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Chapter 23 : Bipolar and Tetrapolar Mating Systems in the Ustilaginales
Category: Fungi and Fungal Pathogenesis
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This chapter discusses species that include Ustilago hordei in particular, that have provided useful comparative information leading to insights into the genetic basis of bipolar versus tetrapolar mating systems in the smut fungi as a group. It first discusses the importance of smut fungi and the interactions of these pathogens with host plants to provide context for appreciating the role of mating in disease. The chapter then focus on the details of the mating system in U. hordei, including the structure and function of the mating-type loci, the genomic organization of these elements, and the sequence of the 527-kb MAT-1 locus. The cloning and analysis of the b genes from U. hordei revealed that they are very similar in structure to those in U. maydis, consisting of a gene complex with divergently transcribed bE and bW genes. The large size of the U. hordei MAT region and the suppression of recombination in this area prompted the authors to propose that the region might function to maintain a set of genes that function together in sexual development and, potentially, in pathogenesis. They therefore characterized t.
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- Restriction Fragment Length Polymorphism
Examples of mating interactions on charcoal medium. Aliquots of cultures of haploid basidiospores having the mating genotype indicated are cospotted on complete medium plates supplemented with activated charcoal ( 28 ). When both the a and the b loci are of different allelic specificity, a straight-growing, dikaryotic hypha is produced (inset) and the ensuing colony will have a white “fuzzy” appearance. Note that for U. hordei there are only two mating-type alleles (MAT-1 and MAT-2) in nature but that for U. maydis a and b specificities can assort in all combinations among progeny from genetic crosses.
Genomic organization of the mating-type loci in U. hordei and U. maydis. In U. hordei the MAT locus is defined as the region delimited by the known a and b mating-type gene complexes, a distance of 526,707 bp for MAT-1. The five BAC clones, yielding a total of 625,845 bp of sequence, are indicated ( 8 ). In both MAT-1 and MAT-2 (430 kb) strains, the locus sits roughly in the middle of the largest chromosome (chromosome I). Note that the MAT-2 region has not been sequenced but that its length and the orientation of the complexes have been determined ( 47 ). In U. maydis, the gene complexes are found on two different chromosomes: the a locus on contig 1.83 (scaffold 4; Broad Institute http://www.broad.mit.edu/annotation/fungi/ustilago_maydis/) harbored by chromosome V, and the b locus on contig 1.13 (scaffold 1) on chromosome I; contig 1.12 is likely linked to contig 1.13 based on the synteny found with U. hordei (represented by the solid black lines). Gene names are as follows: mfa, mating pheromone gene; pra, a1 pheromone receptor gene; pan, pantoate b-alanine ligase gene; bW1, bWest1 gene; bE1, bEast1 gene ( 4 , 6 , 16 ). Drawing is not to scale. Reprinted from Fungal Genetics & Biology (reference 8 , Fig. 1) © 2006, with permission from Elsevier.
Strategy to prove linkage and recombination suppression, and to measure the physical distance between the mating-type gene complexes in U. hordei. The two thick lines represent the MAT chromosomes with the respective locations of the a and b mating-type gene complexes for MAT-1 (A) and MAT-2 (B). The organization of the mating-type gene complexes is enlarged (compare with Fig. 23.2 ) and the positions of the integrated constructs used to tag them are indicated (boxes). I-SceI represents the 18-bp recognition sequence for the rare-cutting, intron-homing enzyme from Saccharomyces cerevisiae which was linked to the selectable markers for phleomycin (phleo) to tag the a complex, and for hygromycin B (hyg) to tag the b complex. Digestion with the enzyme I-SceI led to the estimate of the respective distances as indicated by the double-headed arrows (see the text for details) ( 47 ). See the legend to Fig. 23.2 for explanation of gene names.