Enhanced Desiccation Resistance Research Articles

Does pre-exposure to mild drought enhance desiccation resistance in Collembola?

Does pre-exposure to mild drought enhance desiccation resistance in Collembola?

Metabolic Cost of a Nutritional Symbiont Manifests in Delayed Reproduction in a Grain Pest Beetle.

Simple SummaryAnimals engage in various symbioses. However, these interactions are not always beneficial for the host; they can also incur costs under certain circumstances. The bacterial symbiont supports, on the one hand, the cuticle formation of the sawtoothed grain beetle Oryzaephilus surinamensis, which is extremely beneficial under dry conditions as a thicker and more melanized cuticle prevents desiccation of the insect. On the other hand, under higher humidity, the benefit is strongly reduced. In this study, we investigated whether harboring a symbiont can also be a disadvantage. Therefore, we first measured the number of symbionts throughout the beetles’ life and found a strong increase during the end of metamorphosis, just before beetles reach adulthood. Afterwards, males lose the symbionts again, whereas females retain a stable number. A comparison of beetles with and without symbionts revealed no differences in many life history traits. Larval development took the same time and there was also no difference in adult mortality or lifespan or the number of offspring of females. However, females with symbionts started to reproduce significantly later by one to two weeks, meaning they have a disadvantage in comparison to females without symbionts. Thus, harboring a symbiont is beneficial or costly in a context-dependent manner.Animals engage in a plethora of mutualistic interactions with microorganisms that can confer various benefits to their host but can also incur context-dependent costs. The sawtoothed grain beetle Oryzaephilus surinamensis harbors nutritional, intracellular Bacteroidetes bacteria that supplement precursors for the cuticle synthesis and thereby enhance desiccation resistance of its host. Experimental elimination of the symbiont impairs cuticle formation and reduces fitness under desiccation stress but does not disrupt the host’s life cycle. For this study, we first demonstrated that symbiont populations showed the strongest growth at the end of metamorphosis and then declined continuously in males, but not in females. The symbiont loss neither impacted the development time until adulthood nor adult mortality or lifespan. Furthermore, lifetime reproduction was not influenced by the symbiont presence. However, symbiotic females started to reproduce almost two weeks later than aposymbiotic ones. Thus, symbiont presence incurs a metabolic and context-dependent fitness cost to females, probably due to a nutrient allocation trade-off between symbiont growth and sexual maturation. The O. surinamensis symbiosis thereby represents an experimentally amenable system to study eco-evolutionary dynamics under variable selection pressures.

Transcriptional Control of Quality Differences in the Lipid-Based Cuticle Barrier in Drosophila suzukii and Drosophila melanogaster.

Cuticle barrier efficiency in insects depends largely on cuticular lipids. To learn about the evolution of cuticle barrier function, we compared the basic properties of the cuticle inward and outward barrier function in adults of the fruit flies Drosophila suzukii and Drosophila melanogaster that live on fruits sharing a similar habitat. At low air humidity, D. suzukii flies desiccate faster than D. melanogaster flies. We observed a general trend indicating that in this respect males are less robust than females in both species. Xenobiotics penetration occurs at lower temperatures in D. suzukii than in D. melanogaster. Likewise, D. suzukii flies are more susceptible to contact insecticides than D. melanogaster flies. Thus, both the inward and outward barriers of D. suzukii are less efficient. Consistently, D. suzukii flies have less cuticular hydrocarbons (CHC) that participate as key components of the cuticle barrier. Especially, the relative amounts of branched and desaturated CHCs, known to enhance desiccation resistance, show reduced levels in D. suzukii. Moreover, the expression of snustorr (snu) that encodes an ABC transporter involved in barrier construction and CHC externalization, is strongly suppressed in D. suzukii. Hence, species-specific genetic programs regulate the quality of the lipid-based cuticle barrier in these two Drosophilae. Together, we conclude that the weaker inward and outward barriers of D. suzukii may be partly explained by differences in CHC composition and by a reduced Snu-dependent transport rate of CHCs to the surface. In turn, this suggests that snu is an ecologically adjustable and therefore relevant gene in cuticle barrier efficiency.

