Persistence and invasiveness of high-level heteroplasmy through biparental transmission of a selfish mitochondrion in Drosophila

ABSTRACTHeteroplasmy is the coexistence of more than one type of mitochondria in an organism. Although widespread sequencing has identified several cases of transient or low-level heteroplasmy that primarily occur through mutation or paternal leakage, stable, high-titer heteroplasmy remains rare in animals. In this study we present a unique, stable and high-level heteroplasmy in male and female flies belonging to the neotropical Drosophila paulistorum species complex. We show that mitochondria of D. paulistorum are polyphyletic and form two clades, α and β, with two subclades each. Mitochondria of the α2 subclade appear functional based on their genomic integrity but are exclusively found in heteroplasmic flies and never in homoplasmy, suggesting that they are a secondary mitotype with distinct functionality from the primary mitochondria. Using qPCR, we show that α2 titer do not respond to energetic demands of the cell and are generally higher in males than females. By crossing hetero- and homoplasmic flies, we find that α2 can be transmitted to their offspring via both parents and that levels are dependent on nuclear background. Following α2 mitotype levels during embryogenesis, we demonstrate that this secondary mitotype replicates rapidly just after fertilization of the egg in a period when primary mitochondria are dormant. This so-called “Replication precox” mitochondrial phenotype likely prevents the α2 mitotype from being outcompeted by the primary mitotype – and thereby secures its persistence and further spread as a selfish mitochondrion, we hereby designate “Spartacus”. Finally, we reconstruct the evolutionary history of mitochondria in the willistoni subgroup uncovering signs of multiple mitochondrial losses and introgressions. Our data indicate an α-like mitochondrial ancestor in the willistoni subgroup, with the β mitotype likely acquired via introgression from an unidentified donor. We hypothesize that the selfish characteristics of α2 might have emerged as a response to competition for inheritance with the introgressed β mitotype.