Extrachromosomal genetic elements in fungi.

Abstract

All fungi like eukaryotes possess mitochondria, which are the sites of the oxydative phosphorylation. As eukaryote evolution depends on oxygenic atmosphere, these organisms are primarily aerobic. Except a small group of strict anaerobes (those which lost the capacity of oxydative pathways living in special milieu in association with rumen of grass-eating animals) all fungi can utilize various compounds as carbon sources via oxidative phosphorylation pathways resulting in high energy yield. Certain groups of fungi – i.e. most of the yeasts – under anaerobe conditions, are able to supply themselves with lower levels of fermentation energy, too exhibiting a slow growing capacity utilizing the same amount of carbon source. The mutation of mitochondrial genome or mitochondrial functions encoded by nuclear genes of these fungi might result in a so-calledpetitephenotype producing small colonies on solid media due to their slow growing capacity. These mutants can utilize only fermentable carbon sources. Filamentous fungi have only limited possibilities to produce such phenotypes. ExceptZygomycetes(where the shortage of oxygen induces dimorphic transitions) filamentous fungi can grow and develop their vegetative and sexual reproductive structures only in aerobe milieu. However amongNeurosporaspecies there are several mitochondrial mutations resulting in morphological phenotypes. These are due to the lower energy level provided by the reduced capacity of cytochrome-oxidase enzymes. These mutants (e.g.pokystopper) can be considered aspetiteanalogues. The complete loss of mitochondrial functions – such asrho zerocharacter in yeast – cannot be survived by filamentous fungi.Podospora anserinaand some of its close relatives exhibit a so-calledsenescencephenotype, which means that the growing hyphae in the youngest part of the colonies stop growing and start to die within a short period of time. This phenomenon – discussed below – is also connected to reduced function of mitochondria.The first part of this paper gives a short overview of the genetic organization of mitochondria of fungi, based on the most recent data of three filamentous fungi:Aspergillus nidulansNeurospora crassaandPodospora anserina. Their data are compared to those of the well-characterizedSaccharomyces cerevisiae. In the second part we summarize what we know about other extrachrosomal elements, such as DNA plasmids of various origins and structures, and dsRNAs or virus like particles (VLP). Also discussed are their roles and/or putative functions in the life of the fungi.