Variability in the electrophoretic karyotype of Cochliobolus carbonum
Abstract
The fungus Cochliobolus carbonum is a foliar pathogen of maize which is divided into races based on pathogenic phenotype. Race 1 isolates produce HC-toxin, a cyclic peptide responsible for increased virulence on susceptible maize. Toxin synthesis requires a peptide synthetase encoded by the HTS1 gene in the Tox2 locus, which is absent from non-toxin producing races. This study examined the variability in chromosomes of race 1 isolates, a rather uniform population that is believed to have arisen from a progenitor that acquired Tox2 and, thus, the ability to produce HC-toxin. The Tox2 locus of C. carbonum was found by hybridization with HTS1 sequences to reside on different chromosomes of 2.3 or 4 Mb in different race 1 isolates. Chromosome-specific markers from C. heterostrophus identified a 2.3 Mb chromosome in all isolates, and a 4 Mb marker hybridized with a 2.0 Mb chromosome in isolates where the 4 Mb chromosome is absent. Ascospore progeny from crosses of two isolates having HTS1 on different chromosomes were analyzed for toxin-producing activity, virulence, and chromosomal location of HTS1. All progeny were found to be toxin producers, although they exhibited a range of toxigenic capacity. All progeny were phenotypically race 1, although lesion development was delayed for some of the progeny relative to the parents. HC-toxin production and virulence were not correlated with chromosomal location of HTS1. All progeny contained HTS1 sequences based on PCR amplification with gene-specific primers. Pulsed field gel electrophoresis was employed to separate the chromosomes of several progeny. Hybridization with HTS1 sequences indicated that each of the progeny contained a single HTS1-bearing chromosome, but some showed non-parental chromosomal polymorphisms. These results indicate that there is variability in the electrophoretic karyotype of C. carbonum race 1. The Tox2 locus has either been incorporated into the genome of more than one progenitor isolate on more than one occasion, or the Tox2-containing chromosome has been altered by a translocation or large-scale rearrangement in some isolates.
Degree
Ph.D.
Advisors
Dunkle, Purdue University.
Subject Area
Plant pathology|Genetics|Molecular biology
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