Assessment of genetic diversity of seagrass populations using DNA fingerprinting: implications for population stability and management.

Abstract

Populations of the temperate seagrass, Zostera marina L. (eelgrass), often exist as discontinuous beds in estuaries, harbors, and bays where they can reproduce sexually or vegetatively through donal propagation. We examined the genetic structure of three geographically and morphologically distinct populations from central California (Elkhorn Slough, Tomales Bay, and Del Monte Beach), using multilocus restriction fragment length polymorphisms (DNA fingerprints Seagrasses form extensive meadows along the shores of all but the polar seas (1). Where they are found, these submerged marine angiosperms structure nearshore food webs and are highly productive (2,3). Seagrass systems worldwide serve as important and often critical habitats for a broad diversity of invertebrate and fish species, many of which are economically important (3, 4). In addition, seagrasses protect coastlines by minimizing erosion; increasing sedimentation, leading to enhanced recycling of nutrients; and improving water clarity (5).Zostera marina L., or eelgrass, is the dominant seagrass species in temperate waters and can achieve production rates exceeding 4 g of carbon m-2 day-l (2). Eelgrass reproduces both sexually and vegetatively and can colonize to depths of 30 m in clear waters but typically is restricted to shallow or intertidal depths in many estuaries (1-3, 6, 7). Although eelgrass is ecologically successful in very low-light environments (<100 ILmol of quanta m-2.s1l; refs. 6 and 7), the reduction in light penetration found in most industrialized coastal regions has severely restricted the depth distribution and abundance of eelgrass and other seagrass species (8-11).Losses worldwide of seagrass beds have accelerated at alarming rates in the last two decades because of physical disturbance (e.g., dredging, coastline development, fishing practices) and water quality deterioration most often realized as enhanced light attenuation by the water column because of particle loading, eutrophication, and nuisance algal blooms (8-11). Though some of the proximal causes for seagrass loss are increasingly evident (8)(9)(10)(11)(12), the importance of genetic diversity and gene flow for resource stability is unknown. The poor knowledge of the minimal habitat requirements for seagrass growth, colonization and establishment mechanisms, genetic diversity, and reproductive modes requisite for the maintenance of ecologically successful populations hinders the development of sound management criteria (see ref. 13).Previous investigations examining isozyme polymorphisms revealed essentially no genetic diversity within populations and a low level of genetic distinction between geographically disjunct populations of eelgrass (14,15). These findings, in conjunction with the known vigorous rhizomatous growth of this and other seagrass species, have led to the notion (3, 15) that the wide distribution and general ecological success of seagrasses are based upon a vegetative growth strategy. Consequently, a high degree of genetic similarity within pop...

Genetic uniformity in Amphibolis antarctica, a dioecious seagrass

“…Amphibolis antarctica appears to be genetically analysis, the M13 fingerprinting method may be expected to reveal differences between individuals (Vassart et a!., 1987;Rogstad et at., 1988;Rogstad, 1993;Alberte et at., 1994). However, maternal parent and offspring plant tissue as well as several different seedlings from other parental shoots were tested but gave no detectable differences.…”

“…Female-dominated sex ratios are often documented among apomicts and thus apomixis cannot be discounted among seagrasses, particularly where genetic variability is low (Richards, 1986;Les, 1988;Asker & Jerling, 1992). The few published studies of genetic variability in seagrass populations have led to conflicting generalizations about their genetic diversity and recruitment methods (McMillan, 1982(McMillan, , 1991Les, 1988;Fain et a!., 1992;Laushman, 1993;Alberte et a!., 1994;Waycott, 1995). Most of these studies have been conducted on monoecious and hermaphrodite seagrasses, principally the northern hemisphere species Zostera marina, using allozymes (Gagnon et a!., 1980;Laushman, 1993), RFLPs (Fain et a!., 1992) and DNA fingerprinting (Alberte et al, 1994).…”

Genetic variation within and between populations of Posidonia australis, a hydrophilous, clonal seagrass

“…Species such as Ceratophyllum echinatum are predominantly clonal with little variation (Les, 1991), whereas others such as Zostera marina have genetically variable populations comprising many individuals (Laushman, 1993). Several other studies on hydrophiles have investigated the fine-scale genetic structure of populations, documenting both multiclonal (Les, 1991;Laushman, 1993;Alberte et at., 1994;Lokker et at., 1994;Waycott, 1995) and apparently uniclonal (Les, 1991; Correspondence and present address: Department of Tropical Environment Studies and Geography, James loads (Orth & Moore, 1983;Cambridge & McComb, 1984;Walker & McComb, 1992), increasing attention has been devoted to understanding the ecology and physiology of these organisms.…”

Genetic structure of the seagrass Posidonia oceanica in the Western Mediterranean:ecological implications

“…Previous application of DNA fingerprinting techniques to the seagrasses Posidonia australis from Australia and Zostera marina from California revealed low population genetic similarity. Alberte et al (1994) showed, using minisatellite analysis, (1) low within ( S, = 0.44 to 0.68) and between (Sb = 0.47 to 0.51) population similarities in geographically disjunct populations of Z, marina, (2) Australia (Waycott 1995), and more recently, a short report indicated some polymorphism in P. oceanica along the Thyrrenean coast of Italy (Franconi et al 1995). From our analysis, the genetic structure of Posidonia oceanica in the Western Mediterranean appears to be quite different from that observed in these previous studies on seagrasses.…”