Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


J. Emmett Duffy


Eelgrass (Zostera marina L.) is the northern hemisphere's dominant marine angiosperm, a species with both ecological and economic importance. Initial allozyme surveys of eelgrass populations in Chesapeake Bay (Virginia, USA) revealed substantial amounts of geographically-partitioned genetic variation, which could be the result of nonselective demographic processes, including founder events and drift. However, strong spatial variation in the environment and in eelgrass morphology suggests that differential adaptation of isolated beds to local environmental conditions could also produce these patterns. This dissertation used three sets of studies to investigate microevolutionary processes might produce the observed variation among Chesapeake eelgrass beds: (1) an allozyme survey of genetic diversity within and among twelve beds of different ages and sizes, (2) controlled breeding experiments to characterize the mating system of Z. marina and determine its susceptibility to inbreeding or outbreeding depression, and (3) reciprocal transplants to test for local adaptation within Zostera marina demes. Results showed considerable genetic diversity within beds and strong differentiation among beds but no relationship between genetic diversity and bed age or size, suggesting that founder events or clonal competition do not strongly depress genetic variation in this system. Artificial matings revealed no evidence of inbreeding depression in the 3 beds tested; seed production was significantly higher in selfed crosses than in either outbred or within-bed (inbred) crosses. Finally, reciprocal transplants showed some evidence of local adaptation in shoot density and seed production, but this was inconsistent in space and time. Phenotypic plasticity, perhaps bounded by genetic constraints, appeared to be the primary means by which Chesapeake eelgrass responded to local environmental variation. These studies support the emerging idea that eelgrass is not a panmictic obligate outbreeder, and they support important influences of non-selective processes (restricted gene flow and phenotypic plasticity) on the population structure of Chesapeake Bay eelgrass.



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