A genetic analysis of variation for the ability to fly after exposure to thermal stress in Drosophila mojavensis

Computational Sequence Analysis of Inversion Breakpoint Regions in the Cactophilic Drosophila mojavensis Lineage

We investigated rates of chromosomal evolution in Drosophila mojavensis using whole-genome sequence information from D. mojavensis, Drosophila buzzatii, and Drosophila virilis. Drosophila mojavensis is a cactophilic species of the repleta group living under extreme ecological conditions in the deserts of the Southwestern United States and Northwestern México. The genome of D. buzzatii, another member of the repleta group, was recently sequenced and the largest scaffolds anchored to all chromosomes using diverse procedures. Chromosome organization between D. mojavensis and D. buzzatii was compared using MUMmer and GRIMM software. Our results corroborate previous cytological analyses that indicated chromosome 2 differed between these 2 species by 10 inversions, chromosomes X and 5 differed by one inversion each, and chromosome 4 was homosequential. In contrast, we found that chromosome 3 differed by 5 inversions instead of the expected 2 that were previously inferred by cytological analyses. Thirteen of these inversions occurred in the D. mojavensis lineage: 12 are fixed and one of them is a polymorphic inversion previously described in populations from Sonora and Baja California, México. We previously investigated the breakpoints of chromosome 2 inversions fixed in D. mojavensis. Here we characterized the breakpoint regions of the 5 inversions found in chromosome 3 in order to infer the molecular mechanism that generated each inversion and its putative functional consequences. Overall, our results reveal a number of gene alterations at the inversion breakpoints with putative adaptive consequences that point to natural selection as the cause for fast chromosomal evolution in D. mojavensis.

Thermal acclimation is not induced by habitat-of-origin, maintenance temperature, or acute exposure to low or high temperatures in a pit-building wormlion (Vermileo sp.)

Wormlions are sit-and-wait insect predators that construct pit-traps to capture arthropod prey. They require loose soil and shelter from direct sun, both common in Mediterranean cities, and explaining their high abundance in urban habitats. We studied different aspects of thermal acclimation in wormlions. We compared chill-coma recovery time (CCRT) and heat-shock recovery time (HSRT) of wormlions from urban, semi-urban and natural habitats, expecting those originating from the urban habitat to be more heat tolerant and less cold tolerant. However, no differences were detected among the three habitats. We then examined whether maintenance temperature affects CCRT and HSRT, and expected beneficial acclimation. However, CCRT was unaffected by maintenance temperature, while temperature affected HSRT in an opposite direction to our prediction: wormlions maintained under the higher temperatures took longer to recover. When testing with two successive thermal shocks, wormlions took longer to recover from both cold and heat shock after applying an initial cold shock. We therefore conclude that cold shock inflicts some damage rather than induces acclimation. Finally, both cold- and heat-shocked wormlions constructed smaller pits than wormlions of a control group. Smaller pits probably translate to a lower likelihood of capturing prey and also limit the size of the prey, indicating a concrete cost of thermal shock. In summary, we found no evidence for thermal acclimation related either to the habitat-of-origin or to maintenance temperatures, but, rather, negative effects of unfavorable temperatures.

Temperature fluctuations during development reduce male fitness and may limit adaptive potential in tropical rainforest Drosophila

Understanding the potential for organisms to tolerate thermal stress through physiological or evolutionary responses is crucial given rapid climate change. Although climate models predict increases in both temperature mean and variance, such tolerances are typically assessed under constant conditions. We tested the effects of temperature variability during development on male fitness in the rainforest fly Drosophila birchii, by simulating thermal variation typical of the warm and cool margins of its elevational distribution, and estimated heritabilities and genetic correlations of fitness traits. Reproductive success was reduced for males reared in warm (mean 24 °C) fluctuating (±3 °C) vs. constant conditions but not in cool fluctuating conditions (mean 17 °C), although fluctuations reduced body size at both temperatures. Male reproductive success under warm fluctuating conditions was similar to that at constant 27 °C, indicating that briefly exceeding critical thermal limits has similar fitness costs to continuously stressful conditions. There was substantial heritable variation in all traits. However, reproductive success traits showed no genetic correlation between treatments reflecting temperature variation at elevational extremes, which may constrain evolutionary responses at these ecological margins. Our data suggest that even small increases in temperature variability will threaten tropical ectotherms living close to their upper thermal limits, both through direct effects on fitness and by limiting their adaptive potential.

Ecological genomics of host shifts in Drosophila mojavensis

Advances in next-generation sequencing technologies have liberated our dependency on model laboratory species for answering genomic and transcriptomic level questions. These new techniques have dramatically expanded our breadth of study organisms and have allowed the analysis of species from diverse ecological environments. One such species is the cactophilic Drosophila mojavensis that inhabits the deserts of western North America. These insects feed and develop in the necrotic cacti, feeding largely on the microflora of the necrotic plant tissues. Drosophila mojavensis is composed of four geographically and ecologically separated populations. Each population (Baja California peninsula, mainland Sonoran Desert, Mojave Desert and Santa Catalina Island) utilizes the necrotic tissues of distinct cactus species. The differences in the nutritional and chemical composition of the necroses include a set of toxic compounds to which resident population must adapt. These ecological differences have facilitated many of the life history, behavior, physiological and genetic differences between the cactus host populations. Genomic resources have allowed investigators to examine the genomic and transcriptional level changes associated with the local adaptation of the four D. mojavensis populations, thereby providing further understanding of the genetic mechanism of adaptation and its role in the divergence of ecologically distinct populations.

