- December 3, 2012
- 12:00 pm
Brandon Campitelli, Stinchcombe Lab
Host: John Stinchcombe
Leaf shape is remarkably variable among plants, and hence likely has major consequence for ecological function and fitness. My thesis addresses the ecological significance of clinal variation for a leaf shape polymorphism in Ipomoea hederacea (lobed leaves dominate the north, entire-shaped leaves restricted to the south), and investigates the role of adaptation and demography in shaping its evolutionary history in its eastern North American range. To evaluate the adaptive value of the cline, I surveyed leaf shape genotypes from many populations, and found a steep latitudinal leaf shape cline that was not reflected at neutral genetic markers. Furthermore, the leaf shape
locus was a genomic outlier, implicating divergent selection in generating or maintaining the cline.
I investigated the thermoregulatory and freezing tolerance properties of the leaf shape genotypes, and discovered that lobed leaves remain marginally warmer at night, and a 1html5-dom-document-internal-entity1-deg-endC decrease separated mildly damaged and severely frost damaged tissue, potentially suggesting that a critical ambient temperature could drive differential leaf shape damage. I further explored three additional hypothesized selective agents (insect herbivores, flowering phenology and growth), and showed that these putative agents impose selection on I. hederacea, but do not differentiate between leaf shapes. These studies highlighted the challenge of identifying selective agents, even for a
polymorphic trait with hypothesized selective mechanisms.
To understand the contribution of adaptation and demography in shaping I. hederacea’s evolutionary history, I sequenced several nuclear loci in 192 individuals sampled from 24 populations and characterized patterns of nucleotide diversity. I demonstrated that I. hederacea is genetically structured in patches consistent with long-distance dispersal, is genetically depauperate, and is undergoing range expansion, suggesting a recent founder event or metapopulation dynamics. My thesis represents a comprehensive evaluation of the key processes affecting a polymorphism that influences plant morphology, geographical distribution, and population history.