A reciprocal translocation and possible insertion(s) tightly associated with host-specific virulence in Cochliobolus heterostrophus

A reciprocal translocation and one or more apparent insertions are shown to be tightly associated with Tox1, a locus controlling T-toxin production and host-selective virulence in race T of the maize pathogen Cochliobolus heterostrophus. Chromosome arrangements were examined by separating chromosomal DNAs of a variety of Tox+ and Tox− strains by pulsed-field gel electrophoresis and hybridizing with probes known to detect RFLPs genetically linked to Tox1. The existence of the translocation was demonstrated by chromosome hybridization patterns; the existence of the putative insertion(s) was deduced from chromosome migration rates. Both differences in chromosome arrangement were detected between 8 Tox+ and 8 Tox− near-isogenic laboratory strains, suggesting that the differences are tightly linked to Tox1. The reciprocal translocation was also detected between all 7 Tox+ and 8 Tox− field isolates examined, suggesting that the translocation is common in natural populations. The field isolates may also differ by the insertion(s); however, numerous additional chromosome size polymorphisms in the field isolates prevented a firm conclusion. The tight association of the translocation and insertion(s) with T-toxin production suggests that chromosome rearrangements may have been involved in the evolution of race T and Tox1. These genomic differences may be causally related to the previously reported reduced fitness of race T relative to race O on N-cytoplasm maize. Key words : chromosome rearrangement, southern corn leaf blight, pulsed-field electrophoresis, T-toxin, evolution.