Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus)

A paradigm of evolutionary biology is that adaptation and reproductive isol ation are caused by a nearly infinite number of mutations of individually s mall effect. Here, we test this hypothesis by investigating the genetic bas is of pollinator discrimination in two closely related species of monkeyflo wers that differ in their major pollinators. This system provides a unique opportunity to investigate the genetic architecture of adaptation and speci ation because floral traits that confer pollinator specificity also contrib ute to premating reproductive isolation. We asked: (i) What floral traits c ause pollinator discrimination among plant species? and (ii) What is the ge netic basis of these traits? We examined these questions by using data obta ined from a large-scale field experiment where genetic markers were employe d to determine the genetic basis of pollinator visitation. Observations of F-2 hybrids produced by crossing bee-pollinated Mimulus lewisii with hummin gbird-pollinated Mimulus cardinalis revealed that bees preferred large flow ers low in anthocyanin and carotenoid pigments, whereas hummingbirds favore d nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele t hat increases nectar production doubled hummingbird visitation. These resul ts suggest that genes of large effect on pollinator preference have contrib uted to floral evolution and premating reproductive isolation in these monk eyflowers. This work contributes to growing evidence that adaptation and re productive isolation may often involve major genes.