The data reported below cytogenetically locate opaque-2 (o2) relative to T7-9(8558), T7-9(8659), T7-9(071-1), and T7-9b. Only the opaque seeds were planted in the experiments reported here. The F1 was used as the female parent in all except one case (noted by an asterisk). The ratio of +:o2 seed is given in parentheses following the F1 genotype.
Six plants identified as duplicate-deficiency 7-9b heterozygotes (50% smaller pollen) were self-pollinated and produced the following +:o2 ratios;
178+:278o2, 29+:61o2, 110+:222o2, 101+:148o2, 151+:283o2, 217+:336o2
Grand total = 786+:1328o2
These data suggest that T7-9(071-1) has a breakpoint (7S.76) distal to the o2 locus and the Dp-Df 7-9b chromosome is transmitted through the megaspore about one-half the time. Crosses of heterozygous T7-9b female x o2 o2 male yield no opaque kernels out of a total of 570.
In conclusion, the opaque-2 locus appears to be distal to 7S.55, not beyond 7S.76, and probably near 7S.70. Additional information was reported in Genetics 61:107-116, placing o2 approximately 2.5 map units distal to the 7S.62 breakpoint of T6-7(6885).
R. L. Phillips
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