Gene expression was unaffected by long-term exposure to elevated temperature and pCO2 conditions
Elevated temperature and p CO2 conditions had a negative impact on survival and size of the corals used in this study (Table 1), yet gene expression within an offspring group did not differ between ambient and elevated conditions (Figure 1). Nevertheless, gene expression changes under short-term acute stress are commonly found in coral. This often involves the regulation of genes encoding heat shock proteins, ion transport, apoptosis, immune responses and/or oxidative stress (Barshis et al., 2013; Desalvo et al., 2008; Meyer et al., 2011; Ruiz-Jones & Palumbi, 2017). The absence of DEGs in corals under ambient versus elevated conditions was unexpected and may be due to the relatively mild and long-term nature of the treatments. The elevated conditions of this study (ambient +1 °C, 685 ppm p CO2) were relatively mild compared to many other longer-term studies (e.g., ambient +7 and + 12 ºC, Maor-Landaw et al., 2017; 856-3880 ppmp CO2, Vidal-Dupiol et al., 2013). In addition, gene expression responses of corals under long-term stress have been shown to differ from those under short-term stress. Despite significant differences in CO2 concentration under control and natural CO2 seep sites (i.e., ~355 versus 998 ppm), only 61 DEGs were found in A. millepora from the two sites (Kenkel et al., 2017). Similarly, the expression of calcification-related genes changed significantly in A. millepora subjected to short-term (i.e., 3 days) high p CO2 exposure (Moya et al., 2012, 2015), but far fewer DEGs were found as exposure time increased (Moya et al., 2015; Rocker et al., 2015). Since cellular stress gene expression responses can be transient (Kültz, 2003), certain expression changes may only be detectable during the initial exposure and therefore fewer differentially expressed genes are generally found in long-term studies.