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REVIEW & INTERPRETATION

Measurements of Representativeness Used in Genetic Resources Conservation and Plant Breeding

^a Dep. of Genetics, ESALQ/Universidade de Sao Paulo, Cx. P. 83, 13400-970, Piracicaba, Sao Paulo, Brazil
^b Biometrics and Statistics Unit, International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, México DF, México

^* Corresponding author (j.crossa{at}cgiar.org).

This paper reviews some measurements of representativeness such as the effective population size (N_e) useful in genetic resources conservation and plant breeding research. Some easy-to-use expressions for estimating N_e in a number of practical conditions are given. For the case of several subpopulations, N_e is scarcely influenced by the total inbreeding. Thus, N_e becomes mostly dependent on the allelic diversity among subpopulations and the number of subpopulations sampled. When, under natural conditions, levels of interpopulation allelic divergence are low, it is misleading to believe that a small number of subpopulations will be sufficient to attain adequate effective population sizes. When a single population is considered and seeds being sampled have a family structure, at the limit N_e depends only on the number of seed parents and the coancestry coefficient among sibs within families. Accession regeneration is the case where the reference population is of finite size. Gametic control is a major factor in regeneration. The loss of up to 20% of seeds may be recovered in terms of N_e when female gametic control is applied. This is not attainable with random sampling of seeds. When studying N_e in recurrent selection schemes, results showed that the gain in N_e through gametic control is very small when selection is intensive. When comparing effective population sizes for the single seed descent (SSD) method versus the bulk system, results showed that SSD maintains genetic drift at a low level and offers a much better protection against random loss of alleles during selfing generations. Estimating population parameters, through codominant genetic markers is fundamental for obtaining reliable estimates of effective population size.