Heterologous transposition in Aspergillus nidulans

Aspergillus nidulans is one of the model ascomycete fungi. Transposition ev ents have never been described in this organism. We have determined that th is organism has at least 13 copies of a Fot1-related element. These copies are transcribed, non-methylated and polymorphic in various wild isolates. I n spite of this, we have failed to isolate transposon insertions when the r esident niaD gene is used as a transposon trap. This contrasts with the sit uation described previously in Fusarium oxysporum. We show that two element s of F. oxysporum, Fot1 and impala, transpose efficiently in A. nidulans. W e have developed the impala system by tagging it with the yA gene. This per mits the visual detection of the transposon by the colour of the conidiospo res. We demonstrate that no endogenous transposase of A. nidulans is able t o act in trans on a defective impala element, whereas its own transposase d riven by two different promoters is able to mobilize this element. The freq uency 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 pos itions in the genome. The availability of mitochondrially inherited mutatio ns has allowed us to demonstrate that hybrid dysgenesis is apparently absen t in A. nidulans. The development of this system opens the way to investiga ting the mechanism underlying the paucity of transposition events leading t o visible phenotypes. It should allow us to develop efficient gene-tagging tools, useful in this and other fungi.