Chromosome-level genome assembly and methylome profile yield insights for the conservation of endangered loggerhead sea turtles.

BACKGROUND

Characterizing genetic and epigenetic diversity is crucial for assessing the adaptive potential of threatened populations and species in the face of climate change. Sea turtles are particularly vulnerable due to their temperature-dependent sex determination (TSD) system, which heightens the risk of extreme sex ratio bias and extinction under future climate scenarios. High-quality genomic and epigenomic resources will therefore support conservation efforts for these endangered flagship species with such plastic traits.

FINDINGS

We generated a chromosome-level genome assembly for the loggerhead sea turtle (Caretta caretta) from the globally important Cabo Verde rookery. Using Oxford Nanopore Technology (ONT) and Illumina reads followed by homology-guided scaffolding to the same species, we achieved a contiguous (N50: 129.7 Mbp) and complete (BUSCO: 97.1%) assembly, with 98.9% of the genome scaffolded into 28 chromosomes and 33,887 annotated genes. We also extracted the blood methylome profile from our ONT reads, which was confirmed to be representative of the reference population via whole-genome bisulfite sequencing of 10 additional loggerheads from the same population. Applying our novel resources, we revealed high conservation of synteny between sea turtle species, reconstructed population size fluctuations in line with major climatic events, and identified microchromosomes as key regions for monitoring genetic diversity and epigenetic flexibility. Isolating 199 TSD-linked genes, we further built a large network of functional protein associations and blood-based methylation patterns.

CONCLUSIONS

We present a high-quality loggerhead sea turtle genome and methylome from the globally significant East Atlantic population. By leveraging ONT sequencing, we generate genomic and epigenomic resources simultaneously and showcase the potential of this approach for driving molecular insights for conservation of endangered sea turtles.