Evidence of backcross inviability and mitochondrial DNA paternal leakage in sea turtle hybrids

Hybridization is known to be part of many species' evolutionary history. Sea turtles have a fascinating hybridization system in which species separated by as much as 43 million years are still capable of hybridizing. Indeed, the largest nesting populations in Brazil of loggerheads (Caretta caretta) and hawksbills (Eretmochelys imbricata) have a high incidence of hybrids between these two species. A third species, olive ridleys (Lepidochelys olivacea), is also known to hybridize although at a smaller scale. Here, we used restriction site-associated DNA sequencing (RAD-Seq) markers, mitogenomes, and satellite-telemetry to investigate the patterns of hybridization and introgression in the Brazilian sea turtle population and their relationship with the migratory behaviours between feeding and nesting aggregations. We also explicitly test if the mixing of two divergent genomes in sea turtle hybrids causes mitochondrial paternal leakage. We developed a new species-specific PCR-assay capable of detecting mitochondrial DNA (mtDNA) inheritance from both parental species and performed ultra-deep sequencing to estimate the abundance of each mtDNA type. Our results show that all adult hybrids are first generation (F1) and most display a loggerhead migratory behaviour. We detected paternal leakage in F1 hybrids and different proportions of mitochondria from maternal and paternal species. Although previous studies showed no significant fitness decrease in hatchlings, our results support genetically-related hybrid breakdown possibly caused by cytonuclear incompatibility. Further research on hybrids from other populations in addition to Brazil and between different species will show if backcross inviability and mitochondrial paternal leakage is observed across sea turtle species.