Date of Award

5-31-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Biology - (Ph.D.)

Department

Federated Department of Biological Sciences

First Advisor

Daniel E. Bunker

Second Advisor

Phillip Barden

Third Advisor

Jessica Lee Ware

Fourth Advisor

Claus Holzapfel

Fifth Advisor

Emily Josephs

Abstract

The aim of this study is to quantify the effects of natural selection in shaping Capsella bursa-pastoris populations along an urban-rural gradient in New York City.

A reciprocal transplant experiment with 168 lab-germinated C. bursa-pastoris seedlings from both urban and rural populations are grown in eight paired home and away sites distributed throughout the New York metropolitan area. Sites are visited approximately thirteen times to record plant fitness. There is evidence for local adaptation of urban populations: urban plants have longer reproductive durations and produce more seed pods in urban environments. These findings suggest that urban plants are better adapted to the stressful abiotic conditions found in urban areas.

Water stress laboratory trials test if urban populations are shaped by urban water stress regimes. The trials use 392 lab-germinated seedlings representing urban and rural populations from the New York metropolitan area, and include four water-stress treatments: drought, flood, cyclic drought and flood, and a well-watered control. Leaf traits from plants in the drought and control treatments are quantified to examine their role in water stress response. Both plant types appear unaffected by water stress, and demonstrate plasticity in leaf traits in response to drought. Leaf traits predict final plant size in the drought treatment but not in the control.

A salt stress trial tests if urban populations are shaped by urban soil salt stress. The trial includes 288 plants representing urban and rural populations from the New York metropolitan area. Plants are grown under different salt treatments (0, 20, 40, 50, 60, 100, and 150 mM NaCl) for five weeks. Both plant types demonstrate salt-sensitivity, having high rates of mortality at high salt concentrations. However, plants that survive high salt treatments are significantly larger than controls, indicating some individuals are salt tolerant. Leaf trait analysis demonstrates that different plastic responses occur in plants grown in salt stress compared to those grown in drought.

The reciprocal transplant experiment shows evidence of local adaptation in urban populations, whereas the laboratory trials find that the species is highly plastic in leaf trait responses to drought and salinity.

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