title: The a mating-type alleles of Ustilago maydis are idiomorphs.
abstract: Two unlinked incompatibility loci, a and b, control mating, pathogenicity, and sexual development in Ustilago maydis, a fungal pathogen of corn. Fusion of nonpathogenic haploid cells occurs when alleles of the a locus differ; fusion products that differ at the b locus will be pathogenic and complete sexual development. The two alleles of the a locus have been cloned. The a2 allele was isolated by exploiting the close linkage of the a locus to the genetic marker pan1. Several cosmids from a clonal a2 mating-type library allowed, via DNA-mediated transformation, an a1 mating type, pan1-1 auxotrophic recipient to grow prototrophically on medium lacking pantothenic acid. Cosmids containing functional a2 mating-type regions were identified by their ability to confer a2 mating behavior to the a1 recipient strain. The a1 allele was isolated from a plasmid library using DNA from the a2 region as a hybridization probe. The isolated clones contain a1 or a2 mating-type specificity as demonstrated by a plate mating assay, by pathogenicity tests on corn, and by gene replacement experiments. The a1 and a2 mating-type clones contain nonhomologous DNA segments that are flanked by similar nucleotide (nt) sequences. Restriction fragments containing a mating-type function are embedded within the nonhomologous DNA segments. U. maydis contains a single copy of either the a1 or a2 mating-type sequence within each haploid genome. This structural organization demonstrates that the a mating-type alleles of U. maydis are idiomorphs sensu Metzenberg and Glass (1990), nt sequences mapping to the same genetic locus that share no obvious evolutionary relationship.
authors.affiliation: Plant Disease Resistance Research Unit, U.S. Department of Agriculture-Agricultural Research Service, Madison, WI.
authors.forename: E H
authors.initials: EH
authors.lastname: Froeliger
authors.forename: S A
authors.initials: SA
authors.lastname: Leong
mesh_terms: D000483:Alleles; D003001:Cloning, Molecular; D003433:Crosses, Genetic; D004171:Diploidy; D005816:Genetic Complementation Test; D005838:Genotype; D008561:Membrane Fusion; D009693:Nucleic Acid Hybridization; D015183:Restriction Mapping; D014170:Transformation, Genetic; D014588:Ustilago / Q000235:genetics* / Q000502:physiology