Genetic heterogeneity in natural populations of Drosophila melanogaster for ability to withstand dessication

Sex differences in dispersal syndrome are modulated by environment and evolution

Dispersal syndromes (i.e. suites of phenotypic correlates of dispersal) are potentially important determinants of local adaptation in populations. Species that exhibit sexual dimorphism in their life history or behaviour may exhibit sex-specific differences in their dispersal syndromes. Unfortunately, there is little empirical evidence of sex differences in dispersal syndromes and how they respond to environmental change or dispersal evolution. We investigated these issues using two same-generation studies and a long-term (greater than 70 generations) selection experiment on laboratory populations of Drosophila melanogaster There was a marked difference between the dispersal syndromes of males and females, the extent of which was modulated by nutrition availability. Moreover, dispersal evolution via spatial sorting reversed the direction of dispersal×sex interaction in one trait (desiccation resistance), while eliminating the sex difference in another trait (body size). Thus, we show that sex differences obtained through same-generation trait-associations ('ecological dispersal syndromes') are probably environment-dependent. Moreover, even under constant environments, they are not good predictors of the sex differences in 'evolutionary dispersal syndrome' (i.e. trait-associations shaped during dispersal evolution). Our findings have implications for local adaptation in the context of sex-biased dispersal and habitat-matching, as well as for the use of dispersal syndromes as a proxy of dispersal.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

Pushing the limit: examining factors that affect anoxia tolerance in a single genotype of adult D. melanogaster

Drosophila melanogaster is a promiscuous species that inhabits a large range of harsh environments including flooded habitats and varying temperature changes. To survive these environments, fruit flies have adapted mechanisms of tolerance that allow them to thrive. During exposure to anoxic stress, fruit flies and other poikilotherms enter into a reversible, protective coma. This coma can be manipulated based on controlled environmental conditions inside the laboratory. Here we utilize a common laboratory raised strain of D. melanogaster to characterize adaptation abilities to better understand coma recovery and survival limitations. Our goal is to mimic the fly's natural environments (wet anoxia) and relate findings to a typical gas induced environment (dry anoxia) that is commonly used in a laboratory. Despite the abundance of research regarding acute and chronic anoxic exposure and cold stress, the literature is lacking evidence linking anoxic stress with variable environmental conditions such as animal age and stress duration. We present novel ways to assess coma recovery and survival using readily available laboratory tools. Our findings suggest that younger age, exposure to colder temperatures and wet environments increase resistance to anoxic stress.

Effects of size, sex and teneral resources on the resistance to hydric stress in the tephritid fruit fly Anastrepha ludens

Water availability is recognized as one of the most important factors in the distribution and activity of terrestrial organisms. In the case of insects, hydric stress imposes a major challenge for survival because of the small surface-area-to-volume ratio they exhibit. In general, stress resistance is expected to co-vary positively with size; however, this pattern can become obscured in insects that exhibit sexual size dimorphism, as sexes differ in size and/or shape and have dissimilar resource allocations. In the present study, we use an allometric-based approach to (i) assess the desiccation and starvation stress resistance of teneral Anastrepha ludens flies, (ii) disentangle the relationships between resistance, size and sex and (iii) examine the adult fly body differences in water and lipid contents before and after exposure to stress. After controlling for sexual size dimorphism, an allometric increase of resistance with overall size was observed for all stress-based treatments. The scaling exponents that define the proportion of increase resistance varied with size traits and with type and degree of hydric stress. In this allometric relationship, and also in the relationships between mass and wing length and between size and teneral resources, the sexes maintained similar scaling exponents but differed in the intercepts. Males were more resistant to stress than females; this competitive advantage is probably linked to greater amounts of teneral lipids and more water use during stress.

Effects of larval growth condition and water availability on desiccation resistance and its physiological basis in adult Anopheles gambiae sensu stricto

Background

Natural populations of the malaria mosquito Anopheles gambiae s.s. are exposed to large seasonal and daily fluctuations in relative humidity and temperature, which makes coping with drought a crucial aspect of their ecology.

