Effects of pollinator preference on flower color polymorphism in wild radish, Raphanus raphanistrum (Brassicaceae).

Abstract

Pollinator foraging behavior can be a major selective force on flower color polymorphisms in plant populations. Pollinators may prefer one color morph to another, increasing its relative reproductive success, or they may remain constant to either morph, effecting assortative mating. In wild radish, Raphanus raphanistrum, these behaviors affect not only the frequencies of flower color traits, but also the persistence of crop genes in wild populations, as white-flowered plants in predominantly yellow-flowered populations may indicate past hybridization with crop radish, R. sativus. A diversity of pollinators was observed in two artificial and two natural poulations in northern Michigan, and relative abundances and visitation rates were measured to estimate the relative importance of each taxon as it varied with time and site. Overall pollinator visitation was slightly but significantly biased toward the yellow-flowered morph in one of the two garden plots. Several taxa, including cabbage butterflies and syrphid flies, showed consistent preferences for yellow-flowered plants, although the preferences of most taxa were more variable. The majority also showed consistent but weak constancy toward the yellow-flowered morph. Site-, time- and taxon-specific preferences may thus combine to preserve low frequencies of white flowers in wild radish populations.