A fungal gene for antibiotic resistance on a dispensable ("B") chromosome.

A family of cytochrome P-450 (Pda) genes in the pathogenic fungus Nectria haematococca is responsible for the detoxification of the phytoalexin pisatin, an antimicrobial compound produced by garden pea (Pisum sativum L.). The Pda6 gene was mapped by electrophoretic karyotype analysis to a small meiotically unstable chromosome that is dispensable for normal growth. Such traits are typical of B chromosomes. The strains of Nectria studied here have no sequences that are homologous to the Pda family other than Pda6 and therefore demonstrate that unique, functional genes can be found on B chromosomes. Unstable B chromosomes may be one mechanism for generating pathogenic variation in fungi.