noltii revealed up-regulation
of 28 genes in response to heat in the northern N. noltii population, none of them encoding HSPs or genes of any functional category associated with the term “stress”. To investigate whether the 28 genes were also important during the heat response of the southern population, the normalized expression profiles were compared between all four N. noltii libraries. While the expression of the 28 “heat response” genes was in general strongest during heat in the northern population, they show intermediate expression levels in both southern N. noltii libraries ( Fig. 5; FDR α < 0.05, Fig. S7). This suggests an increased constitutive expression in the southern population for the 28 genes of the northern heat response. Population performance in response to the heat wave was measured using normalized changes in shoot abundance. A generalized linear model (GLM) Dabrafenib approach showed significant treatment and time point effects for both species (p-value < 0.05) with a negative effect of the heat treatment and a greater shoot loss towards the end of the experiment
(Table S3). For Z. marina, the negative effect of the heat treatment was weakest during acute heat on the northern population; the southern population performed better throughout the experiment (p-value < 0.05) (Fig. S8, Table S3). For N. noltii, no significant difference was found in performance between populations (p-value < 0.05, Table S3). The treatment effect was weakest during acute heat in the northern population MRIP (Fig. S8). Short-term reductions in growth were present in both species. In accordance with the expectation of N. noltii being more stress tolerant, we observed a Selleck Navitoclax higher temperature threshold for the induction of heat shock proteins in N. noltii compared to Z. marina, regardless of population origin. Moreover, we identified a higher constitutive expression
of heat responsive (HR) genes in populations from the southern location of both species, suggesting a possible mechanism for local adaptation. Our study supports earlier work on Z. marina showing a largely concordant acute heat stress response between populations from northern and southern European locations and the expected up-regulation of several heat shock proteins upon heat treatment ( Franssen et al., 2011a) (Table S4, Fig S6). Across locations, HSP up-regulation in Z. marina indicates molecular stress during the realistic heat wave scenario at water temperatures of 26 °C (see also Bergmann et al., 2010), which is further supported by detrimental effects on shoot abundance as well as reduction in growth rates and poorer photosynthetic performance shown in previous experiments ( Bergmann et al., 2010, Winters et al., 2011 and Gu et al., 2012). Heat stress responses, however, involve many thermal tolerance processes other than induction of HSP genes (Krebs, 1999, Larkindale et al., 2005, Wahid et al., 2007, Kotak et al., 2007 and Gu et al., 2012).