Evolutionary persistence of a highly prevalent multicopy mitochondrial-derived nuclear insertion (Mega-NUMT) in Neotropical Drosophila flies

Although strict maternal transmission of mitochondria is a general feature of animals for ensuring homogeneity in mitochondrial DNA (mtDNA) across generations, exceptions were reported in the recent past. For example, some extremely rare but spectacular cases of heteroplasmy and paternal transmission in humans have questioned the universal evolutionary principle. Hence, as an alternative, the Mega-NUMT concept was coined to explain this discovery and was thereafter partly proven to exist. This concept expands on the quite common transfer of mtDNA fragments to the nucleus (NUMTs) by considering the existence of multicopy mitochondrial nuclear insertions. Mega-NUMT reports are currently restricted to a few cases in animals, including humans. However, their detailed genomic organization, natural prevalence, and potential biological functions remain unclear. Here, we discovered that up to 60 full-sized mitochondrial genomes are integrated into the nuclear genome of the neotropical Drosophila paulistorum using long-read sequencing and in situ hybridization. The copies are organized in one cluster on chromosome 3, which we designated the "Dpau Mega-NUMT". Contrary to the rarity in humans, this Mega-NUMT is found at high prevalence (40%) in both laboratory lines and natural D. paulistorum populations of different semispecies. Additionally, the Mega-NUMT copies are phylogenetically separated from the current mitotypes of D. paulistorum. Together, these observations suggest long-term maintenance of the Mega-NUMT in nature. Hence, we propose that the Dpau Mega-NUMT may have been transferred to the nuclear genome before the D. paulistorum semispecies radiation and speculate based on these findings that it possibly is maintained at relatively high prevalence in nature by balancing selection or due to a yet undetermined function.