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.