The chicken or the egg? Adaptation to desiccation and salinity tolerance in a lineage of water beetles.

Transitions from fresh to saline habitats are restricted to a handful of insect lineages, as the colonization of saline waters requires specialized mechanisms to deal with osmotic stress. Previous studies have suggested that tolerance to salinity and desiccation could be mechanistically and evolutionarily linked, but the temporal sequence of these adaptations is not well established for individual lineages. We combined molecular, physiological and ecological data to explore the evolution of desiccation resistance, hyporegulation ability (i.e., the ability to osmoregulate in hyperosmotic media) and habitat transitions in the water beetle genus Enochrus subgenus Lumetus (Hydrophilidae). We tested whether enhanced desiccation resistance evolved before increases in hyporegulation ability or vice versa, or whether the two mechanisms evolved in parallel. The most recent ancestor of Lumetus was inferred to have high desiccation resistance and moderate hyporegulation ability. There were repeated shifts between habitats with differing levels of salinity in the radiation of the group, those to the most saline habitats generally occurring more rapidly than those to less saline ones. Significant and accelerated changes in hyporegulation ability evolved in parallel with smaller and more progressive increases in desiccation resistance across the phylogeny, associated with the colonization of meso- and hypersaline waters during global aridification events. All species with high hyporegulation ability were also desiccation-resistant, but not vice versa. Overall, results are consistent with the hypothesis that desiccation resistance mechanisms evolved first and provided the physiological basis for the development of hyporegulation ability, allowing these insects to colonize and diversify across meso- and hypersaline habitats.

Temperature and photoperiod affect stress resistance traits in Drosophila melanogaster

AbstractThe long‐term survival of species and populations depends on their ability to adjust phenotypic values to environmental conditions. In particular, the capability of dealing with environmental stress to buffer detrimental effects on fitness is considered to be of pivotal importance. Resistance traits are readily modulated by a wide range of environmental factors. In the present study, Drosophila melanogaster Meigen is used to investigate plastic responses to temperature and photoperiod in stress resistance traits. The results reveal that stress resistance traits (cold, heat, starvation and desiccation resistance) are affected by the factors temperature and sex predominantly. Cooler temperatures compared with warmer temperatures increase cold tolerance, desiccation and starvation resistance, whereas they reduce heat tolerance. Except for heat resistance, females are more stress‐resistant than males. Stress resistance traits are also affected by photoperiod. Shorter photoperiods decrease cold tolerance, whereas longer photoperiods enhance desiccation resistance. Overall, thermal effects are pervasive throughout all measured resistance traits, whereas photoperiodic effects are of limited importance in the directly developing (i.e. nondiapausing) flies used here, suggesting that pronounced photoperiodic effects on stress resistance traits may be largely limited to, and triggered by, diapause‐inducing effects.

Retention and adaptiveness of photoperiodic EGG diapause in Florida populations of invasive Aedes albopictus.

Female Aedes albopictus, F2-F3 descendents from individuals collected as immatures at 6 geographic sites in the USA during 2008, exposed to short daylengths (10 h of light and 14 h of darkness at 21 degrees C) laid eggs in diapause, whose frequency depended upon population origin. Diapause responses in northern Florida and Illinois were strong, as had been reported approximately 10 years earlier for Ae. albopictus from these regions. For southern Florida, the diapause response was polymorphic, and its mean incidence decreased at 2 of 3 collection sites compared to 10 years earlier. Exposure in the field for 2- to 4-wk intervals in Vero Beach (lat 27 degrees 35'N) during January 2009 revealed that eggs laid by short-day females had significantly higher survivorship, even though <50% were estimated, from laboratory results, to be in diapause. Enhanced desiccation resistance may select for retention of diapause in southern Florida.

