Heterologous transposition in Aspergillus nidulans

Abstract

Aspergillus nidulans is one of the model ascomycete fungi. Transposition events have never been described in this organism. We have determined that this organism has at least 13 copies of a Fot1-related element. These copies are transcribed, non-methylated and polymorphic in various wild isolates. In spite of this, we have failed to isolate transposon insertions when the resident niaD gene is used as a transposon trap. This contrasts with the situation described previously in Fusarium oxysporum. We show that two elements of F. oxysporum, Fot1 and impala, transpose efficiently in A. nidulans. We have developed the impala system by tagging it with the yA gene. This permits the visual detection of the transposon by the colour of the conidiospores. We demonstrate that no endogenous transposase of A. nidulans is able to act in trans on a defective impala element, whereas its own transposase driven by two different promoters is able to mobilize this element. The frequency of excision of these modified elements is between 10(-4) and 10(-5). Loss of the transposable element occurs in about 10% of all excision events. In the remaining 90%, the transposon seems to be integrated at random positions in the genome. The availability of mitochondrially inherited mutations has allowed us to demonstrate that hybrid dysgenesis is apparently absent in A. nidulans. The development of this system opens the way to investigating the mechanism underlying the paucity of transposition events leading to visible phenotypes. It should allow us to develop efficient gene-tagging tools, useful in this and other fungi.

Genome size evolution: Within-species variation in genome size

“…Transposition of the LINE (Long Interspersed Nuclear Element: non-LTR retrotransposon) MGL (M. grisea LINE) was also observed in another experiment in which two M. grisea isolates from rice were crossed (Nishimura et al, 2000), and the Ds-like element, Ascot-1, has been shown to excise during mating in Ascobolus immersus b2 (Colot et al, 1998). No evidence of TE amplification was, however, found in crosses between A. nidulans strains (Li Destri Nicosia et al, 2001). These findings strongly suggest that the transposition of TEs may be enhanced during mating in fungi, but only some TE families are involved.…”

Minos as a novel Tc1/mariner-type transposable element for functional genomic analysis in Aspergillus nidulans

“…The impala element belongs to the Tc1-mariner family (Daboussi et al, 1992;Daboussi, 1997). Impala has proved to be an efficient tool for gene-tagging and functional analysis, mainly because of its relatively high frequency (approximately 10 À5 ) of transposition and little host specificity within ascomycetes (Li Destri Nicosia et al, 2001;Villalba et al, 2001;Firon et al, 2003;de Queiroz and Daboussi, 2003;Carr et al, 2010;Cecchetto et al, 2004). However, impala also shows a trend for transposition within introns or 5 0 regions, rather than in ORFs or essential regulatory regions (Deschamps et al, 1999).…”

Heterologous transposition in Ustilago maydis

“…Interestingly, in A. nidulans it was found that the heterologous transposons impala and Fot1 also show a marked tendency to integrate outside of ORFs (Li Destri Nicosia et al 2001). In this case it was suggested that this phenomenon is not based on target preferences of the transposon, but on a general 'immunity' of genes to foreign DNA.…”

“…In F. oxysporum the nitrate reductase gene from A. nidulans was first used as a transposon trap and the tagged gene then served in A. nidulans and F. monoliforme to monitor excision of the transposon. The reinsertion rate was estimated to range between 25% (for F. monoliforme) and 90% (for A. nidulans) (Hua- Li Destri Nicosia et al 2001). In contrast to these systems, the counterselectable plasmid system used for U. maydis does not allow us to calculate the frequency of excision.…”

“…Class II transposons have been efficiently utilized as mutagenic agents in many different systems, including plants, nematodes, insects and higher vertebrates (Cooley et al 1988;Zwaal et al 1993;Bhatt et al 1996;Yant et al 2000). The successful use of DNA transposons in fungi has recently been described for ascomycete species such as Saccharomyces cerevisiae, Magnaporthe grisea, Fusarium oxysporum and Aspergillus nidulans (Migheli et al 1999;Weil and Kunze 2000;Hua-Van et al 2001;Li Destri Nicosia et al 2001;Villalba et al 2001) Lack of host specificity is of central importance for the use of transposons in heterologous systems. One transposon group that is especially well suited for this purpose is the Tc1/mariner superfamily (Plasterk et al 1999).…”