The effect of TB-4a and TB-8a on intergenic recombination of chromosome nine

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.

Tables 1 and 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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