PLASTICITY AND EVOLUTIONARY POTENTIAL OF ALLIARIA PETIOLATA LIFE HISTORY AND LEAF CHEMISTRY TRAITS IN DIFFERENT COMPETITIVE ENVIRONMENTS
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Alliaria petiolata is an invasive biennial herb in North America where it has detrimental impacts on flora and fauna. Allelopathy, the chemical inhibition of plant competitors, is one explanation for why A. petiolata is more successful in North America than in Eurasia, where it is native. There is strong laboratory evidence that A. petiolata is allelopathic, and recent evidence has suggested that natural selection acts on allelopathic compounds (i.e., allelochemicals). If selection is acting on allelochemicals, allelopathy should influence survival and reproduction; however, there is not strong evidence of this in field settings. To shed light on this discrepancy, I measured phenotypic selection, plasticity and heritability of putative allelochemicals and life history traits to determine if there is selection on allelochemicals, if allelochemicals can respond to selection, and if selection is due to a direct effect of allelopathy, or due to selection acting on a correlated trait. Using families from 23 A. petiolata populations, I measured glucosinolate, flavonoid and chlorophyll A expression in A. petiolata leaf tissue to estimate selection on these traits in inter- and intraspecific competition and without competition. A separate experiment was also conducted to test the effect of soil nutrients on glucosinolate, flavonoid and chlorophyll A plasticity using 3 different A. petiolata families. I found that A. petiolata exhibited strong plastic responses to both fertilizer application and competition, but most traits exhibited very low heritability. I also found that while selection appeared to act on glucosinolate expression during interspecific competition, there is more evidence to suggest that glucosinolate expression was under indirect selection due to direct selection acting on the correlated trait, chlorophyll A in response to light limitation. There was genetic variation for glucosinolate expression, but it was mostly manifested in the ratio of glucosinolate: chlorophyll A expression, therefore, the heritability of glucosinolate expression is likely to vary among environments and to be higher in nutrient-rich environments. These results highlight the need to account for plasticity and heritability and measure other important traits to account for indirect selection when assessing evolutionary potential.