| The ivyleaf morning glory, (Ipomoea hederacea) has a genetic polymorphism in leaf shape: some plants have lobed leaves, while others possess entire, unlobed leaves. Leaf shape variation of this magnitude may affect plant water relations and herbivore preference, and is thus likely to be experiencing selection. However, no studies have yet demonstrated that leaf morphs which are predicted to be favored in a particular environment are actually favored. I have investigated the action of natural selection on and the causative agents affecting this leaf shape polymorphism. I used three different approaches. As a first step, I identified the geographic pattern of leaf shape variation. Second, to determine if natural selection was responsible for this pattern, I measured the pattern of selection acting at three sites which spanned this geographic variation. Third, to determine the cause of this pattern, I identified the agents of selection.;First, using herbarium collections and population surveys, I determined that leaf shape variation in this species is spatially segregated: only lobed plants are found in the northern part of the species range, while in the south, both lobed and entire plants are found. Second, using perturbation field experiments, I measured the pattern of natural selection over an area that spanned this geographic variation. I found directional selection in a "lobed-only" site one year, and heterozygous advantage at the same site the next year. At two sites further south, I found heterozygous advantage. This pattern of selection is consistent with the geographic pattern, suggesting that natural selection is, in part, responsible for the observed pattern of geographic variation. Third, I measured environmental correlates to leaf shape variation in I. hederacea, to formalize hypotheses of the potential agents of selection. I then explicitly tested the hypotheses that water availability and white rust are agents of selection. Water availability was not found to be an agent of selection on this polymorphism in two experiments designed to explicitly test this hypothesis. However, the white rust fungus, Albugo ipomoeae-panduranae, was found to be an agent of selection. This study suggests that leaf shape has a previously unconsidered function: resistance to pathogens. |