Methods

To better understand natural variation in desiccation resistance in this species, the effects of variation in larval food availability and access to water as an adult on subsequent phenotypic quality and desiccation resistance of adult females of the Mopti chromosomal form were tested experimentally.

Results

It was found that, under normal conditions, larval food availability and adult access to water had only small direct effects on female wet mass, dry mass, and water, glycogen and body lipid contents corrected for body size. In contrast, when females subsequently faced a strong desiccation challenge, larval food availability and adult access to water had strong carry-over effects on most measured physiological and metabolic parameters, and affected female survival. Glycogen and water content were the most used physiological reserves in relative terms, but their usage significantly depended on female phenotypic quality. Adult access to water significantly influenced the use of water and body lipid reserves, which subsequently affected desiccation resistance.

Conclusions

These results demonstrate the importance of growth conditions and water availability on adult physiological status and subsequent resistance to desiccation.

Using experimental evolution to study the physiological mechanisms of desiccation resistance in Drosophila melanogaster

Data from populations undergoing experimental evolution can be used to make comparisons between physiologically differentiated populations and to determine evolutionary trajectories. Comparisons of long-established laboratory populations of Drosophila melanogaster that are strongly differentiated with respect to desiccation resistance are used to test alternative hypotheses concerning the mechanisms that fruit flies use to survive bouts of extreme desiccation. This comparative study supports the hypothesis that, in at least one case, D. melanogaster can evolve increased resistance to desiccation by decreasing water loss rates and by increasing bulk water content but not by increasing metabolic water content or dehydration tolerance. While glycogen was involved in water storage, its primary role was in water binding, not the production of metabolic water. Measurement of the trajectories of these component mechanisms during selection for desiccation resistance is used to demonstrate that water loss rate quickly plateaus in response to selection, while water content continues to improve. This disparity reveals the value of studying evolutionary trajectories and the need for longer-term selection studies in evolutionary physiology.

Levels of variation in stress resistance in drosophila among strains, local populations, and geographic regions: patterns for desiccation, starvation, cold resistance, and associated traits

Stress resistance traits in Drosophila often show clinal variation. Although these patterns suggest selection, there is generally no attempt to test how large differences at the geographical level are relative to levels of variation within and between local populations. Here we compare these levels in D. melanogaster from temperate Tasmania versus tropical northern Queensland by focusing on adult resistance to desiccation, cold and starvation stress, as well as associated traits (size, lipid content). For starvation and desiccation resistance, levels of variation were highest among strains from the same population. whereas there was little differentiation among local populations and a low level of differentiation at the geographic level. For adult cold resistance, there was local differentiation and strain variation but no geographic variation. For size (thorax length), geographic differentiation was higher despite some overlap among strains from the tropical and temperate locations. Finally, for lipid levels there was only evidence for variation among strains. The low level of differentiation among geographic locations for stress resistance was further verified with the characterization of isofemale strains from 18 locations along a coastal transect extending from Tasmania to northern Queensland. Crosses among some of the isofemale strains showed that results were not confounded by inbreeding effects. Strains derived from a cross between a tropical and temperate strain differed for all traits, and variation among strains for body size was higher than strain variation within the geographic regions. Unlike in previous studies, lipid content and starvation resistance were not correlated in any set of strains, but there was a correlation between cold resistance and lipid content. There was also a correlation between desiccation resistance and size but only in the geographic cross strains. These findings suggest a large amount of variation in stress resistance at the population level and inconsistent correlation patterns across experimental approaches.

Desiccation and starvation resistance in Drosophila: patterns of variation at the species, population and intrapopulation levels

