Supplementary methods and results from Mitochondrial DNA content in eggs as a maternal effect

The transmission of detrimental mutations in animal mitochondrial DNA (mtDNA) to the next generation is avoided by a high level of mtDNA content in mature oocytes. Thus, this maternal genetic material has the potential to mediate adaptive maternal effects if mothers change mtDNA level in oocytes in response to their environment or body condition. Here, we show that increased mtDNA abundance in mature oocytes was associated with fast somatic growth during early development but at the cost of increased mortality in three-spined sticklebacks. We also examined whether oocyte mtDNA and sperm DNA damage levels have interacting effects because they can determine the integrity of mitochondrial and nuclear genes in offspring. The level of oxidative DNA damage in sperm negatively affected fertility, but there was no interacting effect of oocyte mtDNA abundance and sperm DNA damage. Oocyte mtDNA level increased toward the end of the breeding season, and the females exposed to warmer temperatures during winter produced eggs with increased mtDNA copies. Our results suggest that oocyte mtDNA level can vary according to the expected energy demands for offspring during the embryogenesis and early growth. Thus, mothers can affect offspring development and viability through the context-dependent effects of oocyte mtDNA abundance.