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,

No abstract

The b mating-type locus of the fungal plant pathogen Ustilago maydis encodes two multiallelic gene products, bE and bW, that control the formation and maintenance of the infectious cell type. Dimerization via the N-terminal regions of bE and bW proteins encoded by alleles of different specificities establishes a homeodomain-containing transcription factor. The bE and bW products encoded by alleles of like specificities fail to dimerize. We constructed sets of chimeric alleles for the bE1 and bE2 genes and for the bW1 and bW2 genes to identify sequences that control specificity. The mating behavior of strains carrying chimeric alleles identified three classes of specificity: b2 (class I), specificity different from either parental type (class II), and b1 (class III). Crosses between strains carrying bE and bW chimeric alleles identified two short blocks of amino acids that influence specificity and that are located in the N-terminal variable regions of the b proteins. Comparisons of pairs of chimeric alleles encoding polypeptides differing in specificity and differing at single amino acid positions identified 16 codon positions that influence the interaction between bE and bW. Fifteen of these positions lie within the blocks of amino acids identified by crosses between the strains carrying chimeric alleles. Overall, this work provides insight into the organization of the regions that control recognition.Recognition mediated by protein-protein interactions plays a fundamental role in many biological processes. Well-characterized examples include antibody-antigen interactions (8, 9, 23), ligand-receptor binding (22,35), and the establishment and maintenance of tissue integrity by cadherins (19). The proteins involved in sexual reproduction and incompatibility in fungi provide relatively simple examples of determinants of self versus nonself recognition. In this paper, we describe a molecular genetic approach to identify the determinants of recognition for the proteins encoded by the b mating-type locus of the fungal corn pathogen Ustilago maydis.U. maydis is commonly found in nature as black diploid teliospores on infected corn plants (6). The teliospores germinate, and meiosis occurs to produce haploid, yeast-like progeny. Nonself recognition between compatible haploid mating partners is a prerequisite to the establishment of an infectious, dikaryotic cell type, and the genes at the a and b mating-type loci are considered pathogenicity factors (reviewed in references 2 and 18). The a locus, with alternate specificities a1 and a2, encodes pheromones and pheromone receptors and controls recognition of mating partners at the level of cell fusion (3,11,31). The b locus controls the formation and maintenance of the infectious cell type after cell fusion has occurred. If the cells participating in mating have different specificities (nonself) at the b locus, a vigorous, straight dikaryotic filament is formed and this cell type will be infectious. In contrast, mating partners that carry b sequences of like specificities ...

The effectiveness of diatomaceous earth (DE) as a treatment against parasites and to increase feed efficiency and egg production of organically raised free-range layer hens was evaluated in 2 breeds of commercial egg layers [Bovan Brown (BB) and Lowmann Brown (LB)] that differ in their resistance to internal parasitic infections. Half the hens of each breed were fed diets supplemented with DE (2%). Their internal parasite loads were assessed by biweekly fecal egg counts (FEC) and by postmortem examination of the gastrointestinal tract. Supplementing DE in diets of LB hens, the more parasite-resistant breed, did not significantly affect their FEC and adult parasite load. However, BB hens treated with dietary DE had significantly lower Capillaria FEC, slightly lower Eimeria FEC, fewer birds infected with Heterakis, and significantly lower Heterakis worm burden than control BB hens. Both BB and LB hens fed the diet containing DE were significantly heavier, laid more eggs, and consumed more feed than hens fed the control diet, but feed efficiency did not differ between the 2 dietary treatments. Additionally, BB hens consuming the DE diet laid larger eggs containing more albumen and yolk than hens consuming the control diet. In a subsequent experiment, the effectiveness of DE to treat a Northern fowl mite (Ornithonyssus sylviarum) infestation was tested. Relative to controls, both breeds of hens that were dusted with DE had reduced number of mites. The results of this study indicate the DE has the potential to be an effective treatment to help control parasites and improve production of organically raised, free-range layer hens.

Mating between compatible haploid cells of the corn smut fungus, Ustilago maydis, results in a switch from budding to filamentous growth. We are analyzing the multiallelic b incompatibility locus that governs maintenance of the infectious, filamentous dikaryon. Specificity regions have been identified in the N-terminal portions of the bE and bW genes at the b locus. In addition, we have found that heterozygosity at the b locus attenuates fusion. This result suggests that b gene products may exert a negative influence on some processes, in addition to their generally recognized role in maintaining filamentous growth. The b genes have also been characterized in Ustilago hordei. This species has a bipolar mating system in which the b genes are linked to genes required for pheromone production and response to form one large mating-type region (MAT locus) with two allelic specificities. In a separate study, we have discovered that defects in adenylate cyclase result in constitutive filamentous growth and greatly reduce the virulence of U. maydis on corn seedlings. Mutations have been identified that suppress the adenylate cyclase defect and restore budding growth. Sequence analysis revealed that one of the suppressor mutations is in the gene encoding the regulatory subunit of cAMP-dependent protein kinase (PKA). Mutants altered in PKA activity have the interesting phenotype of multiple budding and frequent mislocalization of the bud site. Key words: smut, dimorphism, b locus, sex.