Degenerate MAGGY elements in a subgroup of Pyricularia grisea: a possible example of successful capture of a genetic invader by a fungal genome

Abstract

The LTR-retrotransposon MAGGY is found sporadically in isolates of Pyricularia grisea (Magnaporthe grisea). Based on a dendrogram constructed by RFLP analysis of rDNA, isolates that carry MAGGY elements were classified into a single cluster that comprised four rDNA types. However, in a few members of this cluster, exemplified by isolates from common millet (Panicum miliaceum), the MAGGY element has distinct features. Southern analysis suggested that these isolates possessed a single copy of a MAGGY-related sequence whose restriction map differed from that of MAGGY itself. Sequence analysis revealed that the MAGGY-related sequence was a degenerate form of MAGGY, characterized by numerous C:G to T:A transitions, which have often been reported to result from RIP (Repeat-induced point mutation) or RIP-like processes. However, the favored target site for C:G to T:A transitions in this fungus, determined by examining a total of 501 sites, was (A/T)pCp(A/T), which differs from that for the RIP process originally reported in Neurospora (CpA), and from that reported in Aspergillus (CpG). The fact that certain members of the cluster of MAGGY carriers retain a single copy of a degenerate MAGGY element implies that the ancestor of these isolates successfully “captured” the invading MAGGY element.