The curious case of northeast Brazil’s cross-breeding sea turtles | Horizon

Sea turtles have been around since dinosaurs roamed the Earth, stretching back about 110 million years. Yet now their existence is at risk, with six of today’s seven species classified as threatened or endangered.

While efforts are ongoing to protect turtles, part of the puzzle that requires more knowledge concerns the effects of interspecies mating, or hybridisation – particularly between hawksbills (Eretmochelys imbricata) and loggerheads (Caretta caretta) in one area of Brazil.

Hybridisation has been observed in five of the world’s existing species, yet much of this has been at low levels. However, in Brazil’s northeastern state of Bahia, genetic analysis has uncovered that as many as 30 to 40% of female hawksbills at some nesting sites are, in fact, first-generation hybrids resulting from cross-breeding with loggerheads.

These hybrids can then produce viable second-generation offspring by interbreeding, or ‘backcrossing’, with members of either parent species.

Understanding why this is happening may be important for informing conservation efforts because hybridisation can have consequences for species’ survival. It could also shed an intriguing light on the potential role played by hybridisation in long-term evolution, given that hawksbills and loggerheads are estimated to have diverged genetically as long as 30 million years ago.

‘You don’t see that a lot in the animal kingdom – species that have diverged so long ago and they’re capable of still hybridising,’ said Dr Sibelle Torres Vilaça, a population geneticist at the University of Ferrara in Italy. ‘There’s evolutionarily an interesting question there: what happens when these genomes meet again?’

Some cases of turtle hybridisation involve crosses between species that diverged more than 60 million years ago, which is very unusual. ‘If you think about primates, it would be something like humans and lemurs hybridising,’ said Dr Vilaça.


Dr Vilaça is investigating the causes of this hybridisation and its implications in the TurtleHyb project.

She says that interbreeding is aided in Bahia by the overlapping nesting seasons of hawksbills and loggerheads. Taking into account that the two species mature between the ages of 20 and 40, while two hybrid generations have been observed so far, her team thinks high-frequency cross-breeding started before 1980.[…]

Continue reading: The curious case of northeast Brazil’s cross-breeding sea turtles

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