Complementation of a stable Met2-1 mutant of the zygomycete Absidia glauca by the corresponding wild-type allele of the mycoparasite Parasitella parasitica, transferred during infection.

Compared with prokaryotes, where horizontal gene transfer events are frequently found and can be studied in the laboratory at the mechanistic level, few systems are known that allow direct experimental access to parasexual phenomena in eukaryotes. In zygomycetes, a basal lineage of fungi, several mycoparasitic fungi are known that inevitably form a cytoplasmic continuum with their hosts during infection. We provide evidence that, corresponding to the expectation suggested by the morphology of the infection process, gene transfer occurs from the parasite to the host. For analysing this parasexual system at the DNA level, we characterized interspecific recombinants obtained by infecting a stable methionine-auxotrophic Absidia glauca mutant with heavy rearrangements at the Met2-1 locus, which encodes homoserine acetyltransferase. Recipients were shown to be complemented by part of the corresponding gene from Parasitella parasitica. This foreign DNA is neither integrated at the putative Met2-2 locus in the recipient strain nor integrated at Met2-1, a locus encoding a hypothetical protein with amino acid similarity but with unknown function. Based on hybridization studies and on the phenotype of recipients that bear some mitotic instability of the acquired prototrophy, we propose that P. parasitica DNA is established in A. glauca recipients as extrachromosomally located replicons.