A substantial number of Drosophila studies have investigated variation in desiccation and starvation resistance, providing an opportunity to test for consistent patterns of direct and correlated responses across studies and across the species and population levels. In general, responses to laboratory selection for these traits in D. melanogaster are rapid and indicate abundant genetic variation in populations. However, slower responses to selection for desiccation resistance occur in other species including D. simulans. Clines suggest adaptive divergence although specific selection pressures have not been documented empirically. Drosophila species differ markedly in desiccation and starvation resistance and there is also marked variation within species for desiccation resistance that may be linked to local climatic conditions. Laboratory selection experiments on starvation resistance in D. melanogaster suggest that changes in lipid content are largely responsible for resistance variation but this factor may be less important in explaining variation among species. For desiccation, lines with increased resistance show reduced rates of water loss but no changes in the minimum water content that flies can tolerate. Changes in life history traits are sometimes associated with altered levels of stress resistance. Increased starvation resistance is associated with longer development time and reduced early age reproduction in different studies. However, other associations are inconsistent between studies as in the case of stress resistance changing following selection for longevity. Multiple mechanisms may underlie genetic variation in stress resistance and future studies should address the evolutionary importance of the different mechanisms at the population and species levels.

Desiccation resistance in interspecific Drosophila crosses. Genetic interactions and trait correlations

We used crosses between two closely related Drosophila species, Drosophila serrata and D. birchii, to examine the genetic basis of desiccation resistance and correlations between resistance, physiological traits, and life-history traits. D. serrata is more resistant to desiccation than D. birchii, and this may help to explain the broader geographical range of the former species. A comparison of F2's from reciprocal crosses indicated higher resistance levels when F2's originated from D. birchii mothers compared to D. serrata mothers. However, backcrosses had a resistance level similar to that of the parental species, suggesting an interaction between X-linked effects in D. serrata that reduce resistance and autosomal effects that increase resistance. Reciprocal differences persisted in hybrid lines set up from the different reciprocal crosses and tested at later generations. Increased desiccation resistance was associated with an increased body size in two sets of hybrid lines and in half-sib groups set up from the F4's after crossing the two species, but size associations were inconsistent in the F2's. None of the crosses provided evidence for a positive association between desiccation resistance and glycogen levels, or evidence for a tradeoff between desiccation resistance and early fecundity. However, fecundity was positively correlated with body size at both the genetic and phenotypic levels. This study illustrates how interspecific crosses may provide information on genetic interactions between traits following adaptive divergence, as well as on the genetic basis of the traits.

Relationships between water balance properties and habitat characteristics in the sibling Hawaiian Drosophilids, D. mimica and D. kambysellisi

Four populations of Drosophila mimica and 1 population of D. kambysellisi collected at sites which differed in wetness were examined for several water balance characteristics. Net water loss per hour increased as a_v (relative humidity/100) decreased in all populations, but the rate of increase was lower in populations from dry sites. When exposed to 0.70 a_v, D. kambysellisi, which were from a rain forest, lost water faster and died sooner than did D. mimica. Two D. mimica collecting sites were divided into smaller units based on substrate type at one site and on litter wettness at the other site. The D. mimica at the first site were homogeneous with respect to the water balance properties studied here, but in the second site, there was evidence of population differentiation associated with litter wettness.

Adaptive strategies in natural populations of Drosophila : Ethanol tolerance, desiccation resistance, and development times in climatically optimal and extreme environments

Adult tolerance of ethanol vapour in a closed system containing 12% ethanol in solution, decreases in a cline from southern to northern Australia. However a Darwin population is more tolerant than predicted from its latitude. Ethanol tolerance races in Australia have almost certainly evolved within the last 100-150 years, because of resource unavailability prior to that time. Within populations, variation among isofemale strains is lowest in the climatically extreme southern Melbourne (37°S) and northern Darwin and Melville I. (11-12°S) populations. This suggests low resource diversity within extreme populations compared with the climatically less extreme Brisbane (28°S) and especially Townsville (19°S) populations. For desiccation resistance, the population rankings are: Darwin Melbourne > Townsville > Brisbane Melville I. and for development time, rankings are similar: Darwin Melbourne < Townsville < Brisbane Melville I.Therefore resource utilization heterogeneity is greatest in populations not greatly stressed by desiccation and where development times are extended. In total therefore, the utilization of a diversity of resources is a feature of populations tending somewhat towards a K-strategy; this is emphasized by the relative heterogeneity among isofemale strains of these populations for desiccation resistance and to a lesser extent development times.The D. melanogaster gene pool can be viewed as made up of climate-associated races. Since the ethanol tolerances of adjacent (and climatically similar) Darwin and Melville I. are very different, resource utilization races may occur within climatic races. Such a mosaic of resource utilization races are more likely in climatically extreme than in optimal habitats.

