Construction of chimeric alleles with altered specificity at the b incompatibility locus of Ustilago maydis.

Abstract

Multiallelic incompatibility systems found in many fungi and plants function to limit inbreeding by mediating self versus nonself recognition. The plant pathogenic fungus Ustilago maydis has a locus caled b that governs incompatibility. Two multiallelic genes, bE and bW, are present at the b locus. Fusion of haploid strains carrying different alleles at bE and bW establishes an infectious dikaryon capable of pathogenesis on maize (Zea mays). Cells carrying a single type of b locus, whether haploid or dikaryotic, are nonpathogenic. To identify sequences within the bE gene that determine allelic specificity, targeted gene replacement was employed to produce a series of chimeras between the blE and b2E alleles. Incompatibility tests with strains carrying the chimeric alleles identified a 30-to 48-amino acid region responsible for specificity. Surprisingly, the chimeras with recombination points within this region had a specificity different from both parent alleles. Overall, these results define an important domain in bE involved in self versus nonself recognition.The corn smut fungus, Ustilago maydis, incites a disease on corn (Zea mays) typified by the formation of large tumors on plant tissue (1). Interestingly, the parasitic phase ofthe fungal life cycle is also the sexual phase because the fungus must infect the host plant to produce meiotic progeny. Mating interactions between nonpathogenic, haploid strains of the fungus establish the infectious, dikaryotic cell type. This dikaryon proliferates within the plant, induces tumors (galls), and eventually sporulates to fill the tumor tissue with masses of sooty, black teliospores. The teliospores germinate to give the four haploid products of meiosis.The ability of haploid cells to fuse and establish the infectious dikaryon is controlled by two genetic loci, a and b.Two alternate sequences at the a locus (a) and a2) appear to control fusion (2-4). These sequences have recently been demonstrated to be idiomorphs (5) and to encode pheromones and pheromone receptors (6). The b locus, which may have as many as 25 different specificities, controls establishment of the infectious dikaryon once fusion has occurred (7,8). This type of mating system is referred to as tetrapolar to indicate that two loci, each with alternate alleles, govern the interaction of mating partners. It is believed that this type of mating system functions to limit inbreeding and to promote outbreeding. In this regard, the b locus of U. maydis is similar to the incompatibility loci of other basidiomycete fungi such as Schizophyllum commune (9) and Coprinus cinereus (10) and the S incompatibility locus of plants, such as members of the Solanaceae and Brassicaceae (11).The b locus is of particular interest because ofits multiallelic nature, its role in self versus nonself recognition, and its role in determining the morphology and the pathogenicity of the fungus. Cells heterozygous at b, whether haploid or diploid,

Evolution of the Bipolar Mating System of the Mushroom Coprinellus disseminatus From Its Tetrapolar Ancestors Involves Loss of Mating-Type-Specific Pheromone Receptor Function

“…Polymorphism at and near the mating-type locus: Basidiomycete mating-type genes have been shown to have high levels of amino acid polymorphism between alleles with substitutions clustered in the N-terminal regions of the homeodomain proteins, as this region has been determined to function in allele discrimination (Yee and Kronstad 1993;Banham et al 1995;Wu et al 1996;Badrane and May 1999). To determine the extent and location of variability of the putative C. disseminatus mating-type genes we used long PCR to amplify the Aa and Ab subloci in two separate reactions.…”

Dual Sets of Chimeric Alleles Identify Specificity Sequences for the bE and bW Mating and Pathogenicity Genes of Ustilago maydis

“…In previous work, we defined a specificity region for the bE1 and bE2 genes by the construction and analysis of chimeric alleles (37). This work identified a 40-amino-acid sequence within the N-terminal variable region that was thought to contain the residues that control specificity.…”

cAMP regulates morphogenesis in the fungal pathogen Ustilago maydis.

“…Recognition mediated by the products of the b genes is of particular interest because there are at least 25 different specificities at the b locus (Puhalla 1970;Silva 1972). Specificity is mediated by the variable amino-terminal region of bE as demonstrated by the construction of chimeric alleles of the bE1 and bE2 genes (Yee and Kronstad peptides is active in maintaining filamentous growth (Gillissen et al 1992;Yee and Kronstad 1993).…”