Repetitive DNA sequences include retrotransposons in genomes of the Glomeromycota

Abstract

Twenty-five repetitive elements are first described in the genomes of the arbuscular mycorrhizal (AM) fungi Gigaspora margarita, Gig. rosea and Glomus mosseae. Nineteen repetitive DNA sequences isolated by genomic library screening and four by self-priming PCR had no homology to known DNA sequences, except for two Gig. margarita sequences and one Gig. rosea sequence which showed amino acid similarity to retrotransposons. Part of the Gig. rosea sequence was also similar to a DNA transposon. Two other retrotransposon sequences were isolated using PCR targeting of reverse transcriptase and ribonuclease H domains. Evidence is provided for three gypsy-like LTR retrotransposon and two non-LTR retrotransposon sequences in the AM fungal genomes. Four contain stop codons indicating that they cannot be active. Expression of three retrotransposons was not detected in germinating spores or intraradical hyphae of Gig. margarita. Southern blot analyses indicated that these three sequences are dispersed in the genome and that two are methylated. Sequence analysis of different GmarLTR1 copies showed they have undergone mutations by transitions, which may have been induced by cytosine methylation. Transposable elements may have played a major role in shaping genome structure and size during evolution of the Glomeromycota.