For several years we have carried out experiments on practical utilization of parental (androgenetic) haploidy in corn breeding programs. Up to now male-sterile counterparts production through androgenesis has not been widely employed, because of several difficulties connected with the conversion procedure.
The production of sterile counterparts by the haploid method is possible only for inbreds which are maintainers of sterility (cms). At present it is known that 91% of inbreds maintain cmsS, and only 68% maintain cmsC (Frankovskaya et al., 1995). We have no possibility to avoid this difficulty, and the inbreds that restore fertility are transferred into sterile cytoplasm by use of the extensive backcrossing method.
The second difficulty is connected with genetic markers that are used for paternal haploid induction. Unfortunately they are not good enough for practical use in corn breeding. They were created on the basis of different cms types and include the ig gene, which increases the paternal haploidy frequency up to 2% (Kermicle, 1969) or even 9% (Kindiger, 1992). Difficulties in practical use of these markers depend on their low productivity, because of small ears with a large quantity of defective kernels. Besides, low expression of the gene R-nj makes it difficult to select haploid kernels. To overcome this inconvenience special crosses are carried out between samples with different doses of the ig gene and different coloration expression (MNL 71:45). It is essential to have a marker with such coloration expression which gives a chance to identify haploid kernels without difficulties. Their haploid frequency has to be not less than 0.1%. We believe this aim will be achieved.
The third difficulty was quite unexpected. An unusual phenomenon was found in the sterile counterpart of the Kr714S inbred, which was obtained through paternal haploidy (cytoplasm S was taken from marker cmsS ig/+ R-nj, nucleus from original inbred Kr714). Crossing of Kr714S x P502RS was done to determine the restoring of S sterility in Kr714S. Several ears with colored and colorless kernels were found among hybrid F1 progeny. This means that although the phenotype of Kr714S was identical with Kr714 (besides sterility), some part of the female (marker) genetic material was involved in its genotype. Similar facts were reported by other authors (Chalyk, 1970; Tyrnov, 1984). This phenomenon has wrecked our notion that the paternal haploid appears as a result of male parent sperm getting into female cytoplasm and the embryo developing without female genetic material. The case of Kr714S was not the only one. The same thing was observed in inbred Kr82S. Thatís why we have to do some additional crosses to be completely sure of the identity of the line and its sterile counterpart.
While overcoming all difficulties during the last 5 years we obtained
71 paternal haploid plants, which formed normal kernels after pollination
with the corresponding linemaintainer of sterility paternal haploidy
frequency was varied from 0.03% to 0.14%. As a result of this research
we produced sterile counterparts (cmsS or cmsC) for 16 corn lines.
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