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Variability Among Alfalfa Clones in Seed Production. I. Effective Population Size¹

The plant-to-plant variation in gamete production of alfalfa (Medicago sativa L.) plants was shown to increase the levels of inbreeding and probability of significant random change in gene frequencies of six alfalfa populations grown for seed at Reno, NV. The inbreeding effective population sizes [Ne_(i)] and variance effective population sizes [Ne_(v)] were estimated by assuming that gamete production was proportional to number of racemes on a plant. The Ne_(i) averaged 45% of N, the population size, and the Ne_(v) were 59 to 144% of N. Both effective population sizes were far below ideal values which occurs only with equal gametic contributions by each plant to the next generation. Three of the populations were in their 2nd year of seed production while the others were in their 1st year of production. Those transplants in 2nd year of production were much more productive than those in their 1st year but also had a proportionally greater plant-to-plant variation in gamete production. The result was nearly constant effective population sizes over years of production. The effect of limiting the number of seeds collected from each plant effectively reduced between plant variation in female gamete production but had no effect on variation in male gamete production. The increases in effective population sizes were 17 to 35% for Ne_(i) and 47 to 75% for Ne_(v). The maximum increases of effective population sizes occurred with seed collection limits at 60 to 80% of the mean plant seed yields for a population. The common practice of collecting the same small amount of seed from each plant appeared to be an unsatisfactory procedure for controlling inbreeding or random change in gene frequency.

Key Words: Medicago sativa L. • Genetic conservation • Genetic drift • Inbreeding

² Research geneticist, USDA-ARS, College of Agriculture, Rm. 239G, Univ. of Nevada, Reno, NV 89557.

Received for publication August 3, 1984.