Horizontal transfer of genetic elements in the black Aspergilli

The thesis deals with the horizontal transfer of genetic elements in the black Aspergilli . The black Aspergilli form a complex group of asexual species. All share a characteristic black conidiospore color and the ability to efficiently degrade tannin. Selective isolation of all different black Aspergillus types is possible on media with 20% tannin. Tannins can form complexes with proteins, that are difficult to mineralize. Therefore, the strains may have a special niche in the control of the natural nitrogen cycle. Black Aspergilli occur worldwide and especially in warmer regions at high densities. The spores have an efficient aerial distribution, which produces a well-mixed sporebank in soil throughout the world.Under laboratory conditions isogenic lines are capable, after hyphal fusions, to form a heteroplasmic heterokaryon and (transient) diploids. This so-called parasexual cycle can result in recombination via reassortment of chromosomes, mitotic crossing-over and/or exchange of cytoplasmic genetic elements. Most of the natural isolates are heterokaryon incompatible with one another and unable to form a stable heterokaryon. About the exact mechanism of the heterokaryon incompatibility reactions in the black Aspergilli little is known. Confrontations between heterokaryon self-incompatible strains suggest that prefusion genes are involved. The fact that protoplast fusions are partly able to overcome incompatibility reactions suggests that also fusion and postfusion genes are involved.One of the cytoplasmic candidates for horizontal transfer is the mitochondrion. Different mitochondrial haplotypes can be distinguished, corresponding with different black Aspergillus types . No horizontal transfer or recombination of mitochondria was observed in our natural isolates, though in protoplast fusions mitochondria can recombine. In nature new mitochondrial types may result from mutations.Most of the transfer experiments in these thesis were done with cytoplamsic dsRNA mycoviruses. In nature 10% of the population is infected with a variety of different dsRNA fragments of different viral origins. These mycoviruses can cause serious reductions in their host's fitness on traits as spore production and growth rate. Population genetic models predict that deleterious elements should disappear from a population, unless they have an extra way of transfer than just vertical transmission to offspring. Interspecies transfer of mycoviruses with species like Fusarium poae was in our experiments less difficult to achieve (and thus perhaps more likely in nature) than intraspecies transfer between different black Aspergillus types.In a diploid both interchromosomal and intrachromosomal mitotic recombination could take place. However, molecular data suggests that there is little (para)sexual recombination in the black Aspergillus population, in contrast to other presumably asexual fungi tested so far. Recently transposable elements have been found in black Aspergillus strains. These do seem to have transposed between different, quite unrelated strains. Circular intermediates of these transposable elements may also transfer little parts of genomic DNA, which may lead to some recombination. The size of the genetic elements may influence the chance on horizontal transfer during cell contact: no detectable transfer of mitochondria, very little of mycoviruses and some transfer of transposable elements.