Verticillium wilt of crucifers: characterising novel interspecific fungal hybrids

Earlier studies by other workers have suggested that isolates of Verticillium dahliae from crucifers have unusual properties suggesting novel origins. The aims of the current study were to determine whether distinct interspecific hybridisations have given rise to these isolates and to gain a clearer understanding of the origins and most appropriate taxonomy for V. dahliae crucifer isolates. The existence of distinct populations within V. dahliae was established by examining the spore length and DNA content of crucifer isolates from Europe, Japan, Russia and USA and a range of other isolates. These measurements were then correlated with molecular studies on (i) the internal transcribed spacer (ITS) and internal intergenic spacer (IGS) regions of the rRNA genes, (ii) random genomic sequences, (iii) mitochondrial DNA and (iv) the whole genome using amplified restriction fragment length polymorphism (AFLP). On the basis of this work it is suggested that V. dahliae crucifer isolates are natural interspecific hybrids which have arisen via two separate and one probable hybridisation events. Furthermore it is suggested that both the V. alboatrum and V. dahliae parental isolates are distinct from any other isolates of either species previously studied. Vegetative incompatibility studies showed that this is unlikely to be a major barrier to the formation of hybrids, therefore the possible mechanisms by which hybrids arose are considered. The genetic status of these hybrid isolates is also discussed and the term ‘amphihaploid’ (by analogy with amphidiploid as used for plants) is suggested as better than the ill-defined ‘near-diploid' used by some authors. Some isolates are also proposed to be ‘secondary haploids' derived from amphihaploid interspecific hybrids. It is suggested that all amphihaploid hybrid isolates should be included in a single new taxon, V. x dahlatrum, which reflects their mode of origin, parental species and genome complement