, 1999, Sørensen et al., 2003 and Sørensen, 2010). HSP expression is known to be induced by denatured proteins ( Ananthan et al., 1986 and Krebs, 1999). Thus, the lack of HSP up-regulation in N. noltii suggests that 25 °C were too low to induce protein denaturation. A higher temperature threshold for protein denaturation can be achieved through protein stability by 1) intrinsic factors such as amino-acid composition and 2) extrinsic factors besides HSPs such as thermostabilizing solutes ( Fields, 2001), e.g. 2,3-diphosphoglycerate in methanogenic bacteria ( Hensel and König, 1988) or sugars as protective osmolytes in seagrasses ( Gu et al., 2012). While thermostabilizing solutes enable more

plastic responses by increase or decrease of the respective solutes, LY2157299 intrinsic protein properties require a multitude of microevolutionary changes, e.g. changes in amino-acid composition, which only arise PF-01367338 in vitro over much greater time scales ( Fields, 2001). As both species co-occur in a wide range of habitats, extrinsic factors seem more likely to influence protein stability in both species; however, this requires further experimental investigation.

The seagrass populations from northern and southern European locations were chosen not only to provide biological replication to infer species differences, but also to gain insights into population differences from colder (northern) vs. warmer (southern) temperature habitats (Fig. S1). A common-stress-garden setup with a relatively long acclimation phase (~ 50 days) was chosen to minimize non-heritable components induced by the native habitat (Hoffmann ADAMTS5 et al., 2005 and Whitehead and Crawford, 2006). Population responses to heat were similar for Z. marina from both locations with 267 genes concordantly up-regulated during heat and very divergent in N. noltii with 28 genes up-regulated in the northern strongly responding population. The respective heat responsive (HR) genes showed signs for a constitutive up-regulation in the southern population of both species. This suggests that constitutive up-regulation of HR genes

in a species might be an adaptive mechanism of populations from different local temperature regimes to cope with elevated habitat temperatures, which can in general occur over microevolutionary time scales ( Bettencourt et al., 1999). A similar pattern with a higher constitutive expression of HSPs in species from habitats with higher characteristic temperatures was observed among species of lizards (Ulmasov et al., 1992 and Zatsepina et al., 2000) and ants (Gehring and Wehner, 1995), although such a pattern may not be general (e.g. see Bettencourt et al., 1999, Zatsepina et al., 2000 and Barua et al., 2008). Besides the constitutive up-regulation of HR genes, the strength of the inducible response might also play an important role (e.g. Bettencourt et al., 1999 and Feder and Hofmann, 1999). In Z.