A Survey of Mitochondrial Genome Heterogeneity as a Function of Relatedness in Asexual Freshwater Planarians

Development and regeneration is highly dependent on the integrated role of mitochondria in cell death pathways. In particular establishing identical mitochondrial genomes within all cells is essential for coordinated cellular communication and activity during development and homeostasis, and mitochondrial genome instability, i.e. heteroplasmy has been linked to multiple disease states. Typically, the establishment of a single mitochondrial haplotype occurs at the single cell stage of development, although there are exceptions. This cellular bottleneck typically occurs in both sexual and asexual systems. However the exception to the rule is the asexual planarian, Schmidtea mediterranea, which is the model system for regeneration. In this organism propagation is mediated through transverse fission of a large number of cells, thus multiple mitochondrial haplotypes can develop over time. To our knowledge this is the first study to investigate, at the mitochondrial genomic level, the consequences of lacking a single cell bottleneck using Illumina sequencing. Three “related” individuals derived from the same founder and three “unrelated” individuals derived from different founders were sequenced. The genomes were de novo assembled to a length of 16,562 base pairs with an average coverage of 3,009x, and the genomes were annotated and variants were identified. On average variants occurred every 10 base pairs. Interestingly we found that related individuals contained more variation than unrelated individuals. The variation found within the mitochondrial population was substantially greater than that found in other studies, e.g. human tissues or cancers. Thus, this system represents an excellent opportunity to understand mitochondrial population dynamics during regeneration.