O‐acetyltransferase gene neuO is segregated according to phylogenetic background and contributes to environmental desiccation resistance in Escherichia coli K1

Escherichia coli K1 causes disease in humans and birds. Its polysialic acid capsule can be O-acetylated via phase-variable expression of the acetyltransferase NeuO encoded by prophage CUS-3. The role of capsule O-acetylation in ecological adaptation or pathogenic invasion of E. coli K1 is largely unclear. A population genetics approach was performed to study the distribution of neuO among E. coli K1 isolates from human and avian sources. Multilocus sequence typing revealed 39 different sequence types (STs) among 183 E. coli K1 strains. The proportion of the ST95 complex (STC95) was 44%. NeuO was found in 98% of the STC95 strains, but only in 24% of other STs. Grouping of STs and prophage genotypes revealed a segregation of prophage types according to STs, suggesting coevolution of CUS-3 and the E. coli K1 host. Within the STC95, which is known to harbour both human and avian pathogenic isolates, CUS-3 genotypes were shared irrespective of the host species. Functional analysis of a variety of strain pairs revealed that NeuO-mediated K1 capsule O-acetylation enhanced desiccation resistance. In contrast, NeuO expression led to a reduced biofilm formation in biofilm positive E. coli K1 isolates. These findings suggest a delicate ecological balance of neuO'on'/'off' switching.

Inversion 2La is associated with enhanced desiccation resistance in Anopheles gambiae

BackgroundAnopheles gambiae, the principal vector of malignant malaria in Africa, occupies a wide range of habitats. Environmental flexibility may be conferred by a number of chromosomal inversions non-randomly associated with aridity, including 2La. The purpose of this study was to determine the physiological mechanisms associated with the 2La inversion that may result in the preferential survival of its carriers in hygrically-stressful environments.MethodsTwo homokaryotypic populations of A. gambiae (inverted 2La and standard 2L+a) were created from a parental laboratory colony polymorphic for 2La and standard for all other known inversions. Desiccation resistance, water, energy and dry mass of adult females of both populations were compared at several ages and following acclimation to a more arid environment.ResultsFemales carrying 2La were significantly more resistant to desiccation than 2L+a females at emergence and four days post-emergence, for different reasons. Teneral 2La females had lower rates of water loss than their 2L+a counterparts, while at four days, 2La females had higher initial water content. No differences in desiccation resistance were found at eight days, with or without acclimation. However, acclimation resulted in both populations significantly reducing their rates of water loss and increasing their desiccation resistance. Acclimation had contrasting effects on the body characteristics of the two populations: 2La females boosted their glycogen stores and decreased lipids, whereas 2La females did the contrary.ConclusionVariation in rates of water loss and response to acclimation are associated with alternative arrangements of the 2La inversion. Understanding the mechanisms underlying these traits will help explain how inversion polymorphisms permit exploitation of a heterogeneous environment by this disease vector.

Fecundity inDrosophilafollowing Desiccation Is Dependent on Nutrition and Selection Regime

Life-history trade-offs of populations of Drosophila melanogaster selected for enhanced desiccation resistance (D populations) include increased pupal mortality, retarded development, increased longevity, and decreased fecundity. We tested the effects of acute desiccation on fecundity and the effects of various nutritional regimes on fecundity following a bout of desiccation in the D populations and their control populations (C populations). Female flies were desiccated for 8 h (C populations) or 24 h (D populations) and then recovered on one of four treatments: distilled water, a saline solution, a saline and sucrose solution, or food. Each treatment was tested in the presence and absence of a yeast supplement. Fecundity was then measured on an oviposition medium over a 12-h period. Fecundity was also measured in flies that did not undergo a desiccation stress. Yeast increased fecundity under all conditions examined in the C populations but not in the D populations. There was an overall effect of recovery treatment on fecundity but no effect of desiccation stress.

