The bx locus in maize is required for the production of cyclic hydroxamates. Root tips carrying a dominant allele of this locus form a deep blue-purple color complex when they are crushed in the presence of a 0.1N solution of ferric chloride while root tips with the recessive allele will not form a color complex. F1 progeny of a cross between R/r-X1; Bx/Bx female parents and r/r; bx/bx male parents were germinated and screened for the presence of cyclic hydroxamates.

Eighteen out of 8074 r-X1 deficiency-containing (colorless) kernels expressed the recessive bx phenotype, and root-tip chromosome counts indicated that they were monosomic individuals. Measurement of mitotic metaphase chromosomes indicated that the missing chromosome was a long metacentric chromosome, and this information was used to tentatively assign the bx locus to chromosome five (Weber, 1982, pp. 79-83 in Maize for Biological Research, W. F. Sheridan, ed.). However, when these monosomic plants were grown to maturity, the morphology of the monosomic plants was characteristic of plants monosomic for chromosome four, and pachytene analysis of these plants indicated that the univalent chromosome was chromosome four.

To confirm the cytological identification, B-A translocations for both arms of chromosomes 1, 2, 3, 4, and 5 were crossed by a Bx/bx heterozygote, and only TB-4S uncovered the recessive bx allele. Clearly, the bx locus is located distal to the breakpoint of TB-4S on the short arm of chromosome 4. (Supported in part by DOE Contract 79EVO2121.)

Kevin Simcox and David Weber

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