Allen G. Gibbs

Lipid melting and cuticular permeability: new insights into an old problem

The idea that the physical properties of cuticular lipids affect cuticular permeability goes back over 65 years. This proposal has achieved textbook status, despite controversy and the general lack of direct supporting evidence. Recent work supports the standard model, in which lipid melting results in increased cuticular permeability. Surprisingly, although all species studied to date can synthesize lipids that remain in a solid state at environmental temperatures, partial melting often occurs due to the deposition of lipids with low melting points. This will tend to increase water loss; the benefits may include better dispersal of lipids or other compounds across the cuticle or improved communication via cuticular pheromones. In addition, insects with high melting-point lipids are not necessarily less permeable at low temperatures. One likely reason is variation in lipid properties within the cuticle. Surface lipids differ from one region to another, and biophysical studies of model mixtures suggest the occurrence of phase separation between melted and solid lipid fractions. Lipid phase separation may have important implications for insect water balance and chemical communication.

Gene transcription during exposure to, and recovery from, cold and desiccation stress in Drosophila melanogaster

We exposed adult male Drosophila melanogaster to cold, desiccation or starvation, and examined expression of several genes during exposure and recovery. Frost was expressed during recovery from cold, and was up‐regulated during desiccation. Desiccation and starvation (but not cold) elicited increased expression of the senescence‐related gene smp‐30. Desat2 decreased during recovery from desiccation, but not in response to starvation or cold. Hsp70 expression increased after 1 h of recovery from cold exposure, but was unchanged in response to desiccation or starvation stress, and Hsp23 levels did not respond to any of the stressors. We conclude that D. melanogasters responses to cold and desiccation are quite different and that care must be taken to separate exposure and recovery when studying responses to environmental stress.

Discontinuous gas exchange in insects: a clarification of hypotheses and approaches.

Many adult and diapausing pupal insects exchange respiratory gases discontinuously in a three‐phase discontinuous gas exchange cycle (DGC). We summarize the known biophysical characteristics of the DGC and describe current research on the role of convection and diffusion in the DGC, emphasizing control of respiratory water loss. We summarize the main theories for the evolutionary genesis (or, alternatively, nonadaptive genesis) of the DGC: reduction in respiratory water loss (the hygric hypothesis), optimizing gas exchange in hypoxic and hypercapnic environments (the chthonic hypothesis), the hybrid of these two (the chthonic‐hygric hypothesis), reducing the toxic properties of oxygen (the oxidative damage hypothesis), the outcome of interactions between O2 and CO2 control set points (the emergent property hypothesis), and protection against parasitic invaders (the strolling arthropods hypothesis). We describe specific techniques that are being employed to measure respiratory water loss in the presence or absence of the DGC in an attempt to test the hygric hypothesis, such as the hyperoxic switch and H2O/CO2 regression, and summarize specific areas of the field that are likely to be profitable directions for future research.

Effects of starvation and desiccation on energy metabolism in desert and mesic Drosophila

Energy availability can limit the ability of organisms to survive under stressful conditions. In Drosophila, laboratory experiments have revealed that energy storage patterns differ between populations selected for desiccation and starvation. This suggests that flies may use different sources of energy when exposed to these stresses, but the actual substrates used have not been examined. We measured lipid, carbohydrate, and protein content in 16 Drosophila species from arid and mesic habitats. In five species, we measured the rate at which each substrate was metabolized under starvation or desiccation stress. Rates of lipid and protein metabolism were similar during starvation and desiccation, but carbohydrate metabolism was several-fold higher during desiccation. Thus, total energy consumption was lower in starved flies than desiccated ones. Cactophilic Drosophila did not have greater initial amounts of reserves than mesic species, but may have lower metabolic rates that contribute to stress resistance.

RESOURCE ACQUISITION AND THE EVOLUTION OF STRESS RESISTANCE IN DROSOPHILA MELANOGASTER

Resistance to environmental stress is one of the most important forces molding the distribution and abundance of species. We investigated the evolution of desiccation stress resistance using 20 outbred Drosophila melanogaster populations directly selected in the laboratory for adult desiccation resistance (D), postponed senescence (O), and their respective controls (C and B). Both aging and desiccation selection increased desiccation resistance relative to their controls, creating a spectrum of desiccation resistance levels across selection treatments. We employed an integrative approach, merging data on the life histories of these populations with a detailed physiology of water balance. The physiological basis of desiccation resistance may be mechanisms enhancing either resource conservation or resource acquisition and allocation. Desiccation‐resistant populations had increased water and carbohydrate stores, and showed age‐specific patterns of desiccation resistance consistent with the resource accumulation mechanism. A significant proportion of the resources relevant to resistance of the stress were accumulated in the larval stage. Males and females of desiccation‐selected lines exhibited distinctly different patterns of desiccation resistance and resource acquisition, in a manner suggesting intersexual antagonism in the evolution of stress resistance. Preadult viability of stress‐selected populations was lower than that of controls, and development was slowed. Our results suggest that there is a cost to preadult resource acquisition, pointing out a complex trade‐off architecture involving characters distributed across distinct life‐cycle stages.

