Genetic variation in three Ceratocystis species with outcrossing, selfing and asexual reproductive strategies

Abstract

SummaryNuclear and mitochondrial markers were used to examine variation within three closely related species of tree pathogens with differing reproductive strategies. Ceratocystis eucalypti is obligately outcrossing; Ceratocystis virescens is capable of selfing due to unidirectional mating type switching; and Chalara australis is an asexual species, comprised of a single mating type. When the nuclear DNA fingerprinting markers (CAT)5 and (CAG)5 were used as probes against Pst I‐restricted DNA, isolates of C. eucalypti were found to be highly polymorphic, and Ch. australis showed very little polymorphism. The selfing C. virescens showed an intermediate level of variation in the nuclear fingerprint markers, and much of the variation appeared to be due to differences between two forms of the species, one pathogenic to Acer and Liriodendron and another less‐pathogenic form on Fagus and other hardwoods. Mitochondrial DNA (mtDNA) polymorphisms were examined by digesting total DNA with Hae III or CfoI, and C. eucalypti showed somewhat more variation in mtDNA than did C. virescens. The only polymorphism seen in the mtDNA of Ch. australis was associated with a plasmid. Selfing in C. virescens may be common and could explain an intermediate level of diversity when compared to the obligately outcrossing C. eucalypti and the asexual Ch. australis.