Abstract
Since the mitochondrion became an endosymbiont of an ancestral eukaryotic cell, there has been a massive transfer of genetic material from the mitochondrial genome to the nucleus. Essential functions of the mitochondrion now rely on interactions with over 1000 nuclear-encoded genes. Compatibility between the genomes is necessary for efficient mitonuclear interaction, and incompatibility due to divergent lineages of the two genomes coming into contact is referred to as “mitonuclear incompatibility”. Admixed populations provide an opportunity to investigate mitonuclear discordance due to divergent combinations of mitochondrial and nuclear ancestry.
In model organisms, mitonuclear discordance has been linked to reduced reproductive fitness, increased DNA damage, oxidative stress and human neurological diseases such as bipolar disorder. The common brushtail possum (Trichosurus vulpecula), an invasive pest species in New Zealand, shows evidence of hybridisation from distinct Tasmanian and mainland Australia source populations. Development of the reproductive system occurs primarily outside of the uterus in marsupials, making possums an exciting potential model for mitonuclear discordance and its suggested ramifications on reproductive fitness.
I utilised a database of genes known to be mitochondrially associated in model organisms (e.g. humans, mice) alongside the possum genome to identify nuclear-encoded mitochondrially associated genes and their distribution throughout the possum genome. Single nucleotide polymorphisms (SNPs) were then identified within these mitochondrially associated genes based on population-level RNAseq data. SNPs determined to be fixed between one Tasmanian and seven New South Wales reference samples were used to generate estimates of heterozygosity and proportions of SNPs matching the genotype of reference samples for 67 putatively admixed Otago possum samples. A comparison of these results to the mitochondrial haplogroup of the Otago possums showed no evidence of nuclear re-assortment towards mitochondrial background. Nevertheless, I identified 3 SNPs (out of 819) that had a significant association with expression of 3 mitochondrially-encoded genes (COX1, COX2, ATP6), implying that matching mitochondrial background may be important for their expression. The encouraging results of this pilot study should be followed up with larger sample sizes, and a more comprehensive sample of genetic variation among Australian source populations. For the significant associations I did identify, analysis of the potential impact on protein interactions and investigation into potential functional impacts in discordant individuals would also be advantageous. Overall, this research contributes to the development of a novel model system for identifying the genomic underpinning of mitonuclear discordance, which has far-reaching ramifications for human health and pest control.