Physical properties of insect cuticular hydrocarbons: The effects of chain length, methyl-branching and unsaturation

Abstract The waterproofing abilities of insect cuticular lipids, consisting mainly of hydrocarbons, are thought to depend upon their biophysical properties. However, little is known regarding the effects of specific structural changes upon cuticular lipid properties. We examined the phase behavior of pure hydrocarbons differing in chain length, methyl-branching pattern, and unsaturation, using Fourier transform infrared spectroscopy. Melting temperatures ( T m ) of 21–40 carbon n -alkanes increased by 1–3°C for an increase in backbone chain length of one carbon atom. The effects of methyl-branching on hydrocarbon properties depended upon the location of the methyl group along the molecule. Melting temperatures of 25-carbon long methylpentacosanes decreased by over 30°C as the location of the methyl moiety was shifted from the terminal portion of the molecule to more internal positions. Addition of a second methyl branch had additional effects on T m . Unsaturation decreased T m by 50°C or more.

The role of larval fat cells in adult Drosophila melanogaster.

SUMMARY In the life history of holometabolous insects, distinct developmental stages are tightly linked to feeding and non-feeding periods. The larval stage is characterized by extensive feeding, which supports the rapid growth of the animal and allows accumulation of energy stores, primarily in the larval fat body. In Drosophila melanogaster access to these stores during pupal development is possible because the larval fat body is preserved in the pupa as individual fat cells. These larval fat cells are refractive to autophagic cell death that removes most of the larval cells during metamorphosis. The larval fat cells are thought to persist into the adult stage and thus might also have a nutritional role in the young adult. We used cell markers to demonstrate that the fat cells in the young adult are in fact dissociated larval fat body cells, and we present evidence that these cells are eventually removed in the adult by a caspase cascade that leads to cell death. By genetically manipulating the lifespan of the larval fat cells, we demonstrate that these cells are nutritionally important during the early, non-feeding stage of adulthood. We experimentally blocked cell death of larval fat cells using the GAL4/UAS system and found that in newly eclosed adults starvation resistance increased from 58 h to 72 h. Starvation survival was highly correlated with the number of remaining larval fat cells. We discuss the implications of these results in terms of the overall nutritional status of the larva as an important factor in adult survival in environmental stresses such as starvation.

A genetic polymorphism maintained by natural selection in a temporally varying environment

Environments that are crowded with larvae of the fruit fly, Drosophila melanogaster, exhibit a temporal deterioration in quality as waste products accumulate and food is depleted. We show that natural selection in these environments can maintain a genetic polymorphism with one group of genotypes specializing on the early part of the environment and a second group specializing on the late part. These specializations involve trade‐offs in fitness components. The early types emerge first from crowded cultures and have high larval feeding rates, which are positively correlated with competitive ability but exhibit lower absolute viability than the late phenotype, especially in food contaminated with the nitrogenous waste product, ammonia. The late emerging types have reduced feeding rates but higher absolute survival under conditions of severe crowding and high levels of ammonia. Organisms that experience temporal variation within a single generation are not uncommon, and this model system provides some of the first insights into the evolutionary forces at work in these environments.

The role of discontinuous gas exchange in insects: the chthonic hypothesis does not hold water

SUMMARY Insects breathe through valved openings (spiracles) in their cuticle. Many insects open and close their spiracles in a cyclic pattern (discontinuous gas-exchange cycles, or DGC). These cycles were observed over half a century ago, their hypothesized function being to minimize loss of water from the tracheal system. However, numerous recent studies have found that respiration accounts for a small fraction of total water loss, and that insects stop performing DGC at times when this pattern would be most useful. Thus, the importance of cyclic gas exchange for water conservation has been challenged. The leading alternative is the chthonic hypothesis, which proposes that DGC originated in insects from hypercapnic (high CO2) environments (e.g. burrows) to aid in release of carbon dioxide. By keeping the spiracles closed, insects would concentrate CO2 and increase the gradient for outward diffusion of CO2. CO2 would be released rapidly when the spiracles opened, and respiratory water loss would be reduced. The chthonic hypothesis therefore predicts that DGC minimizes the ratio of respiratory water loss to CO2 release relative to other modes of gas exchange. We tested the chthonic hypothesis by simultaneously measuring water loss and CO2 release in reproductive females (queens) of the seed-harvester ant Pogonomyrmex barbatus, a burrowing species from North American deserts. Queens used one of three patterns of gas exchange, discontinuous, cyclic and continuous. We resolved the problem of separating cuticular transpiration and respiratory water loss for individuals that used continuous gas exchange by developing a regression method that can be used across all patterns of gas exchange. The ratio of respiratory water loss to CO2 release did not differ among ants using different patterns of gas exchange, in contrast to the expectation of the chthonic hypothesis. Metabolic rate, however, varied with gas-exchange pattern, and was lowest for individuals that used discontinuous gas exchange, intermediate for individuals using cyclic gas exchange, and highest for individuals using continuous gas exchange.

No place to hide: microclimates of Sonoran Desert Drosophila

Abstract 1. We characterized year-round microclimate conditions (temperature and humidity) in and around necrotic cacti of the Sonoran Desert of southwestern North America. Necrotic cacti serve as host plants for four endemic species of Drosophila. 2. Flies in the field were exposed to high and variable temperatures, sometimes ranging between 40°C in a single 24-h period. Habitat temperatures often exceeded thermal tolerance limits measured in the laboratory. 3. The air inside necroses was more humid than the air outside, but was often warmer during the day than outside air. Thus, air pockets within necroses do not provide a thermal refuge. 4. Necrotic tissues inhabited by Drosophila larvae reached temperatures in excess of 40°C, as did the moist soil in which one of the endemic Drosophila species undergoes larval and pupal development. 5. We conclude that high temperatures provide a significant environmental stress for Sonoran Desert Drosophila, at all developmental stages.