Genetic Diversity in Aquatic Plants
Elly P.H.Best, PhD
U. S. Army Engineer Research and Development Center, Environmental Laboratory
Large-scale Declines and Invasions of Aquatic Vegetation
Native plants can provide water quality and habitat benefits while preventing or delaying the spread of weedy exotic species. Many native aquatic plant populations are in decline largely due to man-made activities resulting in changed water table regimes, increased erosion, decreased water transparency, and increased levels of nutrients and xenobiotics in water and sediments. Millions of acres of non-vegetated littoral environments are currently available as a consequence of large-scale losses of native vegetation, but also newly created by construction of reservoirs and waterways. These nonvegetated areas will eventually support the growth of aquatic vegetation, with a likelihood of invasion by exotic weeds in areas where local native plant propagules are missing or are otherwise inhibited from germination and establishment.
Restoration and Reestablishment of Desired Aquatic Vegetation
To maximize the benefits of restoration and to safeguard water bodies from invasion by weedy species, approaches are needed to establish native plants. Practical methods for large-scale establishment of submersed plants have been under development since 1997, in the Aquatic Plant Control Research Program (APCRP). Important factors determining establishment success on a short term are herbivory, water level fluctuations, light availability, and substrate quality. It is not yet clear which factors are important on a longer term, but a likely candidate is functional-group dependent, critical, level of genetic diversity of the species- or plant.
The Tentative Importance of Genetic Diversity for Plant Population Persistence
A recurring debate in evolutionary biology concerns the extent to which either historical processes, such as the retreat of glaciers during the last ice age and successive recolonization, or contemporary processes, such as gene flow and ecolopical differentiation, have contributed to present-day geographic population structure (Avise 1994). Several theoretical and empirical studies have investigated distribution patterns with regard to the impact of glaciation periods (Hewitt 1996), ecological differentiation and dispersal (Peck, Yearsley, and Waxman 1998) or gene flow and population structure (Wright 1943). Empirical studies on a variety of organisms across Europe have found latitudinal clines of genetic diversity, showing greater genetic variation in the southern than in the northern species range (Hewitt 1996), as well as lower levels of sexual reproduction in southern areas.
Genetic diversity within a plant species and the genetic structure of plant populations probably depend on life history and ecological factors. Genetic diversity on the population level is usually higher in terrestrial than in aquatic plants. Genetic divergence between populations tends to be higher in clonal than in nonclonal species, with the level of within-population genetic variation of the former depending on whether populations consist of many - or few genets. Genetic divergence between populations tends to be higher in emergent than in submersed aquatic plants. In the clonal emergent reed and cordgrass, a relatively high genetic variability can be found in areas impacted by frequent and severe water level fluctuations. In the predominantly clonal submersed American wildcelery and zostera, a relatively low population variability was found. However, in the latter species genetic variability was increased in populations growing in an intertidal area impacted by frequent and severe water level fluctuations compared to that in continuously inundated populations (Ruckelshaus 1998).
Molecular Techniques for Analysis of Genetic Diversity in Plants
DNA fingerprinting techniques can be used to determine genetic diversity in plants. These techniques, developed in the last ten years, have recently been applied to plant, fish, and insect populations. One method, amplified fragment length polymorphisms (AFLP), is being used to rapidly generate hundreds of genetic markers from a wide variety of plants. Such a large number of markers permits detection of differences at or below the subspecies level. AFLP involves isolation of DNA from less than 2 grams of tissue. DNA is cut into discreet pieces using two restriction enzymes. These pieces are then selectively amplified to generate a DNA fingerprint. Data obtained are used to determine biogeographic and clonal distribution, relatedness within and between populations, and to detect effects of management practices and pollutants on genetic diversity.
For example, AFLP was used to determine genetic diversity within and between cordgrass (Spartina) species from contemporary wetlands in the eastern United States. Over 500 genetic markers were detected. Markers of S. alterniflora generally clustered by site, and genetic diversity differed considerably from that of unrelated plants such as soybeans (see figure; 96.4KB). Genetic diversity tended to be higher in native than in constructed wetlands (Dog River, Naval Site, Sea Marsh, and Peck's Shore; see figure; 96.4KB). Genetic diversity was higher in S. alterniflora than in S. patens and S. spartina (unpublished data, 1999). Differences in genetic diversity are currently being related to historical and ecological causes and consequences.
Outlook
A high genetic diversity has proven to be important for long-term survival of animal populations. However, similar evidence for plant populations is lacking. Historical and contemporary plant materials need to be analyzed for population persistence and genetic diversity in order to evaluate the possibility of similar trends. The genetic data generated from contemporary aquatic vegetation will allow needed interpretation of relationships among genetic diversity, population persistence, and resilience to stressors.
Cooperation with other projects on genetic diversity of aquatic plants is actively sought.
References
POC: Dr. Elly P. H. Best
Webdate: September 1999