Dosage analysis on the teosinte branched1 mutation suggests it is an antimorphic dominant mutation
--Hubbard, L and Hake, S

Plants homozygous for the teosinte branched1 mutation in maize exhibit a severe phenotype of extreme tillering and long lateral branches tipped by tassels in upper ear nodes. Previous observations suggest that tb1-ref has a semi-dominant effect on tiller number when heterozygous, displaying a mild tillered phenotype with no obvious effects on the female inflorescence (Schnable, MNL 66:5, 1992).

Dosage analysis of the tb1 locus was begun to further investigate the tb1-ref allele. Crosses were carried out using a line of maize obtained from James Birchler (University of Missouri) that is carrying a transposition of 18% of the long arm of chromosome 1L within chromosome 3L, which we will designate as Tp(1-3). Tp(1-3) encompasses the tb1 locus and thus can be used to generate stocks hyperploid for the normal or mutant alleles of tb1 (Birchler and Levin, Genetics 127:609-618, 1991). A Tp(1-3) heterozygote was crossed by tb1-ref homozygotes and the F1 was backcrossed by tb1-ref homozygotes. The resulting material was assayed for the presence of tb1-ref or the normal tb1 allele by linkage to different adh1 alleles.

Results from the dosage analysis on plants carrying 1 or 2 doses of normal or mutant tb1 alleles show that when tb1-ref allele is present at a higher dose than the normal allele, the severe tb1-ref phenotype is observed (Table 1). These results suggest that the tb1-ref allele actively interferes with functioning of the normal tb1 allele and therefore, this mutation should be interpreted as antimorphic. These findings are consistent with the observation that this mutation may be semi-dominant. Analyses of the mild tillering phenotype in tb/+ relative to +/+ and tb/+/+ (+ representing the normal allele and tb the mutant) are under investigation.

Table 1. Results from crosses in which the female parent is heterozygous for Tp(1-3) and tb1-ref and the male parent is homozygous for tb1-ref.
 

	Severe phenotype*	Normal phenotype
tb/tb	9
tb/+		14
tb/+/+		13
tb/tb/+	9

*Severe phenotype consists of long lateral branches tipped by tassels in the ear position and excessive tillering.

 Note: The stock carrying tb1-ref allele was obtained from Bill Sheridan (University of North Dakota). 

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