The nature of the dominant mutations Tp1 and Tp2 was studied by comparing the expression of these mutations in the presence of different numbers of copies of their wildtype alleles. Plants of the genotype Tp2 g r-/Tp2 g r- and Tp1 gl1/+ + were crossed, respectively, by plants carrying the B-A translocations TB-10L19 and TB-7Lb. Hyperploid, hypoploid and diploid progeny were identified on the basis of kernel (R-scm) or seedling markers (g and gl1), or by chromosome number. Hyperploid progeny from both crosses were clearly mutant in phenotype. This demonstrates that these mutations involve a gain-of-function and confirms an observation made earlier in the case of Tp2 (Poethig, MNL 57:35, 1983). In the case of Tp2, all 3 dose classes (i.e. Tp2/ + / +, Tp2/ +, Tp2/-) had an identical phenotype. Thus Tp2 is either a constitutive mutation, or a mutation that causes Tp2 + to be expressed at greater than twice its normal level. In contrast, Tp1/- individuals were more severely mutant than Tp1/+/+ individuals, suggesting that this mutation in some way antagonizes normal gene activity.

S. Poethig

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