Schemske&Bradshaw PNAS 1999

Schemske&Bradshaw PNAS 1999 - Pollinator preference and...

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Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus) Douglas W. Schemske* ² and H. D. Bradshaw, Jr. *Department of Botany and College of Forest Resources, University of Washington, Seattle, WA 98195 Edited by Barbara Anna Schaal, Washington University, St. Louis, MO, and approved August 11, 1999 (received for review June 10, 1999) A paradigm of evolutionary biology is that adaptation and repro- ductive isolation are caused by a nearly infinite number of muta- tions of individually small effect. Here, we test this hypothesis by investigating the genetic basis of pollinator discrimination in two closely related species of monkeyflowers that differ in their major pollinators. This system provides a unique opportunity to investi- gate the genetic architecture of adaptation and speciation because floral traits that confer pollinator specificity also contribute to premating reproductive isolation. We asked: ( i ) What floral traits cause pollinator discrimination among plant species? and ( ii ) What is the genetic basis of these traits? We examined these questions by using data obtained from a large-scale field experiment where genetic markers were employed to determine the genetic basis of pollinator visitation. Observations of F 2 hybrids produced by cross- ing bee-pollinated Mimulus lewisii with hummingbird-pollinated Mimulus cardinalis revealed that bees preferred large flowers low in anthocyanin and carotenoid pigments, whereas hummingbirds favored nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele that increases nectar production doubled hummingbird visitation. These results suggest that genes of large effect on pollinator preference have contributed to floral evolution and premating reproductive isolation in these monkeyflowers. This work contributes to growing evidence that adaptation and repro- ductive isolation may often involve major genes. reproductive isolation u adaptation u speciation u natural selection u pollination O ne of the principal goals of evolutionary biology is to discover the genetic architecture of adaptation. Fisher’s ‘‘infinitesimal’’ model of evolution proposes that adaptation is due to the fixation of many genes with small individual effects, and is based on the assumption that large-effect mutations move a population farther from, rather than closer to, its phenotypic optimum (1). This micromutationist view of ‘‘adaptive geome- try’’ (2) has had widespread support, but was challenged recently by a theory suggesting that mutations of large effect can often be beneficial during the early stages of adaptation as populations move toward their optimum phenotype (3). There have been too few empirical studies to resolve the debate, and it is therefore important to identify systems in which both the genetic basis and ecological significance of adaptive traits can be identified (4, 5).
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Schemske&Bradshaw PNAS 1999 - Pollinator preference and...

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