Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 24, 2000
Publication Date: July 1, 2001
Citation: Gustine, D.L., Sanderson, M.A. 2001. Molecular Analysis of White Clover Population Structure in Grazed Swards during Two Growing Seasons. Crop Science. 41:1143-1149. Interpretive Summary: Lack of persistence of white clover (Trifolium repens L.) populations in intensively grazed dairy pastures of the northeastern U.S. often reduces profits of dairy farms. White clover is the primary source of legume protein and nutrients for grazing dairy cattle, and is a critical component of their nutritional intake. Our goal is to increase sustainable profitability of U.S. dairy farms through identification of farming practices that will increase longevity of white clover populations. White clover populations persist for years in grazing lands primarily through clonal growth, yet retain high genetic variability. Changes in the clonal and genetic structure of white clover populations were studied on three grazed pastures in central Pennsylvania. Randomly amplified polymorphic DNA (RAPD) profiles, a procedure used in paternity tests, were determined monthly during two consecutive growing seasons for white clover plants in the pastures. We found that two mechanisms of white clover vegetative reproduction are at work to prevent a decrease in genetic variation of populations. The combination of continuously changing genotypes (due to intermittent stolon dormancy) and rare seedling establishment is crucial to retaining genotypic diversity in grazed populations of white clover. We conclude that genetic variability of white clover is dynamic at the local scale, which contributes to its long-term persistence in grazing lands.
Technical Abstract: White clover (Trifolium repens L.) populations persist for years in grazing lands primarily through clonal growth, yet retain high genetic variability. This study was conducted to determine how clone dynamics affected within population genetic variation of white clover during two growing seasons at three pasture sites. Up to 37 trifoliate samples were taken monthly by resampling specific points in four 1.2-x 1.2-m area quadrats from April to September in two years; randomly amplified polymorphic DNA (RAPD) profiles of 1137 and 946 sampled ramets in 1997 and 1998 were analyzed for each year. A mean of 30 and 10 clones (two or more cohorts) in 1987 and 1998, respectively, were produced on the three pastures per sampling date (P=0.01). The majority of clones were not detected more than once during each year. Significantly more clones were produced in 1997 than in 1998. The soil moisture content was not different between the two years (P = 0.17). The number of sampled cohorts per clone ranged from 2 to 8 over both years on the three pastures. Within population analysis of molecular variances (AMOVA) ranged from 36 to 79% and 52 to 80% in 1997 and 1998, respectively, indicating low to medium genetic diversity in the populations. The fraction of clonal sampled ramets relative to the total number of sampled ramets ranged from 0.03 to 0.78 in 1997 and 0.04 to 0.33 in 1998. Higher numbers of clonal ramets appeared to reduce genetic diversity; however, this was offset by rapid turnover of clones. We conclude that genetic variability of white clover is dynamic at the local scale, which contributes to its long-term persistence in grazing lands.