Desiccation stress is an important factor of environmental stress in terrestrial environments affected by climatic change and global warming. The ability to withstand events of low humidity is important in determining the distribution and abundance of organisms (Hoffmann and Parsons, 1991, Hoffmann et al., 2003, Terblanche et al., 2006, Chown et al., 2011). Geographic distribution patterns have been associated with genetic variation in stress resistance traits including desiccation (Sørensen et al., 2005, Kellermann et al., 2009, Boher et al., 2010, Strachan et al., 2011, Rajpurohit et al., 2018). Specialist species with low physiological tolerance to desiccation are restricted to the tropics and may lack the plasticity to increase stress resistance at the micro-evolutionary level (Kellermann et al., 2009, Telonis-Scott et al., 2012).

Typically, severe desiccation causes a rapid death. However, repeated exposure to sub-lethal stress of desiccation may trigger different responses in an organism, mainly by causing alterations in behavior, gene expression, and hardening or acclimation effects (Hoffmann, 1990, Hoffmann, 1991, Telonis-Scott et al., 2012, Wang et al., 2021). Therefore, survival time after sub-lethal episodes of desiccation stress can be an ecologically relevant trait in some environments. The ecology and physiology of desiccation stress is a focus of extensive research in Drosophila as a model insect. Because of their lower surface area to volume ratios, larger flies are expected to be more desiccation resistant than smaller flies. However, this expected relationship with body size is not always significant and other factors including size-related traits of some body parts, in addition to cuticular hydrocarbon composition (e.g., Joshua et al., 2020, Wang et al., 2022), could also be implied in desiccation survival (reviewed in Wang et al., 2021). Recent studies on the trade-off between frons and eye width (FW and EW, Fig. 1), for instance, suggested adaptive consequences of these developmentally-related parts of the Drosophila head (Cowley and Atchley, 1990, Norry et al., 2004, Posnien et al., 2012, Arif et al., 2013, Norry and Gomez, 2017, Keesey et al., 2019, Ramaekers et al., 2019, Gaspar et al., 2020, Casares and McGregor, 2021). In addition, some head structures like antennae, which co-vary in size with frons (Ramaekers et al., 2019, Keesey et al., 2019), play a role in moisture detection (Sun et al., 2018, Wang et al., 2021). Thus, it is interesting to test whether or not head traits in the mentioned trade-off can be associated to lifespan after desiccation stress.

Here, we used an intercontinental set of recombinant inbred lines (RIL), which segregate high variation for thermotolerance, to test for genetic variation in survival of D. melanogaster exposed to three desiccation episodes at relatively young ages. In order to compare the genomic distribution of quantitative trait loci (QTL) between survival time after non-lethal stress of desiccation, hereafter referred to as SSD, versus previously mapped QTL for time of survival under permanent desiccation until death (hereafter SPD), we QTL mapped SSD for comparison to SPD previously studied in the same set of RIL (Gomez et al. (2015). SPD time was typically lower than 4 days in previous work (Gomez et al., 2015), whereas SSD time can be dramatically longer than SPD time, allowing possible responses like hardening or acclimation to increase lifespan against repeated episodes of desiccation (Hoffmann, 1990, Hoffmann, 1991, Wang et al., 2021). Thus, the genetic architecture of both traits (SSD and SPD) might differ, as well as their possible associations with other phetotypic traits, Two main hypotheses were addressed. First, we tested the hypothesis that SSD is genetically variable and affected by large-effect QTLs as previously reported for SPD (Gomez et al., 2015). Second, we additionally tested the hypothesis of any possible associations between SSD time and the two negatively correlated parts of the head capsule in Drosophila (FW and EW, Fig. 1). As mentioned above, recent studies suggested possible ecological consequences of the two developmentally-related parts of the head in adult flies. Survival time after events of desiccation can be an ecologically relevant trait that to the best of our knowledge was never tested for any possible associations with both parts of the head capsule in Drosophila.