Introduction
Understanding the ecological consequences of changing environments and extreme climatic events has grown more prominent as climate change scenarios predict more frequent and pronounced fluctuations in temperature (Vázquez et al. , 2017). The world’s climate is changing dramatically, to such an extent that the 90% probability interval for global warming from 1990 to 2100 predicts an increase in average temperatures ranging from 1.7 °C to 4.9 °C (IPCC (Rahmstorf & Coumou, 2011)). And while it is acknowledged that warming temperatures will have a significant impact on biodiversity (Gitay et al. , 2002; Meehl & Tebaldi, 2004; Dawson et al. , 2011; Shukeret al. , 2016), recent studies have shown that the increase in climatic variability and thermal extremes may also have a major impact on populations through a decrease in growth rates, reproduction and survival (Folguera et al. , 2009; Bozinovic et al. , 2016).
Experimental studies have shown that organisms are able to respond to thermal variability through plastic changes in thermal tolerance (Terblanche et al. , 2010; Chidawanyika et al. , 2017). Within-generation plasticity, which include both developmental and reversible plasticity, might impact future generations accelerating the adaptation to novel or fluctuating environments (Ho & Burggren, 2010; Ezard et al. , 2014). In contrast, transgenerational plasticity refers to phenotypic changes in the offspring generation as a response to environmental inputs experienced by the previous generation (Salinaset al. , 2013; Donelson et al. , 2017). Transgenerational plasticity have been described in several traits, including locomotor performance (Leroi et al. , 1994; Seebacher et al. , 2014; Cavieres et al. , 2019), thermal tolerance (Norouzitallab et al. , 2014) and metabolic rate (Donelson et al. , 2012; Le Royet al. , 2017), and enable the offspring to change adaptively according to parental information and avoid the time-lag between environmental signal and phenotypic response (Baker et al. , 2019).
While heat extreme events induce increased tolerance to high temperatures (Bozinovic et al. , 2011; Estay et al. , 2011), they may cause organisms to reduce energy allocation to reproduction (Ragland & Kingsolver, 2008; Roitberg & Mangel, 2016; Koussoropliset al. , 2017). In this sense, Royama (1992) proposed that the thermal environment can affect demographic parameters through nonlinear changes in fecundity and survival. Thus, high temperatures could negatively impact fitness (Estay et al. , 2011; Clavijo-Baquetet al. , 2014), even if organisms exhibit a seemingly compensatory plastic response in thermal tolerance. Most experimental studies assessing the impact of thermal conditions on animal performance focus on physiological performance, and few studies report those effects in conjunction with Darwinian fitness (Bozinovic et al. , 2011; Nyamukondiwa et al. , 2018).
In this vein, here we quantified within- and transgenerational plasticity of thermal tolerance and demographic parameters in the fruit fly Drosophila melanogaster , and their potential role to ameliorate the negative impact of increased temperature variability. Specifically, we evaluated critical thermal maximum and minimum (CTmax , CTmin ) as indicators of physiological thermal tolerances, and the demographic parameters net reproductive rate (i.e., the average number of offspring produced by an individual during its lifetime,\(R_{0}\), ), and generational time (i.e., the average time between the birth of a female and the birth of her first female offspring, \(T_{g}\)) as direct indicators of fitness of flies exposed to variable and constant thermal conditions (Royama, 1992; Pasztor et al. , 1996) (Figure 1).
Overall, we hypothesized that flies reared in variable environments would exhibit a trade-off between physiological and fitness related-traits, namely an increase in heat tolerance but with adverse effects on demographic parameters. We predicted that the negative effects on fitness might be buffered in the subsequent generation if the offspring encountered the same thermal environment as their parents.