A single hot event that does not affect survival but decreases reproduction in the diamondback moth, Plutella xylostella

Extremely hot events (usually involving a few hours at extreme high temperatures in summer) are expected to increase in frequency in temperate regions under global warming. The impact of these events is generally overlooked in insect population prediction, since they are unlikely to cause widespread mortality, however reproduction may be affected by them. In this study, we examined such stress effects in the diamondback moth, Plutella xylostella. We simulated a single extreme hot day (maximum of 40°C lasting for 3, 4 or 5 h) increasingly experienced under field conditions. This event had no detrimental effects on immediate mortality, copulation duration, mating success, longevity or lifetime fecundity, but stressed females produced 21% (after 3 or 4 h) fewer hatched eggs because of a decline in the number and hatching success of eggs laid on the first two days. These negative effects on reproduction were no longer evident in the following days. Male heat exposure led to a similar but smaller effect on fertile egg production, and exposure extended pre-mating period in both sexes. Our results indicate that a single hot day can have detrimental effects on reproduction, particularly through maternal effects on egg hatching, and thereby influence the population dynamics of diamondback moth.

Effects of exposure to short-term heat stress on male reproductive fitness in a soil arthropod

Ambient temperature is a key environmental factor influencing a variety of aspects of the ecology and evolution of ectotherms. Reproductive traits have been suggested to be more sensitive to thermal stress than other life history traits. This study investigated the direct and indirect effects of heat shock on male reproductive success in the widespread springtail Orchesella cincta. Male springtails were exposed to four temperature treatments: heat hardening (35.2°C for 1h), heat shock (37.2°C for 1h), heat hardening+heat shock (35.2°C for 1h, followed 15h later by 37.2°C for 1h), and control (20°C). The heat shock gene Hsp70 showed high expression in all the heat treatments, indicating that the treatments indeed induced thermal stress. Significant mortality was only found in the treatment with heat shock, both with and without heat hardening. A direct effect of heat treatment was found on time to first reproduction, which was significantly longer after heat shock (with or without heat hardening) than in the control treatment. There was no difference among treatments in the number of spermatophores produced in the first reproductive instar. Heat treatment also had indirect effects on male reproductive success. Females chose significantly more spermatophores from control males than from males that received heat shock, heat hardening or both. A high percentage of spermatophores produced by heat shocked males caused reproductive failure in females, but no significant differences among treatments were found. Our results suggest that not all traits were equally affected by the heat stress. Heat hardening did not protect reproductive traits against the negative effects of heat shock. The indirect effects of heat shock on reproduction may be equally important as the direct effects.

How does heat shock affect the life history traits of adults and progeny of the aphid parasitoid Aphidius avenae (Hymenoptera: Aphidiidae)?

Because insects are ectotherms, their physiology, behaviour and fitness are influenced by the ambient temperature. Any changes in environmental temperatures may impact the fitness and life history traits of insects and, thus, affect population dynamics. Here, we experimentally tested the impact of heat shock on the fitness and life history traits of adults of the aphid parasitoid Aphidius avenae and on the later repercussions for their progeny. Our results show that short exposure (1 h) to an elevated temperature (36 degrees C), which is frequently experienced by parasitoids during the summer, resulted in high mortality rates in a parasitoid population and strongly affected the fitness of survivors by drastically reducing reproductive output and triggering a sex-dependent effect on lifespan. Heat stress resulted in greater longevity in surviving females and in shorter longevity in surviving males in comparison with untreated individuals. Viability and the developmental rates of progeny were also affected in a sex-dependent manner. These results underline the ecological importance of the thermal stress response of parasitoid species, not only for survival, but also for maintaining reproductive activities.

Direct and correlated effects of selection on flight after exposure to thermal stress in Drosophila melanogaster

To demonstrate how insects may adapt to ecologically relevant levels of heat stress, we performed artificial selection on the ability of Drosophila melanogaster to fly after an exposure to a high but non-lethal thermal stress. Both tolerance and intolerance to heat stress arose very quickly, as only a few generations of selection were necessary to cause significant separation between high and low lines for heat tolerance. Estimates of heritability based on the lines artificially selected for increased flight ability ranged from 0.024 to 0.052, while estimates of heritability based on the lines selected for the inability to fly after heat stress varied between 0.035 and 0.091. Reciprocal F1 crosses among these lines revealed strong additive effects of one or more autosomes and a weaker X-chromosome effect. This variation apparently affected flight specifically; neither survival to a more extreme stress nor knockdown by high temperature changed between lines selected for high and low heat tolerance as measured by flight ability. As the well-studied heat-shock response is associated with heat tolerance as measured by survival and knockdown, the aspects of the stress physiology that actually affect flight ability remains unknown.