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Category: Fungi and Fungal Pathogenesis
History of the Mating Types in Ustilago maydis, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815837/9781555814212_Chap21-1.gif /docserver/preview/fulltext/10.1128/9781555815837/9781555814212_Chap21-2.gifAbstract:
This chapter presents some general features of Ustilago maydis and a synopsis of its life cycle. The life cycle of U. maydis is characterized by three cell types: a cigar-shaped haploid, unicellular form that divides by budding and is nonpathogenic; a dikaryotic, filamentous form that grows at the tip cell and is pathogenic; and a diploid spore, the teliospore, formed within the tumors induced by the fungus, that is not capable of vegetative growth but germinates and undergoes meiosis to produce the haploid yeast-like form. Heterothallism and Sex Factors Stakman and Christensen demonstrated for the first time that U. maydis is heterothallic. Six mutants designated bmut (three bGmut, two bDmut, and one bImut) were obtained, all of which induced tumors and produced teliospores when inoculated with their progenitors, with themselves and with each other, and thus behave as “universal” alleles. The b locus of U. maydis was the first multiallelic mating-type locus cloned and understood at the molecular level and provided a framework in which to understand the more complex A multiallelic mating-type locus of other basidiomycetes. Strains with an inactive gene for the pheromone precursor can respond to the pheromone produced by cells of opposite mating type but they cannot induce a response in cells of opposite mating type, and strains with an inactive receptor gene can induce a response in cells of opposite mating type but cannot respond to cells of opposite mating type.
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Morphological transitions in the life cycle of U. maydis. U. maydis exhibits three forms (cell types) during its life cycle: a haploid unicellular form; a filamentous dikaryon; and a diploid spore, the teliospore (reviewed in references 8 , 10 , and 26 ). The transition from one form to the other entails conjugation, karyogamy, and meiosis. These processes are accompanied by changes in ploidy, growth habit, and pathogenicity (see references 8 , 10 , and 26 for details). These morphological transitions are governed by two unlinked mating-type loci, a and b, by environment, by nutrients, and possibly by plant signals (reviewed in references 8 , 10 , 36 , 51 , and 52 ). (From reference 8 , with permission.)
Teliospore germination and meiosis. The teliospore is a diploid spore, resistant to adverse environmental conditions. It is formed within the tumors following a discrete developmental program during which changes in morphology take place and karyogamy occurs ( 13 ). Upon germination, a short filament (the promycelium) emerges, into which the diploid nucleus migrates and undergoes meiosis to form a septate promycelium consisting of four haploid cells ( 68 ) (A). Clones of these primary meiotic products arise by budding to produce yeast-like cells (panel A), which can be removed with the aid of a micromanipulator for single tetrad analysis (panel B). Alternatively, the teliospore is allowed to germinate and form a microcolony, which is then resuspended in water and spread on an agar surface (panel C).
The fuzz reaction of haploid and Fuz– diploid strains on charcoal agar. Haploid and diploid strains carrying different a and b alleles produce a white fuzziness due to the formation of filaments in combination with haploid tester strains ( 16 ). Saturated cultures of tester strains (vertical lines) were cross-streaked versus haploid (top four horizontal lines) and Fuz– diploids (bottom four horizontal lines). The testers are, from left to right, a1 b1 (FB1), a2 b2 (FB2), a1 b2 (FB6b), and a2 b1 (FB6a). The horizontal lines represent, from top to bottom, strains a2 b2, a1 b1, a2 b1, a1 b2, and a1/a2 b1/b1 (class I; FB-D12-3); a1/a2 b2/b2 (class II; FB-D11-21); a1/a1 b1/b2 (class III; FB-D11-7); and a2/a2 b1/b2 (class IV; FB-D12-11) (see reference 16 for further details). (From reference 16 , with permission.)
