Evidence for Local Adaptation Associated With Temperature Despite Weak Latitudinal Structure in a Temperate Seagrass

Understanding genetic diversity and structure in natural populations is important for conservation and developing resilience into restored populations for the future. Populations at the lower latitude (or northern) range edges tend to have lower levels of genetic diversity and are poorly connected, although they may be locally adapted to warmer environments. The temperate seagrass, Posidonia sinuosa, occurs across a latitudinal gradient along the Western Australian coastline. Some meadows have experienced significant decline due to industrialisation and urbanisation of coastal areas. We investigated spatial genetic diversity, structure, and local adaptation using single nucleotide polymorphisms (SNPs). All meadows had similarly low levels of genetic diversity with high levels of inbreeding, despite high clonal diversity. Weak genetic structure was consistent with genetic connectivity, enabled by sea-surface seed dispersal. Approximately 6.1% of SNPs were identified as putatively adaptive and most were associated with temperature variables. Allelic turnover was associated with < 1.4°C temperature change, suggesting plants may be sensitive to small temperature changes. Meadows are currently growing in temperatures below their thermal optima, so may tolerate future projected temperatures up to a point. Importantly, this temperature increase is below the 1.5°C-2.0°C goal of the Paris Agreement and plants are unlikely to tolerate current projections for 2100, which equate to +3.2°C, in addition to extreme heatwave events. Conservation of fragmented, northern, warmer-adapted meadows will be important to assist recovery and persistence of damaged meadows into the future, as translocation of putatively warmer-adapted genotypes that have exhibited historic connectivity may be a viable strategy for restoration of impacted higher latitude meadows.