Signs of local adaptation by genetic selection and isolation promoted by extreme temperature and salinity in the Mediterranean seagrass Posidonia oceanica

Adaptation to local conditions is known to occur in seagrasses, however, knowledge of the genetic basis underlying this phenomenon remains scarce. Here, we analyzed Posidonia oceanica from six sites within and around the Stagnone di Marsala, a semi-enclosed coastal lagoon where salinity and temperature exceed the generally described tolerance thresholds of the species. Sea surface temperatures (SSTs) were measured and plant samples were collected for the assessment of morphology, flowering rate and for screening genome-wide polymorphisms using double digest restriction-site-associated DNA sequencing. Results demonstrated more extreme SSTs and salinity levels inside the lagoon than the outer lagoon regions. Morphological results showed significantly fewer and shorter leaves and reduced rhizome growth of P. oceanica from the inner lagoon and past flowering events were recorded only for a meadow farthest away from the lagoon. Using an array of 51,329 SNPs, we revealed a clear genetic struc..., Ninety-five ddRAD-seq library construction and sequencing were conducted at IGATech (Udine, Italy) using an IGATech custom protocol, with minor modifications with respect to Peterson’s double digest restriction-site associated DNA preparation (Peterson et al., 2012). To ensure the quality of sequencing outcomes, for each site, one sample was randomly selected and sequenced twice. The final number of biological replicates for each site was n = 14 for OpenSea-C and n = 15 for the other sites (i.e. North-basin, South-basin, OpenSea-A, OpenSea-B, and OpenSea-D), respectively (i.e. 89 unique samples + 6 technical replicates). In short, gDNA was double digested with both SphI and MboI endonucleases (New England BioLabs). Fragmented DNA was purified with AMPureXP beads (Agencourt) and subsequently ligated with T4 DNA ligase (New England BioLabs). Samples were pooled on multiplexing batches and bead purified as before and then they were size-selected and underwent several purification steps. dd..., , # Raw sequencing data and VCF files

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This dataset contains raw sequencing files and vcf files of a field experiment on the seagrass *Posidonia oceanic*. Samples were collected from six sites within and around the Stagnone di Marsala, a semi-enclosed coastal lagoon where salinity and temperature exceed the generally described tolerance thresholds of the species. This study aimed at better understanding the genetic basis of local adaptation in *P. oceanica* to extreme temperature and salinity.

The screening of genome-wide polymorphisms using double digest restriction-site-associated DNA sequencing revealed a clear genetic structure among the study sites and confirmed the genetic isolation and high clonality of the innermost site. Fourteen outlier loci were identified and annotated with several proteins including those relate to plant stress response, protein transport and regulators of plant-specific developmental events. Especially, five outlier loci showed maximum allele frequency...