Abstract
To trace the fate of individual pollen grains through pollination processes, we determined genotypes of single pollen grains deposited on Hemerocallis stigmas in an experimental mixed-species array. Hemerocallis fulva, pollinated by butterflies, has diurnal, reddish and unscented flowers, and H. citrina, pollinated by hawkmoths, has nocturnal, yellowish and sweet scent flowers. We observed pollinator visits to an experimental array of 24 H. fulva and 12 F2 hybrids between the two species (H. fulva and H. citrina) and collected stigmas after every trip bout of swallowtail butterflies or hawkmoths. We then measured selection by swallowtail butterflies or hawkmoths through male and female components of pollination success as determined by single pollen genotyping. As expected, swallowtail butterflies imposed selection on reddish color and weak scent: the number of outcross pollen grains acquired is a quadratic function of flower color with the maximum at reddish color, and the combined pollination success was maximal at weak scent (almost unrecognizable for human). This explains why H. fulva, with reddish flowers and no recognizable scent, is mainly pollinated by swallowtail butterflies. However, we found no evidence of hawkmoths-mediated selection on flower color or scent. Our findings do not support a hypothesis that yellow flower color and strong scent intensity, the distinctive floral characteristics of H. citrina, having evolved in adaptations to hawkmoths. We suggest that the key trait that triggers the evolution of nocturnal flowers is flowering time rather than flower color and scent.
Highlights
Plants exhibit a tremendous diversity of floral traits that are often highly differentiated among closely related species
It is widely believed that flower color, floral scent and floral morphology have evolved under pollinator-mediated selection on those traits [10,11]
The number of seeds sired estimated by paternity analysis is considered as a more reliable estimate of male fitness, it is affected by pollination process, and by fertilization processes where available resources and pollen-pistil interaction are required for the fertilization success
Summary
Plants exhibit a tremendous diversity of floral traits that are often highly differentiated among closely related species Among these floral traits, flower color [1,2,3] and floral scent [4,5,6] function to attract particular pollinators, and floral morphology, such as corolla shape and anther-stigma distance, determines the efficiency of pollen transfer [7,8,9]. To measure pollinator-mediated selection on floral traits, it is desirable to estimate paternal success by excluding the influences of resource limitation and pollen-pistil interaction. To obtain such estimates, we employ microsatellite genotypes of single pollen grains [23] that allows us to determine the donor of each pollen grain deposited on stigmas
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