Joshua Russell Christie
I intend to examine whether intra-individual genomic conflict results in detrimental fitness effects at the organismal level. Genomic information is not limited to the nucleus, with certain cytoplasmic organelles, namely mitochondria and chloroplasts (in plants), contain independent, albeit much smaller, genomes. While sexually produced offspring receive nuclear DNA from both parents, mitochondrial DNA (mtDNA) is almost always inherited from a single parent. Why is this?
The answer may lie in the differing agendas of cytoplasmic and nuclear DNA. The propagation of a specific allele at a polymorphic locus is primarily dependent upon how the fitness of that allele compares to other variants of that allele in the population. Conversely, in a hypothetical cell that exhibits biparental mtDNA inheritance and contains multiple mitochondrial lineages, the most significant factor affecting the spread of a mitochondrial type within the cell is the speed at which it replicates compared to other mitochondrial variants. A conflict between the nuclear and mitochondrial genomes may arise when ‘selfish’, fast-replicating mitochondrial types hurt fitness at the level of the cell. This conflict is well illustrated in Saccharomyces cerevisia, where small, fast-replicating mitochondria can outcompete wild type mitochondria in anaerobic conditions. However, these mutant mitochondria have respiration deficiencies and cells containing these mutants are at a selective disadvantage in aerobic conditions, where they are duly outcompeted by wild type strains.
Selection on the nuclear genome should act to oppose the unrestrained transmission of these selfish elements. The conflict hypothesis proposes that the uniparental transmission of mtDNA is an adaptation by the nuclear genome to mitigate the threat of selfish mitochondrial lineages, which might harm the fitness of the cell. I intend to test this theory using the slime mould, Physarum polycephalum.
Physarum polycephalum forms a unicellular, multinucleate diploid plasmodium during one stage of its life cycle. Although nuclei and mitochondria continue to divide, no cell division takes place, and thus the plasmodium increases in size and nuclei and mitochondria can mix freely. The mode of mtDNA transmission in Physarum polycephalum is established post-fusion and based upon a hierarchy of mating alleles/types (of which there are at least 16). Significantly, a number of these crosses result in the biparental transmission of mtDNA. Thus, Physarum polycephalum provides an environment in which multiple mitochondrial lineages can exist and compete within the single cell, and it is an excellent model in which to examine the fitness consequences of biparental mtDNA inheritance.