Transgressive segregation in the progeny of a cross between two inducers
of maize maternal haploids
--S. T. Chalyk, V. G. Bylich and O D. Chebotar
The production of new inducers of maternal haploids is of some importance
to maize genetics and breeding. This work may be successful, provided the
number of genes responsible for the induction of haploids is known. To
determine this number of genes, the following test was carried out. The
parental stocks used were two lines: a haploid inducer, Korichnevy Marker
Saratovsky (KMS), and a usual commercial line, MK01, in the progeny of
which no haploids have ever occurred. Previously it has been established
that the yield of maternal haploids ranges from 0.75 to 2.94% when KMS
is used as a pollen parent. Long-term experiments have shown that the range
of variation of the KMS haploid-inducing capacity is largely dependent
on the female parent genotype (Tyrnov and Zavalishina, 1984). A backcross
(MK01 x KMS) x KMS, was made. The backcross plants were selfed and the
resulting seeds were sown in a field plot to determine the frequency of
haploids. Haploid and diploid plants were identified during flowering by
a set of morphological traits. Previously it has been found that morphological
identification is more reliable than cytological analysis. This is due
to the fact that a small proportion of cells with diploid chromosome sets
occur in almost every tissue of haploid plants, which can lead to distorted
results (Khokhlov et al., 1976). A total of 54 progenies by selfing were
examined. The results are listed in the table below. The upper row of the
table shows the percentage of haploids recovered, the bottom one indicates
the number of progenies.
Thirty progenies either showed no haploids, or their frequency of haploids was very low, tending to zero. Twenty-four progenies by selfing backcross plants had the proportion of haploids which corresponded to that of the KMS and varied between 0.75 and 3.25%. The segregation ratio 30:24 is rather close to 1:1. Ideally, the ratio should be 27:27. Chi-square was 0.67, indicating close agreement with the expected segregation. It can be suggested, therefore, that, with respect to the trait concerned, KMS only differs from the usual MK01 line by a single gene. Lack of haploids is dominant, although the dominance is incomplete. It can be inferred from these findings that the development of new haploid inducers is unlikely to be a very difficult task.
In addition to KMS, we have extensively used in our work the inducer line Zarodyshevy Marker Saratovsky (ZMS). Its haploid-inducing capacity closely approximates that of KMS, ranging from 1.7 to 3.4% in different female parent genetic backgrounds. To test for allelism between the genetic factors controlling haploid-inducing capacity of KMS and ZMS, a cross between these two lines was made. The resulting hybrid plants were selfed to produce the F2 progeny. Segregation analysis was based on the frequency of haploids resulting from selfing of the F2 plants. Haploids were identified according to the method described above. A total of 22 progenies were examined. Out of these, 15 have been found to correspond to parental lines in the trait concerned, their haploid frequencies varying between 0.93 and 3.77%. Three progenies lacked haploids (0%). The third group of genotypes are of particular interest. These obviously were superior to parental lines KMS and ZMS. Their frequency of haploids varied between 6.94 and 8.75%. This is more than two times higher than in the parental inducer lines. There were four such progenies. Unfortunately, only 22 progenies were examined, and the F2 segregation ratio allows no definitive conclusions to be made. However, the presence of obviously transgressive forms suggests that KMS and ZMS differ by no less than two genes. In the future, homozygosity of the four progenies superior to the parental lines should be tested.
Both ZMS and KMS exhibit haploid-inducing potential. So it can be assumed that one of the differing genes should be carried by KMS and the other or others by ZMS. This assumption is confirmed by the fact that one of the genes considered was found, in the above experiment, to be carried by KMS.
Thus, it can be suggested that KMS and ZMS are sources of no less than two different genes whose joint action may result in 7 to 9% yield of haploids.
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