Mitochondrial inheritance in filamentous fungi

Abstract

Because of the ease with which fungal strains and life cycles can be manipulated, fungal research has made important contributions to the present understanding of mitochondrial inheritance generally. Several mitochondrial DNA molecules have been completely sequenced and this information has provided useful probes for study of other mitochondrial genomes. Mapping and sequencing has shown that the mtDNA is circular, butin vivo many linear derivatives are observed whose significance is unclear. The genes carried in mtDNA are for components of the electron transport system, ATP synthesis and mitochondrial protein synthesis. Multigenic transcripts are processed to arrive at mRNAs. The mitochondrial genetic code shows several differences compared to the universal code. MtDNA shows a high level of neutral variation due to length mutations and restriction site differences: these are useful in measuring genetic distances in evolutionary and taxonomic studies. Mutations affecting gene function lead to slow/erratic growth or drug resistance. Many mutations are suppressive, rapidly dominating the culture. Mitochondrial plasmids are common in natural isolates of fungi. Strains can contain mixtures of linear and circular types. Some plasmid families show widespread distribution, across different species and genera. Linear plasmids generally have genes for presumptive DNA and RNA polymerases. Circular plasmids have a single gene for a DNA polymerase or a reverse transcriptase. Several plasmids recombine with mtDNA; the most striking cases are the senescence plasmids, which integrate into mtDNA and cause death through insertional mutagenesis. Genetic elements in the mitochondria, and the phenotypes determined by them, are inherited maternally or uniparentally in sexual crosses. Mitochondrial elements and phenotypes are transmitted by cytoplasmic contact between cells. In heteroplasmons the mitochondrial mixtures often segregate during somatic propagation, resulting in pure derivatives. Heteroplasmons also offer the opportunity for recombination between different mtDNA molecules.