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.

The degenerate DNA transposon Pat and repeat-induced point mutation (RIP) in Podospora anserina

“…Some sequence characteristics, like the observation that most of the putative mutations appear to be transitions, are in good agreement with the assumption that RIP-like processes are involved in the degeneration of these transposons. However, the mutations do not occur preferentially at CpA:TpG dinucleotides, as is characteristic for RIP in N. crassa (Cambareri et al 1989;Grayburn and Selker 1989), but are found preferentially at CpG (Neuveglise et al 1996), CpG or CpA (Hua- Van et al 1998) or at (A/Tp)Cp(A/T) (Nakayashiki et al 1999) sites. Furthermore, in none of the fungi mentioned is there any evidence for the occurrence of a sexual cycle under natural conditions; a sexual cycle is a prerequisite for RIP in N. crassa.…”

“…The residue numbers refer to the position in the GRISEA protein (Osiewacz and Nuber 1996) mechanisms have been restricted to N. crassa and A. immersus. However, the search for fungal transposons and their characterization has revealed some clues suggesting ± but not proving ± that RIP-like mechanisms might be responsible for the inactivation of formerly active transposons in fungi like F. oxysporum (Julien et al 1992;Hua-Van et al 1998), A. fumigatus (Neuveglise et al 1996) and Pyricularia grisea (Nakayashiki et al 1999). Some sequence characteristics, like the observation that most of the putative mutations appear to be transitions, are in good agreement with the assumption that RIP-like processes are involved in the degeneration of these transposons.…”

Organization and Distribution Pattern of MGLR-3, a Novel Retrotransposon in the Rice Blast Fungus Magnaporthe grisea

“…MAGGY exhibits a wider distribution pattern (Farman et al, 1996b;Nakayashiki et al, 1999), being present in isolates from such hosts as rice, Panicum spp., Setaria glauca, and Paspalum districhum. However, the isolates carrying MAGGY form a single genetic cluster (Nakayashiki et al, 1999), suggesting a horizontal transfer of MAGGY to their common ancestral strain(s).…”

A family of DNA repeats in Aspergillus nidulans has assimilated degenerated retrotransposons

“…In N. crassa, the C to T mutations characteristic of RIP occur predominantly (75%) at CpA (Watters et al 1999), and in Podospora anserina mutation occurs indiscriminately at CpA and CpT (Hamann et al 2000). RIP-like phenomena in fungi lacking a sexual cycle have shown a bias for CpG in A. fumigatus (Neuveglise et al 1996), CpG and CpA in Fusarium oxysporum (Hua- Van et al 1998) and (A/Tp)Cp(A/T) sites in M. grisea (Nakayashiki et al 1999). A. nidulans, however, has a well-characterized sexual cycle.…”