Data from: 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 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 at the innermost site, likely reflecting
adaptation to the extreme temperature and salinity regimes, possibly due
to the selection of more resistant genotypes and the progressive
restriction of gene flow. Overall, this study helps us to disentangle the
genetic basis of seagrass adaptation to local environmental conditions and
may support future works on assisted evolution in seagrasses.