Plant senescence cues entry into diapause in the gall flyEurosta solidaginis: resulting metabolic depression is critical for water conservation

Mechanisms and possible cues for seasonal increases in desiccation resistance in larvae of the goldenrod gall fly Eurosta solidaginis, were examined before and after natural and premature plant senescence, or after being removed from their gall and placed in either 100, 95 or 75% relative humidity (RH). Rates of water loss were 8.6-fold lower, averaging 0.7+/-0.2 microg mm(-2) h(-1), in larvae from senescent gall tissue and after all RH treatments than in control larvae from pre-senescent plants. Enhanced desiccation resistance occurred quickly, within 3 days of removal from their gall. Contrary to most previous reports, a large majority of the increased desiccation resistance (approximately 85%) was due to reduced respiratory transpiration with the remainder being the result of a lowered cuticular permeability. Rates of cuticular water loss were reduced by the presence of a vapor pressure gradient between the larval hemolymph and environmental water vapor and were probably due to increases in cuticular lipids and/or production of the cryoprotectant glycerol. Metabolic rate was reduced by over fourfold, averaging 0.07+/-0.01 microl CO2 g(-1) h(-1), in larvae from senescent gall tissue and all RH treatments compared to larvae from pre-senescent plants. The magnitude of the reduction in metabolic rates indicated that these larvae had entered diapause. In addition, larvae entered diapause in response to removal from, or degeneration of, the gall tissue they feed, on rather than seasonal changes in temperature or photoperiod. The low metabolic rates of the diapausing larvae probably allowed them to dramatically reduce their respiratory transpiration and total rate of water loss compared with non-diapausing controls. Thus, diapause, with its associated lowered metabolic rate, may be essential for conserving water in overwintering temperate insects, which may be dormant for six or more months of the year.

Analyses of physiological evolutionary response.

Selection studies are useful if they can provide us with insights into the patterns and processes of evolution in populations under controlled conditions. In this context it is particularly valuable to be able to analyze the limitations of and constraints on evolutionary responses to allow predictions concerning evolutionary change. The concept of a selection pathway is presented as a means of visualizing this predictive process and the constraints that help define the population's response to selection. As pointed out by Gould and Lewontin, history and chance are confounding forces that can mask or distort the adaptive response. Students of the evolutionary responses of organisms are very interested in the effects of these confounding forces, since they play a critical role not only in the laboratory but also in natural selection in the field. In this article, we describe some methods that are a bit different from those used in most studies for examining data from laboratory selection studies. These analytical methods are intended to provide insights into the physiological mechanisms by which evolutionary responses to the environment proceed. Interestingly, selection studies often exhibit disparate responses in replicate populations. We offer methods for analyzing these disparate responses in replicate populations to better understand this very important source of variability in the evolutionary response. We review the techniques of Travisano et al. and show that these approaches can be used to investigate the relative roles of adaptation, history, and chance in the evolutionary responses of populations of Drosophila melanogaster to selection for enhanced desiccation resistance. We anticipate that a wider application of these techniques will provide valuable insights into the organismal, genetic, and molecular nature of the constraints, as well as the factors that serve to enhance or, conversely, to mask the effects of chance. Such studies should help to provide a more detailed understanding of the processes producing evolutionary change in populations.

Evolved patterns and rates of water loss and ion regulation in laboratory-selected populations of Drosophila melanogaster.

We have investigated water loss from, and ion regulation within, the hemolymph and tissues of five replicate populations of Drosophila melanogaster that have undergone laboratory selection for enhanced desiccation resistance (i.e. the D populations). We compared the patterns and rates of water loss and the ion content of the D populations prior to and during desiccation with those of five replicate control (C) populations. The net rate of water loss in the C flies was approximately 3-fold greater than that of the D flies during the initial hours of desiccation. After 8 h, both C and D flies had considerable reductions in water loss rate. During 24 h of desiccation, the tissue water content of the D flies was conserved, while the C flies were faced with significant loss of tissue water during the initial 8 h of desiccation. We propose that the increased hemolymph volume of the D flies plays a role in buffering water loss from the tissues. One consequence of this large hemolymph pool is that the hydrated D flies contained approximately seven times more sodium within the hemolymph than did the hydrated C flies. Despite a continual loss of hemolymph volume in the D flies during lengthy periods of desiccation, the sodium content of the hemolymph was significantly reduced only during a single event. We provide evidence that the regulation of extracellular sodium, as well as chloride, occurred via excretory processes during desiccation. In addition, whole-body potassium was not significantly decreased in the D flies during desiccation but was reduced (i.e. excreted) in the C flies; hence, we suggest that the potassium content paralleled tissue water level.