Two mutant recessive alleles were produced at the ora2 locus (orange endosperm) in inbred line Oh43, namely ora2-13 and ora2-15. Pleiotropic vigor effects in populations of +/- and -/- genotypes were compared with +/+ genotypes in tests in the inbred for each allele (the plants in populations of a given genotype are quite uniform).
There are some similarities and some differences in the arrays of pleiotropic effects involving the two alleles. Flowering time behaves in much the same manner in both cases, i.e. the homozygotes flower later and the heterozygotes earlier than the +/+ homozygotes. Perhaps the most striking difference is in plant size as measured by plant height. In the case of the ora2-13 allele, the homozygotes are shorter and the heterozygotes the same height as the + / + homozygotes, in contrast to the case of the ora2-15 allele where both the homozygotes and heterozygotes are taller than the + / + homozygotes. In the case of the ora2-13 allele, there was a decrease in grain yield for the -/- homozygotes and an increase in grain yield for the + /- heterozygotes when compared with the + / + homozygotes. In the case of the ora2 -15 allele, there were no significant differences in grain yield for the -/- or + /- genotypes compared with the + / + genotypes. The genetic reasons for the differences are not clear at this time.
Perhaps the most likely possibilities as to the genetic nature of the pleiotropic effects would be (1) a block of closely linked genes produced by the mutational event which also produced the recessive allele, or (2) mutations spreading along the chromosome produced by the same mutational event which produced the recessive allele.
E. J. Dollinger
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