, 2007 and Kotak et al., 2007). Two hypotheses may explain the lack of HSP up-regulation in N. noltii. First, HSP expression may have been up-regulated earlier in the heat wave experiment and decreased while

the Quizartinib ic50 stress-temperatures continued; or secondly, the critical temperature threshold was not reached. Evidence supporting the first hypothesis has been found in N. noltii (and A. thaliana) at 38 °C, where HSP expression returned to pre-stress levels within several hours or days after heat stress was initiated (but before it was removed) ( Massa et al., 2011). Conversely, HSP up-regulation in Z. marina can persist for 1–3 weeks with a constant applied stress at only 26 °C ( Bergmann et al., 2010 and Franssen et al., 2011a). The mechanisms behind recovery to pre-stress Wnt inhibitor HSP expression levels during stress exposure vs. ongoing induction are not well studied and it is not known to what extent this effect depends on the strength of the applied heat stress. Regarding the second hypothesis, the lack

of HSP induction for N. noltii is due to a higher temperature threshold for HSP up-regulation relative to Z. marina. This correlation between habitat temperature and HSP up-regulation might be an indicator for different ecological niches, a phenomenon commonly observed between species pairs (summarized in Feder and Hofmann, 1999). Numerous examples include fucoid seaweeds ( Jueterbock et al., 2014), mussels (Mytilus), marine snails (Tegula), fruit flies (Drosophila), ants (Cataglyphis and Formica), yeast (Saccharomyces) ( Feder Sitaxentan and Hofmann, 1999), lizards ( Ulmasov et al., 1992) and shrubs (Prunus and Ceanothus) ( Knight, 2010), where congeners and/or related species occur in different ecological niches such as upper vs. lower intertidal areas ( Feder and Hofmann, 1999), south vs. north facing slopes ( Knight, 2010) or different climatic zones ( Ulmasov et al., 1992, Gehring and

Wehner, 1995, Hofmann and Somero, 1996 and Krebs, 1999). In each case, the species naturally occurring in the environment with higher temperatures have higher HSP induction thresholds, which usually differ by 2–7 °C ( Ulmasov et al., 1992, Hofmann and Somero, 1996 and Feder and Hofmann, 1999). For the Z. marina and N. noltii species pair, where long term heat treatment at 25 °C showed over-expression of HSPs in Z. marina (also see Bergmann et al., 2010; Franssen et al., 2011a), but not in N. noltii, the only additional study on N. noltii showed HSP up-regulation in response to a simulated low tide at ~ 38 °C ( Massa et al., 2011). Thus, the exact difference in HSP induction thresholds in Z. marina and N. noltii remains unknown. The lack of HSP induction in N. noltii at 26 °C, in contrast to Z. marina, may be adaptive.