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Repetitive sequences from the genomes of AM fungi

The nuclei of most arbuscular mycorrhizal (AM) fungi (Glomeromycota) have previously been estimated to contain between 0.1 to 1 pg DNA per nucleus. This large genome size may be due to the presence of a high proportion of repeated DNA.

Since repetitive genomic sequences have been used to promote integration of transgenes into genomes in other organisms, we isolated and identified repeated sequences of Gig. rosea, G. mosseae and Gig. margarita to select for transformation vector constructs. A total of 23 repetitive elements were obtained by screening l-ZAPII genomic libraries of the three fungi, obtained within the GENOMYCA project, with DNA from spores. Four were identified from Gig. rosea, 6 from G. mosseae, and 13 from Gig. margarita. All sequences are AT rich and most show no homology to database sequences.

Two from Gig. margarita and one from Gig. rosea have significant similarities to retrotransposable-like elements, with protein sequences found in LTR (long terminal repeats) or non-LTR retrotransposons. This is the first evidence of retrotransposable-like elements in the genome of AM fungi. Their presence may account for the large genome size and the high genetic polymorphism that has been described for this group of organisms.

One sequence which was studied in detail (GmarRT1) is present in about 2000 copies which are dispersed in the genome of Gig. rosea, G. mosseae and Gig. margarita. Different sequences in this same retrotransposon family were obtained and restriction enzyme analyses showed them to be methylated. Methylation of retrotransposons is known in many organisms as a mechanism by which genomes inactivate retrotransposons, and a similar process is associated with the inactivation of transgenes. This, together with the observation that introduction of GmarRT1 into vector constructs did not improve transgene frequency in bombarded spores of Gig. rosea, suggests that the use of a retrotransposon sequence in transformation vectors is not an appropriate strategy for transgene integration in AM fungal genomes.

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Reported by

Institut national de la recherche agronomique
21065 Dijon