ADAPTIVE SIGNIFICANCE OF FLOWER COLOR AND INTER-TRAIT CORRELATIONS INAN IPOMOPSIS HYBRID ZONE

Flower color is often viewed as a trait that signals rewards to pollin ators, such that the relationship between flower color and plant fitne ss might result from its association with another trait. We used exper imental manipulations of flower color and nectar reward to dissociate the natural character correlations present in a hybrid zone between Ip omopsis aggregata and Ipomopsis tenuituba. Isozyme markers were used t o follow the male and female reproductive success of these engineered phenotypes. One field experiment compared fitnesses of I. aggregata pl ants that varied only in flower color. Plants with flowers painted red received more hummingbird visits and sired more seeds than did plants with flowers painted pink or white to match those of hybrids and I. t enuituba. Our second held experiment compared fitnesses of I. aggregat a, I. tenuituba, and hybrid plants in an unmanipulated array and in a second array where all flowers were painted red. In the unmanipulated array, I. aggregata received more hummingbird visits, set more seeds p er flower, and sired more seeds per flower. These fitness differences largely disappeared when the color differences were eliminated. The hi gher male fitness of I. aggregata was due to its very high success at siring seeds on conspecific recipients. On both I. tenuituba and hybri d recipients, hybrid plants sired the most seeds, despite showing lowe r pollen fertility than I. aggregata in mixed donor pollinations in th e greenhouse. Ipomopsis tenuituba had a fitness of only 13% relative t o I. aggregata when traits varied naturally, compared to a fitness of 36% for white relative to red flowers when other traits were held cons tant.