New scheme of haploid recurrent selection in maize --Chalyk, ST, Rotarenco, VA, Bylich, VG In a previous publication, we suggested utilization of haploid plants in recurrent selection (Chalyk and Rotarenco, MNL 1999). Subsequent testing of synthetic populations has shown high efficiency of haploid sib recurrent selection (HSRS) (Chalyk and Rotarenco, Rus J Genet 2001).

In the scheme we used, selection was only carried out for haploid plants. We presume this allowed an increased frequency of genes with nonallelic interactions.

In order to create high-yield maize hybrids, it is necessary to select genotypes with all types of gene interactions contributing to the yield formation. Genes with allelic interactions should be combined with nonallelic ones. To improve synthetic populations more efficiently and to increase frequency of genes with both allelic and nonallelic interactions, a new scheme of haploid recurrent selection, haploid & diploid recurrent selection (H&DRS), is used in the Institute of Genetics, Moldova. In this scheme, selection of haploid plants is combined with selection of diploid plants. Diploid plants are selected based on their combining ability. The new scheme, H&DRS, is a two-step scheme, the same as HSRS.

For comparison purposes, below we present both schemes of recurrent selection � HSRS and H&DRS.

HSRS scheme:
Step 1: Haploids are obtained from a synthetic population.

Step 2: The haploids are pollinated with pollen taken from diploid plants of the improving synthetic population. Selection is carried out only for haploid plants.

The new scheme, H&DRS, differs from HSRS in that in Step 2 diploid plants are used twice. First, diploid plants serve as a source of pollen for haploids. Second, combining ability of diploid plants is tested. In order to test the combining ability, diploid plants from the improving synthetic population are self-pollinated and at the same time crossed with a tester. Hybrids obtained as a result of crossing are tested in Step 1 of the following cycle of selection.

H&DRS scheme: First Cycle of Selection
Step 1: Haploids are obtained from a synthetic population.

Step 2: a) The haploids are pollinated with pollen of diploid plants of the same synthetic population. The best haploids are selected.

b) Diploid plants, which serve as a source of pollen for the haploids, are self-pollinated and crossed with a tester. The tester is the maternal component of the cross.

Second Cycle of Selection
Step 1: a) New haploids are obtained from the population grown from seeds of the haploid plants selected in the previous cycle of selection.

b) Hybrids are tested, which were obtained in the previous cycle of selection.

Step 2: a) Haploids, obtained in Step 1, are pollinated with a mixture of pollen taken from progeny of the diploid plants, which displayed high combining ability. The best haploids are selected.

b) Diploid plants, which are a source of pollen for haploids, are self- pollinated and crossed with a tester in order to assess their combining ability.

Subsequent selection is performed the same as the second cycle of selection. The advantage of the H&DRS scheme over conventional recurrent selection schemes is that it allows the intensive selection of genotypes with additive and epistatic effects at the haploid plants level. Genotypes with high effects of overdominance and dominance are selected at the diploid level based on testing of hybrids. When haploids are pollinated with pollen of diploid plants, genes with high effects of overdominance and dominance are added to those with high additive and epistatic effects. The combination of these types of gene interactions must ensure high efficiency of the synthetic population improvement.

Determination of the combining ability of diploid plants in the H&DRS scheme is quite common for a recurrent selection. Lines, synthetic populations or hybrids can be used as a tester. In our work, a reciprocal version of the H&DRS scheme is used. The work is conducted with two synthetic populations � SP and SA. When combining ability is tested, they serve as each other�s tester.

Diploid plants, which are selected for their combining ability and are used as a source of pollen for haploid plants, are assessed and selected based on many other traits. Before their pollen is used to pollinate haploids, they are selected based on their maturity group, resistance to lodging, resistance to diseases, etc.

When haploid plants are used, it is possible to obtain homozygous lines rapidly. During each cycle of selection, we treat a portion of haploid plants with colchicine in order to double their chromosome number. Thus, each cycle of the selection improves synthetic populations and at the same time produces homozygous lines by an accelerated method.

The authors believe that both HSRS and H&DRS schemes will be useful for maize breeders.
 
 

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