Identification and use of major genes controlling quantitative and qualitative traits have always been of great importance in maize breeding. The employment of diploid plants for this purpose is often fraught with difficulties. This is due to the fact that in segregating populations most of the plants exhibit heterosis and are not comparable with homozygous parental lines. To overcome these difficulties, use of maternal haploids has been suggested (Chebotar and Chalyk, 1996). Employment of matroclinal haploids makes genetic analysis much easier and helps in identifying individual major genes controlling these traits.
The objective of the present study was to explore the possibilities of using matroclinal haploids for ascertaining the number of genes controlling plant height by which maize lines 092 and A619 are differentiated. To produce maternal haploids from lines 092 and A619 and from hybrids between these (092 x A619 and A619 x 092), a haploid-inducer line was used. The resulting haploids were examined in the field. The experiments were performed during 1993, 1994 and 1996. In 1993, the environmental conditions during vegetation were favourable: good rainfall was accompanied by moderate air temperatures. The years 1994 and 1996 differed significantly from 1993: they were very hot and dry. In 1993, it was found that a proportion of haploids derived from hybrids were considerably superior in plant height to haploids produced from parental lines. The occurrence of transgressive haploids suggested that parental lines were differentiated by no less than two genes for plant height. The genes of line 092 were arbitrarily designated as A1B1 and those of line A619 as A2B2.
Of interest are transgressive haploids A1B2 and A2B1. These were superior in plant height to parental genotypes A1B1 and A2B2. If the interaction between genes A and B were additive, then transgressive haploids would occur on each side of distribution of haploids derived from parental lines. However, in the segregating population of haploids derived from hybrids no plants were found which would be inferior in height to haploids produced from parental lines. Therefore, genes A and B were assumed to show the additive x additive epistatic interaction.
Applying the method of Powers (1951), theoretical distributions of all the four genotypes in the segregating population - A1B1, A2B2, A1B2, and A2B1 - were calculated. Summing up the frequencies of these genotypes, allowing for the fact that the proportion of each of them in the population was 25%, made it possible to calculate theoretical distributions of haploids derived from the 092 x A619 and A619 x 092 hybrids. Comparison of the observed and expected distributions showed a good agreement between the two, suggesting that lines 092 and A619 are indeed differentiated by two major genes which exhibited epistatic interactions in 1993.
In 1994, lines 092 and A619 were also found to be differentiated by two major genes concerned with plant height. However, the pattern of plant-height distribution of haploids was somewhat different from that in 1993. This altered distribution appears to have resulted from differential drought resistance of transgressive genotypes A1B2 and A2B1. One of the transgressive genotypes (this can be arbitrarily taken to be A1B2) was, as in 1993, superior in plant height to haploids derived from parental lines. However, plants of the other transgressive genotype, A2B1, shifted from the right to the left side of the distribution. It may be suggested that the epistatic interaction of genes A2 and B1 is unstable and that it is lacking completely under adverse environmental conditions. Therefore, selection of genotypes carrying precisely genes A1 and B2 may be of prime importance in maize improvement.
The severe drought of 1994 prevented us from producing enough seed for carrying out an experiment in 1995. The experiment, therefore, was continued in 1996. There was a severe drought again in the first half of the summer in 1996, resulting in the distributions of haploids being virtually the same as in 1994. Parental lines were found to be differentiated by two major genes. Transgressive genotypes A1B2 and A2B1 were distributed nearly symmetrically in relation to the haploids derived from parental lines A1B1 and A2B2. One transgressive genotype, presumably A2B1, exhibited the lowest plant height. Another transgressive genotype, probably A1B2, was on average superior in plant height to the haploids derived from parental lines. The best A1B2 plants were quite appealing as candidates for breeding. They were pollinated with pollen of diploid plants from a synthetic population, SA, which is targeted for improvement.
Unfortunately, we have no possibility to use molecular markers for mapping genes A1 and B2. Collaboration with researchers from other scientific centres will hopefully assist us in solving this problem.
In conclusion, it is hoped that the use of maternal haploids of maize will prove a useful tool in identifying major genes concerned with traits of breeding value.
In our experiment, two major genes were identified which strongly affect
plant height in maize. The number of genes responsible for the trait in
question showed no variation over years according to environmental conditions,
but the gene interaction varied. In the "favourable" year, the genes showed
additive x additive epistatic interaction, whereas the segregation frequency
of haploids observed under severe drought conditions is attributable to
additive gene interactions.
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