VARIANCE EFFECTIVE POPULATION-SIZE FOR 2-STAGE SAMPLING OF MONOECIOUSSPECIES

During the process of collecting, storing, and regenerating germplasm, random changes in allele frequency due to sampling (genetic drift) oc curs and affect the genetic representativeness of the population. Drif t Is quantified and predicted through the variance effective populatio n size, N-e(nu). When sampling a population, drift may occur at two st ages: (i) when parents are sampled (selected) for reproduction and (ii ) when gametes (offspring) are subsequently sampled from these parents . Hence, N-e(nu) measures should include drift due to sampling of zygo tes, relative to the original population, and that due to gametic samp ling, relative to the set of parents. In this paper, the variance of t he number of contributed gametes, V-k and N-e(nu) were derived, under a two-stage sampling scheme for monoecious species. An alternative der ivation, solely based on the theory of random sampling within finite p opulations, is also given for comparison with the previous result. Pra ctical procedures involving full-sibs (FS), half-sibs (HS), and half-s ibs stemming from a restricted set of parents CHS') for decreasing V-k , or increasing N-e(nu), for field collection of germplasm and accessi on regeneration are described. For germplasm collection, results showe d the advantage of the HS scheme over a wide range of situations. In t he HS system, collectors should take, from a random set of plants, an equal number of seeds from as many seed parents as possible. When the number of seed parent plants is small compared with the total number o f plants, the FS system is always better than the HS' scheme. When the reference population is a germplasm accession, the FS is clearly supe rior to the HS and HS' alternatives. Under the proposed model, measure s of N-e(nu) require clear definition of (i) the reference population and the proportion of effective parents, (ii) the mating scheme, (iii) the number of parents and offspring sampled, and (iv) the level of fe male and male gametic control. The combination of these factors will d etermine the final N-e(nu) value.