Gene action and the bottleneck effect in relation to sample size for maintenance of cross-pollinated populations

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Germplasm conservation aims to preserve at least one copy of each allele in a population. The sample size for regeneration depends on the frequency and number of alleles at a locus. Modes of gene action and interaction (neutral, recessive, dominant, codominant, or overdominant) affect the distribution of allelic frequencies in a finite population. The objectives were to: (1) study the distribution of neutral and overdominant allelic frequencies in a finite population and the relationship of the distribution to the sample size and mutation rate in seed regeneration and (2) examine the effect of a founder event (bottleneck) on the allelic frequency and number of alleles present during regeneration of small collections. Finite populations contain, on the average, up to 11 neutral alleles at low frequency and from one to three neutral alleles at intermediate frequency. In general there is no more than one neutral allele at high frequency. For a high mutation rate (5 × 10 −3), samples of 100 to 250 individuals will have a large proportion of the neutral alleles at low frequency and smaller samples will contain a lesser degree of neutral polymorphism. For a mutation rate of 5 × 10 −4 collections of more than 250 individuals are required for a high degree of neutral polymorphism at a single locus. Overdominance is inefficient for maintaining a large number of alleles in a finite population. The usual sample for regeneration of 100–300 individuals would not maintain many overdominant alleles if the regenerations were affected by selection. If the original collection is small (less than 10 individuals), a bottleneck effect will result in the loss of some alleles present in the original population at low frequency. Therefore, the first priority is to insure appropriate sampling during collection. A sample of at least 100 individuals from the original population should be sufficient for this purpose. It is recommend that 150 to 250 individuals and crossing procedures that maximize effective population size be utilized for the regeneration of collections of cross pollinated species containing a high level of heterozygosity. A balanced bulk should be prepared for use in further regenerations, thus avoiding changes in allele frequency caused by selection and the effects of overdominant alleles on seed production. Mutation is not a significant factor in regenerations based on 150–250 plants.