Vegetative compatibility and heterokaryon stability in Fusarium oxysporum f.sp radicis-lycopersici from Italy

Fusarium crown and root rot, caused by Fusarium oxysporum f.sp. radicis-lycopersici (Forl), is one of the most destructive soilborne diseases of tomato in Italy. Chlorate-resistant, nitrate-nonutilizing (nit) mutants were used to determine vegetative compatibility among 191 isolates of Forl collected in five geographic regions (Calabria, Emilia-Romagna, Liguria, Sardinia, Sicily) in Italy. The isolates were assigned to five vegetative compatibility groups (VCGs): 65 isolates to VCG 0090; 99 to VCG 0091; 23 to VCG 0092; two to VCG 0093; and two to VCG 0096. The population structure of Forl in Italy is similar to that reported for Israel, and differs from that found in North Atlantic European countries, where VCG 0094 is predominant. The stability of prototrophic heterokaryons originating from hyphal anastomosis between compatible complementary nit mutants was assessed through conidial analysis and mycelial mass transfer. Most monoconidial cultures (84%) recovered from 117 prototrophic heterokaryons were nit mutants, indicating that heterokaryons generally do not proliferate well through conidiation; most of the 177 prototrophic heterokaryons examined were unstable, and only 9% sustained prototrophic growth through the tenth mycelial transfer upon subculturing. The prototrophic growth is proposed to be maintained through restoration of the heterokaryotic state by continual anastomosis between adjacent homokaryotic hyphae. Since heterokaryosis is a prerequisite for parasexual recombination, we speculate that this mechanism is unlikely to play a major role in generating the VCG diversity found among Forl or other strains of F. oxysporum.