Combined use of mitochondrial and nuclear genetic markers further reveal immature marine turtle hybrids along the South Western Atlantic

Abstract

Marine turtle hybridization is usually sporadic and involves reports of only a few individuals; however, Brazilian populations have high hybridization rates. Here we investigated the presence of hybrids in morphologically identified immature hawksbills (Eretmochelys imbricata) along the South Western Atlantic (SWA). We sequenced one mitochondrial (D-Loop) and three nuclear DNA (RAG1, RAG2, and CMOS) markers to better understand the patterns and characteristics of hybrids. We identified 22 hybrids (n = 270), 11 of them at the extreme South of the SWA. Uruguay had the highest hybrid frequency in the SWA (~37.5%) followed by southern Brazil with 30%. These are common areas for loggerheads (Caretta caretta) but uncommon for hawksbills, and these hybrids may be adopting the behavior of loggerheads. By analyzing nuclear markers, we can infer that 50% of the sampled hybrids are first generation (F1) and 36% are the result of backcrosses between hybrids and pure E. imbricata (> F1). We also report for the first time immature E. imbricata x Lepidochelys olivacea hybrids at the Brazilian coast. Considering the high frequency of hybrids in the SWA, continuous monitoring should be performed to assess the fitness, genetic integrity, and extent of changes in the gene pools of involved populations.

Divergence and hybridization in sea turtles: Inferences from genome data show evidence of ancient gene flow between species

“…Sea turtles are known to hybridize frequently, especially along the Brazilian coast (Brito et al, 2020 ; Vilaça et al, 2012 ). Taken together, our phylogenomic results, ABBA‐BABA patterns, topology weighting throughout the genome, and hPSMC point towards ancestral gene flow between different sea turtle species.…”

“…The patterns of D between hawksbills/loggerheads and greens/loggerheads indicate that hybridization, albeit corresponding to a small percentage of the genome, was part of sea turtle's genome evolution. Although hybridization involving green turtles in Brazil is rarely seen (Vilaça et al, 2012 ), greens/loggerheads hybridize in other parts of the world (Brito et al, 2020 ), and ancient gene flow might have happened and left signals in the genomes of both species. Furthermore, the significant D values between hawksbills/loggerheads seem to be in agreement with the presence of hybrids in Brazil between these two species (Vilaça et al, 2012 ) and might suggest that hybridization between these two species has long been part of their evolutionary history.…”

“…In turn, the slow‐paced evolution may delay the onset of genomic incompatibilities after divergence, and this hypothesis is supported by the fact that turtle species with overlapping geographic distributions often hybridize (Buskirk et al, 2005 ; Fritz et al, 2008 ; Karl et al, 1995 ; Vilaça et al, 2012 ). In sea turtles, so far six hybrid combinations have been found between five species (olive ridleys, Kemp's ridleys, loggerheads, hawksbills and green turtles) and in diverse parts of the world including Canada, Brazil, Japan, and Australia (Brito et al, 2020 ). No in‐depth population studies have been performed in all six hybrid combinations to assess if they are fertile beyond first generation (F1) hybrids.…”

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

“…Genetic studies in the Praia do Forte population have been done since the 1980s and the presence of hybrids has been reported in all studies of this rookery (Arantes, Vilaça, et al, 2020;Bass et al, 1996;Brito et al, 2020;Conceição, Levy, Marins, & Marcovaldi, 1990;Karl et al, 1995;Lara-Ruiz et al, 2006;Proietti et al, 2014;Soares et al, 2018Soares et al, , 2017Vilaça et al, 2013Vilaça et al, , 2012. However, no adult backcross has been reported yet, indicating that there is evidence of strong hybrid breakdown in the second generation.…”

“…Other less common hybrids in Bahia include those between loggerheads x olive ridleys, hawksbills x olive ridleys (Soares et al, 2018(Soares et al, , 2021(Soares et al, , 2017Vilaça et al, 2012), and greens x loggerheads (Karl et al, 1995). Interestingly, most molecular studies involving morphologicallyidentified hawksbills found hybrids with loggerhead mitochondrial DNA (mtDNA), and very few hybrid individuals displayed hawksbill mtDNA (Bass et al, 1996;Brito et al, 2020;Lara-Ruiz et al, 2006;Proietti et al, 2014;Soares et al, 2018Soares et al, , 2017.…”