Characterisation and phylogeny of repeated elements giving rise to exceptional length of ITS2 in several downy mildew genera (Peronosporaceae).

In downy mildews with pyriform haustoria (DMPH), ITS lengths between 1121 and 2587kb were observed. The extreme length of ITS is due to sequence repeats of a poorly conserved part of the ITS2. Secondary structure analysis suggests that individual repeats may form long hairpin structures. The presence of these types of sequence repeats appears to be a synapomorphy for the DMPH. In another group of downy mildews, represented by Hyaloperonospora, similar REs sporadically occur, although with element lengths that are shorter than those in the DMPH group. Two characteristic motifs could be identified and their role in the evolution of the REs is briefly discussed. Dotplot analysis and molecular phylogenetic reconstructions for the repeated elements of representative species show distinct patterns that may suggest how they evolved. The high amount of sequence variability makes the repeated units a useful tool to study speciation and radiation processes in an economically important group of plant pathogens, which may form a model system for studying other coevolutionary host-pathogen systems.