The molecular structure and organization of the b locus. The b locus is the major determinant of filamentous growth and pathogencity (reviewed in references 8 , 10 , 26 , and 51 ) and is estimated to have 25 naturally occurring alleles ( 70 , 78 ), each of which consists of two divergently transcribed genes, bW and bE, separated by an intergenic region of approximately 200 bp ( 40 , 82 ). Each gene codes for a polypeptide containing a homeodomain-related motif ( 40 , 82 ). The bW and bE polypeptides have the same structural organization, a variable region at the amino terminus and a constant region for the remainder of the polypeptide, but show no similarity except for the homeodomain region ( 40 , 56 , 82 ). The active b protein is found only in the dikaryon and consists of a bW and a bE subunit encoded by different b alleles, for example, bW1-bE2 or bW2-bE1 ( 40 , 54 ). Each dikaryon has two different active heterodimers, but only one is necessary for activity ( 40 ). The variable region is the specificity determinant and controls dimerization of these polypeptides ( 54 , 96 , 97 ). The homeodomain of both bE and bW is required for activity ( 81 ). (From reference 8 with permission.)
Amino acid sequence comparison of bE and bW polypeptides. The amino acid sequences of seven bE and four bW polypeptides are shown ( 40 , 56 , 82 ). The variable region of bE and bW encompasses the first 110 and 140 amino acids, respectively. The remainder of the polypeptide is the constant region (not shown in its entirety) and contains the homeodomain. The three helices of the homeodomain are shaded. The homeodomain of bE belongs to the atypical class of homeodomains, whereas that of bW belongs to the typical class (see reference 25 ).
Deletion analysis of the b locus. The bE or bW or both genes were deleted in haploid strains ( 40 ), and the resulting mutants were tested for b activity (filament formation [Fuz phenotype] and tumor induction [Tum phenotype]) by cospotting on charcoal agar and inoculation into corn plants, respectively. The strains listed under strain 1 are derivatives of a1 b1, and those under strain 2 are derivatives of a2 b2. Mixtures of wild-type a1 b1 and a2 b2 strains result in a Fuz+ Tum+ phenotype (row 1). Deletion of one bE gene or one bW gene in one of the partners does not affect b activity (rows 2 and 3, respectively). On the other hand, b activity is abolished by deletion of both bE and bW in one of the partners (row 4), both bE and bW genes in both partners (row 5), bE in both partners (row 6), or bW in both partners (row 7). In contrast, deletion of bE1 in one partner and bW2 in the other (row 8) or of bW1 in one partner and bE2 in the other (row 9) does not abolish b activity and demonstrates that bW and bE from different b alleles are necessary for b activity ( 40 ). The dikaryon contains two active heterodimers, but only one is necessary for function (rows 8 and 9).
b1/b2 chimeric alleles delimit the specificity region. Chimeras between bE1 and bE2 or between bW1 and bW2 were generated by homologous recombination at the b2 locus ( 96 , 97 ). Three classes of chimeras were generated by the crossover events indicated by the lines. Class I chimeras exhibit b2 specificity, class II chimeras exhibit b1 and b2 specificity, and class III chimeras have switched from b2 to b1 specificity ( 96 , 97 ). N2 and C2 refer to the borders of the b2 specificity region; N1 and C1 refer to the borders of the b1 specificity region. The class II chimeras have N1 C2 borders, that is, they share one border with b1 and the other with b2. Because class II chimeras have both b1 and b2 specificity, it has been proposed that differences at only one border are sufficient for dimerization and generation of the active b heterodimer ( 96 ). The b specificity region for bE1/bE2 encompasses a region of approximately 40 amino acids, and that of bW1/bW2 encompasses a region of approximately 70 amino acids ( 96 , 97 ). Similar conclusions were reached by Kämper et al. ( 54 ) from yeast two-hybrid interactions and analysis of bE2mut alleles.
Molecular structure and organization of the a locus. The a locus has two alleles, a1 and a2 ( 78 ), and codes for components of a pheromone response pathway ( 21 ). Both a1 and a2 contain a pheromone precursor and a receptor gene. The pheromone precursor genes code for polypeptides of 40 and 38 amino acids for a1 and a2, respectively, and the pheromone precursors contain a CAAX box at the C terminus ( 21 ). This sequence is a substrate for prenylation and carboxymethylation, and the modification is necessary for full activity of the mature pheromones ( 86 ). The modified pheromone precursors are processed further to yield lipopeptides of 13 and 9 amino acids for a1 and a2, respectively. The a2 allele contains additional genes: lga2 and rga2, and a pseudopheromone gene, mfa ( 91 ), not present in the a1 allele. (From reference 8 with permission.)
The b locus is multiallelic a
Compatibility reactions among progeny from cross 10A4 × 17D4 and from solopathogen 410qq a