Robertson (Genetics 107:117, 1984) reported a dramatic effect of B-A translocations on crossing over in the male meiosis. He showed that the hypoploid (A AB) constitution gives a markedly higher rate of crossing over than the standard (A A) constitution for regions near the translocation breakpoint. The effect seems to be a general one, since a number of different translocations were tested.
Several explanations were suggested for the hypoploid effect. One possibility was a difference in recovery of gametic classes between male and female, based on the heteromorphic nature of A AB bivalents. Another explanation was that heterochromatin from the B chromosome may enhance male crossing over on AB chromosomes. It is also possible that the partial hemizygosity in A AB plants affects crossing over.
The alternative explanations were tested using TB-9Sb. The 9B chromosome contains most of the distal B heterochromatin. The ability of a standard 9B to enhance crossing over was compared to the effect of a modified 9B The latter 9B (designation 9B -2150) is a deletion derivative that lacks virtually all of the B heterochromatin (Fig. 2, Carlson & Curtis, Can. J. Genet. Cytol. 28:1034). A family was constructed which contained two types of plants: 9 9B (standard) B9 and 9 9B (deletion) B9. Five plants of each type were tested for crossing over in the wx-v interval, which is adjacent to the translocation breakpoint. In each plant, chromosome 9 carried wx v and 9B had Wx V. Reciprocal testcrosses were made, with the results listed below.
The number of plants tested is small and needs to be expanded. Nevertheless, the data indicate that crossing over in the male meiosis is considerably higher in the presence of a standard 9B compared to the heterochromatin deficient 9B . The range of crossover rates in the male is 6.3-16.8% for standard 9B vs. 1.6-3.9% for the deficient 9B . The rates for crossing over in the female meiosis are much closer between the two groups and there is overlap in the ranges: 4.9-8.7% for standard 9B vs. 2.9-6.8% for the deficient 9B.
The findings suggest that Robertson's crossover effect is due in part to the presence of B heterochromatin on the AB chromosome. The effect is not as strong as that found by Robertson, suggesting that other factors may also be involved (note that tests were conducted with balanced heterozygotes rather than hypoploids, and consequently any effect of hypoploidy on crossing over was not measured.) However, it is most interesting that three classes of maize heterochromatin (B, knob and centromeric) have now been implicated in effects on male vs. female crossover rates (Rhoades, Maize Breeding and Genetics, 1978).
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