A fungal gene for antibiotic resistance on a dispensable (“B”) chromosome
Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Science
(Impact Factor: 33.61).
01/1992; 254(5039):1773-6. DOI: 10.1126/science.1763326
A family of cytochrome P-450 (Pda) genes in the pathogenic fungus Nectria haematococca is responsible for the detoxification
of the phytoalexin pisatin, an antimicrobial compound produced by garden pea (Pisum sativum L.). The Pda6 gene was mapped
by electrophoretic karyotype analysis to a small meiotically unstable chromosome that is dispensable for normal growth. Such
traits are typical of B chromosomes. The strains of Nectria studied here have no sequences that are homologous to the Pda
family other than Pda6 and therefore demonstrate that unique, functional genes can be found on B chromosomes. Unstable B chromosomes
may be one mechanism for generating pathogenic variation in fungi.
Available from: Ronny Kellner
- "Our data leave open the question of the relevance of ACs for infectivity in particular. Determinants of host specificity and virulence have been identified on ACs of other fungal plant pathogens, including the AC-encoded enzymes for detoxification of a plant phytoalexin in Nectria haematococca (Miao et al. 1991; Coleman et al. 2011) and the virulence-associated host-specific effector genes on lineage-specific chromosomes of Fusarium oxysporum (Ma et al. 2010). Though we detected no host-specific gene expression of AC genes at the very early stages of infection, 25 AC-encoded genes were upregulated during infection of wheat at 13 dpi (Yang et al. 2013). "
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ABSTRACT: Host specialization by pathogens requires a repertoire of virulence factors as well as fine-tuned regulation of gene expression.
The fungal wheat pathogen Zymoseptoria tritici (synonym Mycosphaerella graminicola) is a powerful model system for the discovery of genetic elements that underlie virulence and host specialization. We transcriptionally
profiled the early stages of Z. tritici infection of a compatible host (wheat) and a noncompatible host (Brachypodium distachyon). The results revealed infection regulatory programs common to both hosts and genes with striking wheat-specific expression,
with many of the latter showing sequence signatures of positive selection along the Z. tritici lineage. Genes specifically regulated during infection of wheat populated two large clusters of coregulated genes that may
represent candidate pathogenicity islands. On evolutionarily labile, repeat-rich accessory chromosomes (ACs), we identified
hundreds of highly expressed genes with signatures of evolutionary constraint and putative biological function. Phylogenetic
analyses suggested that gene duplication events on these ACs were rare and largely preceded the diversification of Zymoseptoria species. Together, our data highlight the likely relevance for fungal growth and virulence of hundreds of Z. tritici genes, deepening the annotation and functional inference of the genes of this model pathogen.
Available from: Dylan Short
- "It is also possible that other unorthodox mechanisms may be at play in generating genetic diversity in F. keratoplasticum. Species that possess small conditionally dispensible (CD) chromosomes are known to be present within the FSSC  as well as in other fusaria . The genome of F. solani f. sp pisi includes three CD chromosomes, which disproportionately harbor genes with no clear Fusarium orthologues, leading to the hypothesis that they were gained through unknown horizontal gene transfer (HGT) events . "
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ABSTRACT: Recent work has shown that Fusarium species and genotypes most commonly associated with human infections, particularly of the cornea (mycotic keratitis), are the same as those most commonly isolated from plumbing systems. The species most dominant in plumbing biofilms is Fusarium keratoplasticum, a cosmopolitan fungus known almost exclusively from animal infections and biofilms. To better understand its diversity and population dynamics, we developed and utilized a nine-locus sequence-based typing system to make inferences about clonality, recombination, population structure, species boundaries and hybridization.
High levels of genetic diversity and evidence for recombination and clonality were detected among 75 clinical and 156 environmental isolates of Fusarium keratoplasticum. The multilocus sequence typing system (MLST) resolved 111 unique nine-locus sequence types (STs). The single locus bifactorial determinants of mating compatibility (mating types MAT1-1 and MAT1-2), were found in a ratio of 70:30. All but one of the 49 isolates of the most common ST (FSSC 2d-2) came from human infections, mostly of the cornea, and from biofilms associated with contact lenses and plumbing surfaces. Significant levels of phylogenetic incongruence were found among loci. Putative clonal relationships among genotypes were estimated, showing a mixture of large clonal complexes and unrelated singletons. Discordance between the nuclear ribosomal rRNA and other gene genealogies is consistent with introgression of ribosomal RNA alleles of phylogenetic species FSSC 9 into F. keratoplasticum. No significant population subdivision based on clinical versus non-clinical sources was found.
Incongruent phylogenetic trees and the presence of both mating types within otherwise identical STs were observed, providing evidence for sexuality in F. keratoplasticum. Cryptic speciation suggested in a published three-locus MLST system was not supported with the addition of new loci, but evidence of introgression of ribosomal RNA genes from another strongly supported phylogenetic species (FSSC 9), also known from plumbing systems and human infections, was detected in two isolates. Overall, F. keratoplasticum is a diverse and geographically unstructured species with a mixed clonal and recombinant life history.
Available from: Fábio Porto-Foresti
- "In some cases, B chromosomes appear to provide some advantage to the host genome and can thus be considered heterotic , . Examples of advantageous B chromosomes were already reported in the Nectria haematococca with B chromosomes favouring this fungus pathogenicity  and in the chive Allium schenoprassum, in which the B chromosomes increase viability from seed to seedling . "
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ABSTRACT: Supernumerary (B) chromosomes have been shown to contain a wide variety of repetitive sequences. For this reason, fluorescent in situ hybridisation (FISH) is a useful tool for ascertaining the origin of these genomic elements, especially when combined with painting from microdissected B chromosomes. In order to investigate the origin of B chromosomes in the fish species Astyanax paranae, these two approaches were used along with PCR amplification of specific DNA sequences obtained from the B chromosomes and its comparison with those residing in the A chromosomes. Remarkably, chromosome painting with the one-arm metacentric B chromosome probe showed hybridization signals on entire B chromosome, while FISH mapping revealed the presence of H1 histone and 18S rDNA genes symmetrically placed in both arms of the B chromosome. These results support the hypothesis that the B chromosome of A. paranae is an isochromosome. Additionally, the chromosome pairs Nos. 2 or 23 are considered the possible B chromosome ancestors since both contain syntenic H1 and 18S rRNA sequences. The analysis of DNA sequence fragments of the histone and rRNA genes obtained from the microdissected B chromosomes showed high similarity with those obtained from 0B individuals, which supports the intraspecific origin of B chromosomes in A. paranae. Finally, the population hereby analysed showed a female-biased B chromosome presence suggesting that B chromosomes in this species could influence sex determinism.
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