to geographic variation in floral divergence between allopatric and sympatric populations. In this study, we investigated variation in floral traits and its implications on the capacity of autonomous selfing in both allopatric and sympatric populations of two closely related Centaurium species(Gentianaceae) across two distinct geographic regions(UK and mainland Europe). Although the magnitude and direction of floral differentiation varied between regions, sympatric populations were always significantly more divergent in floral traits and the capacity to self autonomously than allopatric populations. These results indicate that mating systems can vary substantially within a species and that the joint occurrence of plant species can have a major impact on floral morphology and capacity of autonomous selfing, most likely as a way to reduce the probability of interspecific interference.
Highlights
Heterospecific mating and played an important contribution to total reproductive isolation among three closely related Centaurium species[24]
Region had by far the largest effect on overall morphology (Table 2), followed by species and their interaction, indicating that the two species differed significantly in floral morphology depending on the region where they were growing
Results for populations in the UK showed that petal length (F1,240 = 40.17; P < 0.001) and width (F1,240 = 49.80; P < 0.001) of the flowers were significantly larger in sympatric than in allopatric populations for C. erythraea populations, while C. littorale flowers showed a significant decrease from allopatric to sympatric flowers for petal width (F1,243 = 44.25; P < 0.001) (Fig. 3, Table S2)
Summary
Heterospecific mating and played an important contribution to total reproductive isolation among three closely related Centaurium species[24]. Due to the interplay of large-scale geographic variation in floral traits and small-scale variation resulting from species interactions in secondary contact zones, it can be predicted that a complex mosaic of floral traits and mating systems arises across large geographic areas To test this prediction, we studied variation in floral traits and the capacity of autonomous selfing in both sympatric and allopatric populations of Centaurium erythraea and C. littorale, and investigated whether the same patterns hold across two distinct geographic regions. Since heterospecific pollen transfer between C. erythraea and C. littorale and hybrid fertilization have been shown to be costly, in terms of both seed production and progeny fitness[33], we hypothesized that in order to minimize the risk of hybridization, sympatric populations will tend to show stronger differentiation in floral morphology and selfing capacity than allopatric populations To test this hypothesis, we compared floral traits and the capacity of autonomous selfing between allopatric and sympatric populations of both species. To investigate in more detail the extent to which differences in floral and mating system traits between allopatric and sympatric populations were affected by geographical location, we quantified geographic variation in floral traits and the contribution of autonomous selfing to total seed set in two contrasting regions, namely mainland Europe and the UK
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