Practical considerations for maintaining germplasm in maize.

The main goals of genetic resource management are to acquire, maintain, distribute, characterize, regenerate, preserve, evaluate, and utilize the genetic diversity of crops and their wild relatives. The objectives of this study for ex-situ conservation of maize (Zea mays L.) are to review and describe: (1) practical regeneration methods that are based on population genetic theory; (2) practical problems encountered in choosing core subsets of a maize collection. Whenever possible, regeneration procedures should control the number of pollen parents (male gametes; through controlled hand pollination) and the number of female parent gametes (by harvesting equal numbers of kernels from each seed plant). When the number of pollen and seed parents are controlled during regeneration, the effective population size (Ne) is twice the size of the original population (N). Examples of practical methods for controlling the number of male and female parents are presented. The procedure involves random-paired plant crosses and taking equal numbers of seeds from each maize ear. To form a core subset, accessions of a maize race are subdivided through a stratified sampling procedure. Delineation of a core subset from a Tuxpeño racial collection is described as an example.

Statistical genetic considerations for maintaining germ plasm collections.

One objective of the regeneration of genetic populations is to maintain at least one copy of each allele present in the original population. Genetic diversity within populations depends on the number and frequency of alleles across all loci. The objectives of this study on outbreeding crops are: (1) to use probability models to determine optimal sample sizes for the regeneration for a number of alleles at independent loci; and (2) to examine theoretical considerations in choosing core subsets of a collection. If we assume that k-1 alleles occur at an identical low frequency of p0 and that the k(th) allele occurs at a frequency of 1-[(k-1)p0], for loci with two, three, or four alleles, each with a p0 of 0.05, 89-110 additional individuals are required if at least one allele at each of 10 loci is to be retained with a 90% probability; if 100 loci are involved, 134-155 individuals are required. For two, three, or four alleles, when p0 is 0.03 at each of 10 loci, the sample size required to include at least one of the alleles from each class in each locus is 150-186 individuals; if 100 loci are involved, 75 additional individuals are required. Sample sizes of 160-210 plants are required to capture alleles at frequencies of 0.05 or higher in each of 150 loci, with a 90-95% probability. For rare alleles widespread throughout the collection, most alleles with frequencies of 0.03 and 0.05 per locus will be included in a core subset of 25-100 accessions.

Developing hybrids from two improved maize populations.

The performance of a group of 49 hybrids from seven elite maize (Zea mays L.) lines selected from each of the two populations, BSK(S)C5 and BSSS(R)C5, was compared with the varietal-cross hybrid of the source populations and with elite single-cross hybrids. Both populations had been improved by five cycles of recurrent selection. BSK(S)C5 was improved by S 1 selection from BSK; whereas BSSS(R)C5 was improved by reciprocal recurrent selection from BSSS with BSCB1 as the tester.The mean yield of all hybrids was statistically higher than the mean yield of the varietal-cross hybrid and approached the yields of the checks, indicating that early testing in phase 1 was effective. Nine hybrids were significantly higher yielding than the varietal cross, and their yields were similar to those of the checks. The best hybrid outyielded the varietal cross by 18%.The data obtained from this study indicate that improved populations developed from recurrent selection programs should be useful sources for improved conventional hybrids when the varietal cross yields at least 90% as much as the better current hybrids.