Use of nitrate non-utilizing mutants to study vegetative incompatibility in Fusarium solani ( Nectria haematococca), especially members of mating populations I, V and VI

Nitrate non-utilizing ( nit) mutants were obtained from 89 (out of a sample of 94) strains of Fusarium solani isolated from several plant hosts in Japan, New Zealand and North America. Nit1 (unable to utilize nitrate) and nit3 (unable to utilize nitrate or nitrite), mutants were readily recovered from all 89 strains when these were grown on a minimal medium containing KCLO 3 at 30 g l −1 (MMC), but NitM (unable to utilize nitrate, hypoxanthine or uric acid) mutants were only obtained from 53 strains on this medium. Modified MMC containing l-threonine instead of l-asparagine enabled recovery of NitM mutants from an additional 11 strains. Sulphate non-utilizing mutants ( sul) were generated for a few strains that did not produce NitM mutants. Based on complementation between nit1 (or nit3) and NitM (or sul) mutants, 57 strains were heterokaryon self-compatible, of which 31 were assigned to single-strain vegetative compatibility groups (VCGs). Nine multi-strain VCGs were identified among New Zealand strains: here members of each VCG came from the same host and usually from the same location even though they were often isolated at different times. Two pea strains from North America were in the same VCG, and a third North American pea strain was compatible wih a pea strain from New Zealand. Although the VCG results were consistent and repeatable the vigour of individual heterokaryons varied widely. Frequently the intra-strain complementations produced heterokaryons of greater vigour from those that formed between compatible strains. There was a high frequency of self-incompatibility due either to simple or complex genetic mechanisms among Nectria haematococca MPI strains.