Interspecific Gene Flow and Potential Hybrid Breakdown in South Atlantic Sea Turtles.

Interspecific hybridisation can play an important evolutionary role, generating new taxa or outbreeding depression, which can also lead to behavioural reproductive isolation through reinforcement. Still, there are few studies investigating how divergence time relates to hybridisation rates and its impact on introgression. Sea turtles are known to hybridise, despite their long divergence times and behavioural, ecological and morphological differences. Several genetic studies have attempted to determine the extent of hybridisation between four of the seven extant sea turtle species (Caretta caretta, Eretmochelys imbricata, Lepidochelys olivacea and Chelonia mydas) in the South Atlantic coast of Brazil, an area known to include nesting and feeding sites for all these species. In this work, we combined new and published genomic data of all four sea turtle species and their hybrids in a detailed population genomics study. Although hybridisation does occur in Brazilian nesting sites between three of the four species, it appears to be limited to first-generation hybrids and few backcrosses, which points to some postzygotic barriers between species and hybrid breakdown. Moreover, our evidence suggests a moderately high level of relatedness among hybrids, indicating a reduced number of independent hybridisation events. Despite this fact, we found evidence of backcrosses involving different species, and an F2 progeny between Ca. caretta x E. imbricata F1 hybrids, which suggests hybrid male fertility. Limited interspecific gene flow appears to be part of the long history of divergence of sea turtles, as low levels of deep time introgression were inferred. In summary, this work contributes important evidence regarding the rate and success of hybridisation at large scales of divergence, and how this seemingly equates to low rates of introgression, pointing to temporal limits to gene flow.