I have been studying the effects of monosomy on intergenic recombination in the maize genome (Weber 1971, MGNL 45:32-35; 1976, Genetics 83:s81). I recently determined that intergenic recombination in the sh-wx region of chromosome 9 is consistently and significantly lower in monosomic 4 plants (11.2 ± 0.8%) and monosomic 8 plants (17.1 ± 0.9%) than in diploid control plants (23.0 ± 0.8%) (Weber, 1976). It appears to be unchanged in monosomic 7 plants (21.8 ± 1.8%); thus, monosomy per se does not alter intergenic recombination. In these experiments the monosomics and diploids were testcrossed as males. I also determined the effect of trisomy of chromosome 4 on intergenic recombination in this same region and found that trisomy of chromosome 4 increased recombination slightly in both the male and female parents, but the increases were not significant (Weber 1976, MGNL 50:35-36). Thus, monosomy alters genetic recombination in the region analyzed far more than trisomy. I am attempting to localize these sites of monosomic and trisomic-induced recombinational alterations utilizing TB translocations. Initial results of this work are reported in this communication.
As indicated above (Weber 1976, Genetics 83:s81), monosomy of chromosome 4 decreased recombination in the sh-wx region of chromosome 9 to only 49.7% of that found in diploid control plants. To further localize the site of the recombinational alteration, hyperploid TB-4a plants were crossed as male parents by a c sh wx tester inbred line. Sibling hyperploid and hypoploid progeny of this cross were test-crossed as male parents and the results of these crosses are presented in Table 1.
It can be seen that recombination in the sh-wx region in hypoploid TB-4a plants was not lower, and possibly higher than in hyperploid sibling plants. Therefore it can be concluded that the factor(s) altering recombination in monosomic 4 plants is not in the segment distal to the breakpoint in TB-4a. The factor(s) must therefore lie proximal to the breakpoint in the short arm or in the long arm of chromosome 4.
In monosomic 8 plants, the percent recombination between sh and wx on chromosome 9 is only 74.3% of the value found in diploid control plants. TB-8a hyperploid plants were also crossed as male parents by a yg sh bz wx tester inbred line, and the sibling hyperploid and hypoploid progeny of this cross were testcrossed as male parents. The results are presented in Table 2.
It is clear that recombination in the sh-wx region in hypoploid TB-8a plants is not lower than in hyperploid plants. There is a slight increase in recombination in the hyperploid plants. It is concluded that the region(s) on chromosome 8 that reduces recombination in monosomic 8 plants is not missing in the hypoploid TB-8a plants. The factor(s) affecting recombination on chromosome 8 must be located between the breakpoint and the centromere in the long arm of chromosome 8 or in the short arm of chromosome 8.
It is well established that B chromosomes increase intergenic recombination in the maize genome. Hyperploid plants contain two additional copies of the segment of the B chromosome proximal to the breakpoint on the B chromosome. If these additional B chromosome segments increased recombination, recombination would be higher in the hyperploid plants than in hypoploid plants. Exactly the opposite result was found. If the factor(s) on chromosome 4 or 8 that alter recombination in monosomic 4 or 8 plants were distal to the TB breakpoints, recombination in hyperploid plants would also be higher in hyperploid TB-4a and TB-8a plants than in their hypoploid counterparts. However, recombination in the hyperploid plants was lower than in their sibling hypoploids in both cases. Thus, the influence of B chromosomes on recombination did not interfere with the results of this study, and it can clearly be concluded that the factors on chromosomes 4 and 8 are not distal to the breakpoints in TB-4a or TB-8a. (This work was supported in part by a contract from the USERDA, EY-76-S-02-2121; we would also like to express our appreciation to Funk's Seeds International for generously providing field space in a summer nursery and in a winter nursery in which this work was carried out).
David F. Weber
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