| Direct effects of flower color on reproductive success clearly play a major role in floral color evolution. However, evidence is accumulating that genes affecting floral pigmentation may have pleiotropic effects on fitness. This is not surprising as flavonoids, the major class of compounds determining floral color, are thought to have numerous functions. Effects of mutations in the flavonoid pathway are likely to differ between genes, since different genes will affect different biochemical products and/or have different expression patterns. Consistent with this expectation, wild populations of Ipomoea purpurea have two white-flowered morphs which occur at different frequencies. These morphs are due to categorically different mutations—a regulatory and a structural gene mutation.; I examine the effects of the structural gene mutant, the CHS-D locus, for direct and pleiotropic effects on plant fitness, and compare these effects to previously observed effects of the regulatory, or W-locus, mutation. In controlled experiments, I show that homozygous aa individuals exhibit reduced fertilization success, compared to AA individuals, at high temperatures but not at low temperatures. This demonstrates a previously unknown effect of the flavonoids, alleviation of heat stress on fertilization success.; Examination of selection acting on the a-locus mutant under field conditions shows that the mutation has a deleterious pleiotropic effect on survival which is sufficient to counteract the observed fitness advantage due to increased self-fertilization. Increased self-fertilization in the a-locus whites is not due to reduced pollinator visitation, but instead appears to be due to altered growth of floral filaments.; While selection experiments are able to show the current pattern of selection, they may not reflect the historical pattern of selection. I thus examine molecular sequence of the mutant for evidence of origin, age, and history. The pattern of molecular variation and estimates of allele age for the a allele do not support a hypothesis of balancing selection. Instead, they are consistent with a recent horticultural origin, not only of the mutant allele, but of the I. purpurea populations across the entire southeastern US. Estimates of allele and population age coincide with the earliest known records of the mutant in historical literature. |
