Flower size-number trade-offs and the evolution of floral display in animal-pollinated plants

grantor: University of TorontoThe variety of flowers in animal-pollinated plants suggests a history of strong and diverse selection. One neglected aspect of this diversity is 'floral display', the size, number, and arrangement of flowers. The evolution of floral display should reflect pollinator-mediated selection and life-history trade-offs imposed by finite resources. Although flower size is expected to vary negatively with flower number, variable resources could cause positive relations between these traits. Such positive relations may reflect the hierarchical nature of resource allocation, with resources first divided between somatic growth and reproduction, and then further subdivision of reproductive resources. I investigated the evolutionary significance of trade-offs between flower size and number using theoretical, experimental, and comparative approaches. A quantitative-genetic model involving hierarchical allocation indicated that high genetic variation in early allocation masked trade-offs, and altered the direction of evolution in the traits involved. Relations between flower size and number at the individual and population-level were examined in the annual herb, 'Eichhornia paniculata'. The size and number of flowers within inflorescences decreased rapidly in response to defoliation and seed production, emphasizing the dynamic nature of floral allocation. Quantitative-genetic analyses of two glasshouse-grown populations demonstrated heritable variation in flower size and number. Genetic correlations between flower size and number ranged from negative to positive, depending on population and developmental stage. Accounting for variation in resource status removed positive correlations, but did not reveal additional negative correlations. Two generations of artificial selection on flower size and number partially upheld the expectation that size should increase at the expense of number. Finally, comparative data from 43 species in the perennial genus, ' Narcissus', supported flower size-number trade-offs. However, in ' N. dubius' these traits were positively related, and unrelated when variation in resource status was accounted for. These results illustrated the influence of variable resource levels on traits involved in trade-offs. Flower size and number were negatively correlated when variation in floral traits was high, e.g., among species. However, variable floral resources often caused positive relations between these traits. Thus trade-offs between flower size and number will not always constrain the evolution of floral display, as often assumed in theoretical models.Ph.D