Possible nonrandom disjunction of the B9 chromosome during meiosis

In 1967, Robertson reported on meiotic segregation of the B-9b translocation heterozygote (Genetics 55:433). One conclusion was that the B9 chromosome segregates randomly at first division with respect to its pairing partner, chromosome 9. That pairing occurred in Robertson's study was insured by the selection of c-sh crossovers between 9 and B9 for analysis.

The experiment of Robertson with 9 9B B9 plants has essentially been repeated, using the longer yg2 bz interval. Testcrosses were performed as follows: 9 yg2 bz, 9B Wx, B9 Yg2 Bz x yg2 yg2 bz bz. In the offspring, three crossover classes are important: yg2 Bz Wx, yg2 Bz wx, Yg2 bz Wx. The yg2 Bz Wx class results from yg2-bz crossing over, followed by disjunction of the 9 and B9. The yg2 Bz wx class can occur in two ways. Following crossing over between yg2 and bz, migration of the B9 to the same pole as chromosome 9 can produce yg2 Bz wx. However, yg2 Bz wx may also be produced by a double crossover (cent.-bz and bz-yg2) followed by disjunction of the 9 and B9. The third genetic class, Yg2 bz Wx, is also a double crossover product and is the reciprocal to the yg2 Bz wx double.

To compare types of segregation, the yg2 Bz Wx class is used as an indicator of 9-B9 disjunction and the yg2 Bz wx kernels represent nondisjunction of the paired chromosomes. A correction must be made to remove double crossovers from the yg2 Bz wx class, since they result from 9-B9 disjunction. The corrected term is yg2 Bz wx Yg2 bz Wx. For random migration of the B9, one might expect that yg2 Bz Wx yg2 Bz wx - Yg2 bz Wx. However, the nondisjunction class is only expected at one-half the frequency of yg2 Bz Wx since it is derived from the anaphase I chromosome combination 9 yg2 bz wx/Yg2 bz wx + B9 Yg2 Bz/yg2 Bz. Chromatid segregation at AII will allow recovery of the yg2 Bz wx phenotype in only one-half the cases.

For random migration of the B9 in 9 9B B9 heterozygotes, one expects:

yg2 Bz Wx

yg2 Bz wx - Yg2 bz Wx

= 2/1

The data found for the standard TB-9b were 150/(67-17) = 3.0. The results suggest that the 9 and B9have some tendency to disjoin from each other, although there is considerable migration to the same pole, as noted by Robertson. A collection of modified TB-9b's was also analyzed. The modified translocations lack the ability to undergo nondisjunction at the second pollen mitosis (Chromosoma 42:127). It was considered possible that they might also be altered in meiotic behavior, and the following data were gathered.
Modified TB-9b Ratio
#1855 (telocentric B9) 216/(81-20) = 3.54
#1866 (deletion on B9) 69/(43-4) = 1.77
#10 (deletion on 9B) 98/(35-8) = 3.6
#2150 (deletion on 9B) 165/(75-13) = 2.66

Results are quite variable and may reflect differing genetic backgrounds rather than any substantive deviation from the pattern found with standard TB-9b. Tests with inbred lines are planned. In conclusion, it appears that the B9 chromosome may not disjoin completely at random with respect to chromosome 9 in a translocation heterozygote.

Wayne R. Carlson

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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