CHROMOSOMAL-POLYMORPHISM IN FUSARIUM-OXYSPORUM F-SP NIVEUM

Genome size and chromosome number in seven geographically separated is olates representing six mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP) groups (RFLPG I-VI) and the three known pa thological races of Fusarium oxysporum f. sp. niveum, causal agent of Fusarium wilt of watermelon, were examined by transverse alternating f ield 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 k aryotypes were detected among the seven isolates. Two representative i solates from mtDNA RFLPG I had two karyotypes, whereas the representat ive isolates from mtDNA RFLPGs IV and V had the same karyotype. Putati ve chromosome number varied from five to 10 among the isolates and ran ged 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 s ize 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 extra cted, labeled, and used as probes, 30 of the 40 randomly selected geno mic 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 als o to several chromosomes from each of the other isolates, indicating t he presence of common repeated sequences dispersed throughout the geno mes.