The design of mazes to study Drosophila behavior

Although mazes have been widely used in studying phototaxis, geotaxis, and, more recently, learning in Drosophila, there is no uniformity in maze design, and little is known about the effects such apparatus differences may have on behavior. The new maze design described here is based on T-junctions, molded individually in acrylic, and provides an inexpensive and standardized means of building mazes to any desired specification. The need for uniformity in maze design is demonstrated with an experiment on three variables at the start of a maze that affect the subsequent response of four strains of D. melanogaster in different ways. Some implications for future Drosophila research using mazes are considered.

The genetics of resistance to long-term exposure to CO2 in Drosophila melanogaster; an environmental stress leading to anoxia

Genetic heterogeneity in populations of D. melanogaster has been described for resistance to long-term exposure to CO2 (4 to 5 hours). Crosses between inbred strains, and between strains set up from single inseminated females collected in the wild show the importance of additive genes. Genetic activity for resistance and sensitivity was found on the X, 2 and 3 chromosomes.The mechanism of resistance was shown to be an anoxia effect since the effect of an N2 atmosphere was the same as that of CO2. A study of 18 strains collected in the wild revealed a positive correlation between metabolic rate as measured by O2 uptake and mortality under CO2, and negative correlations were found between body weight, and both mortality under CO2 and metabolic rate. These results are consistent with an anoxia effect. A further variable correlated with body weight is resistance to desiccation. Thus the anoxia effect is correlated with factors determining the distribution of the species in the wild.

Ether resistance in Drosophila melanogaster

Strains set up from single inseminated females of D. melanogaster from the wild differ in their resistance to the anaesthetics, ether and chloroform. The main differences between four selected extreme strains could be explained by additive genes, which in the case of ether resistance were located to regions of chromosomes 2 and 3. The lack of correspondence between ether and chloroform resistance between strains indicates that although the type of genetic architecture controlling the traits is similar, the actual genes differ, which is reasonable in view of their differing chemical structures. Quite high heritabilities were found for resistance to ether based on five inbred strains. No significant associations between resistance to ether and body weight, developmental rate or longevity were found.It is clear that resistance to both anaesthetics would be amenable to more detailed genetic analyses. It is pointed out that the general conclusions reached from such studies will have implications with respect to the effect of chemicals such as insecticides, not naturally present in nature.

Association of karyotype with body weight and resistance to desiccation in Drosophila pseudoobscura

The effect of karyotype on body weight and resistance to desiccation has been examined for the Standard (ST) and Chiricahua (CH) gene arrangements of Drosophila pseudoobscura obtained from Mather, California. Mean body weights fall in the order ST/CH > CH/CH > ST/ST, and variability of body weight in the order CH/CH > ST/ST > ST/CH in adult flies of both sexes. The relative resistance to desiccation in mass survival tests at 0% relative humidity, without food, is [Formula: see text] in males and [Formula: see text] in females. Karyotype has an effect on survival under desiccation, apart from its effect on body size.The data provide a further example of pronounced heterosis associated with a gene-arrangement polymorphism under extreme environmental conditions.

Genetic heterogeneity among the founders of laboratory populations of Drosophila melanogaster : V. Sternopleural and abdominal chaetae in the same strains

1. Single inseminated females of Drosophila melanogaster derived from the same population led to discrete strains for both Sternopleural and abdominal chaeta number in agreement with other work, which indicates that the wild populations have genes polymorphic for these traits. Variability varied significantly between strains for Sternopleural but not abdominal chaeta number. 2. Overall, the data show a weak correlation between the two traits. Considering this by strain, the correlations go from negative (but not significantly < 0) to positive (but significantly > 0). Therefore just as the rate of response to directional selection has been shown to be extremely rapid when based on strains extreme in the direction of the required selection response, it seems that a similar process may be useful for directional selection based on two traits simultaneously, by using only those strains showing a positive correlation.