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Chromosomal Polymorphism in Fusarium oxysporum f. sp. niveum. D. H. Kim, Former graduate research assistant, Department of Plant Pathology and Microbiology, Texas A & M University, College Station 77843; R. D. Martyn, and C. W. Magill. Professors, Department of Plant Pathology and Microbiology, Texas A & M University, College Station 77843. Phytopathology 83:1209-1216. Accepted for publication 23 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1209.

Genome size and chromosome number in seven geographically separated isolates representing six mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP) groups (RFLPG IVI) and the three known pathological races of Fusarium oxysporum f. sp. niveum, causal agent of Fusarium wilt of watermelon, were examined by transverse alternating field electrophoresis (TAFE). Chromosome-size DNA was separated on 0.8% agarose gel at 50 or 80 V with five switch intervals for 168 h. Six karyotypes were detected among the seven isolates. Two representative isolates from mtDNA RFLPG I had two karyotypes, whereas the representative isolates from mtDNA RFLPGs IV and V had the same karyotype. Putative chromosome number varied from five to 10 among the isolates and ranged in size from approximately 900 to 4,400 kb. Either one or both of the two largest putative chromosomes (3.6 and 4.4 Mb) were present in all isolates; however, smaller chromosomes varied both in number and size among isolates. Minimum genome size among the six isolates of F. o. niveum ranged from 15.8 to 26.0 Mb. When individual bands were extracted, labeled, and used as probes, 30 of the 40 randomly selected genomic cosmid clones from a reference strain hybridized to more than one chromosome band. When the procedure was reversed, three of 10 labeled clones hybridized to all chromosomes from the reference strain and also to several chromosomes from each of the other isolates, indicating the presence of common repeated sequences dispersed throughout the genomes.

Additional keywords: electrophoretic karyotype.