Quantifying Spatial and Temporal Genotypic Changes in White Clover Populations by RAPD Technology

As a plant produces more stolons and stolon branches it expands in area and is made up of clonal members. In established white clover (Trifolium repens L.) populations, seedBecause of decay of older stolons and environmental ling recruitment is rare and plants spread by clonal growth, which could lead to low intraspecific genetic diversity. To assess how populadisturbance, a clone frequently fragments into smaller tions of white clover maintain high genetic variability in grazed swards clones. Therefore, physically separate, but genetically during a growing season, we examined spatial and temporal changes identical clonal plants may be found at the subhectare in white clover clones by identifying genotypes by RAPD (random (local) and hectare (field) scales. In theory, any clone amplified polymorphic DNA) profiles. Trifoliate leaf samples were could potentially become a dominant genotype, productaken from plants at up to 37 specific sampling points in each of four ing patches covering a large area within grassland (Cahn 2.8-m2 permanent quadrats in a grazed pasture from April to October and Harper, 1976; Harberd, 1963), and thus reduce the 1997. RAPD profiles (genotype) based on 28 markers were detergenetic variability within the white clover population. mined for genomic DNA prepared from leaf samples. The number Despite the potential for one clone to dominate a of sampled clones per quadrat ranged from zero to 10 and the sampled white clover population, the reported area of clonal members per clone ranged from two to 34 over the growing season. Some clones were found more than once. During the study, numbers patches in the field is from several centimeters to several of clonal members increased or decreased, clones were found on one meters in maximum width. Cahn and Harper (1976) did to three dates, and clones were found in one to two quadrats. The not find high genetic variation and local domination by clonal diversity index D (probability that any two sampled plants have one or more clones. The magnitude of genetic variation different genotypes) ranged from 0.89 to 0.99 for the eight dates, among phenotypically distinct clones sampled in a 50showing that genotypic diversity of populations was affected by numyr-old grassland in North Wales was comparable to that ber of clones and number of clonal members. We report here the expected between clonal “populations taken from disfirst use of dominant RAPD markers to document dynamic changes tinctly different environments” (Burdon, 1980). Gustine of white clover population structure. These data show that white and Huff (1999) also found high genetic variation within clover clones undergo dynamic spatial and temporal variability at a and among white clover populations at 18 locations in local scale, which accounts for much of the genetic diversity within field populations. three northeastern U.S. states using RAPD markers. Previously, Gustine and Huff (1999) found few white clover clones when they sampled plants at 10-m intervals and they found dramatic changes in genetic makeup of W clover is an important functional component populations within a period of 6 wk. Since white clover of temperate grazed ecosystems because of its N clones occur at much smaller scales, we wanted to invesfixing ability and its high nutritional quality as animal tigate clonal dynamics at a scale of 20 to 30 cm and feed. White clover, an obligately outcrossing tetraploid we wanted to know whether white clover populations species, flowers prolifically during the growing season change over much shorter time intervals of 2 to 4 wk. and produces significant amounts of seed that end up RAPD profiles have been used to characterize genetic in the viable seed pool (Chapman and Anderson, 1987; variability, clonal structure, and population structure in Charlton, 1977; Silvertown and Lovett Doust, 1993). In several plant species (Buso et al., 1998; Huff et al., 1998; spite of the high viable seed count in the soil (Tracy Palacios and Gonzales-Candelas, 1997; Sydes and Peaand Sanderson, 2000), few seeds germinate under field kall, 1998). Our objective was to quantify spatial and conditions, and few of those seedlings establish as plants temporal changes in genetic variation at the local scale (Barratt and Silander, 1992; Brink et al., 1999; Fothergill over a growing season in a rotationally stocked sward. et al., 1997; Grime et al., 1988). Chapman and Anderson (1987) also found that buried seed are an ineffective MATERIALS AND METHODS source of new white clover plants in grazed swards. White clover populations are maintained for many White clover populations used in this study formed part of decades in grazed swards at northern mid-latitudes by a managed permanent pasture, in the ridge and valley physiorare seedling recruitment (Chapman, 1983; Fothergill graphic region of central Pennsylvania (408389 N, 778389 W; 231 m asl). The grazed sward, on a Hagerstown silty clay loam et al., 1997; Grime et al., 1988), which results in a new (fine, mixed mesic Typic, Hapludalf; USDA, 1981), was not white clover plant, and by clonal growth. The life span treated with chemical fertilizers. The cumulative rainfall durof a tap rooted white clover plant is about 1 to 2 yr ing March through October was 711 mm (data from the Lew(Pederson, 1995). Plants spread by stoloniferous propaistown, PA, weather station, 9.7 km to the south), which was gation throughout the growing season (Chapman, 1983). 8 mm below normal. The average temperature for that time was 14.88C, 0.88C below normal (NOAA, 1997). The field USDA-ARS, Pasture Systems and Watershed Management Research capacity for the Hagerstown soil was 0.16 to 0.24 m m2 of Unit, Curtin Road, Building 3702, University Park, PA 16802-3702. Received 7 Jan. 2000. *Corresponding author ([email protected]). Abbreviations: PCR, polymerase chain reaction; RAPD, random amplified polymorphic DNA. Published in Crop Sci. 41:143–148 (2001).