The Society for Integrative and Comparative Biology
Adaptive Evolution in the Lab: Unique Phenotypes in Fruit Flies Comprise a Fertile Field of Study1
1 Department of Ecology and Evolutionary Biology, 321 Steinhaus Hall, University of California, Irvine, Irvine, California 92697-2525
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 SYNOPSIS INTRODUCTIONREDUCED LIPID CONTENT (AND...INCREASED HEMOLYMPH VOLUMEREGULATION OF SODIUMLARGE BODY MASS AND...CONCLUSIONReferences |
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Laboratory selection for desiccation resistance, which has been imposed on five replicate populations of Drosophila melanogaster for >200 generations, has resulted in enhanced survivability during periods of extreme water stress. The ability of these populations to persistently resist the fatal effects of desiccation is correlated with evolved physiological traits, namely preferential storage of carbohydrates (associated with reduced lipid reserves) and a dramatic increase in blood volume, which has led to a significant increase in extracellular sodium and chloride content, as well as body mass. When compared to other populations of this drosophilid species, these adaptive traits are unique. While some may argue against the value of evolved traits that have not been found in natural populations, we counter that such traits are of considerable value to the analyses of physiological functions, as well as the underlying mechanisms and evolutionary trajectories of these functions. We propose that multiple physiological consequences almost certainly derive from the evolution of these singular traits; and, furthermore, we discuss future directions for the elucidation of such consequences.
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INTRODUCTION |
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Laboratory selection studies have provided a wealth of biological insights. They allow specific conditions to be imposed on replicate populations, permitting evolutionary changes to occur in a constrained, or relatively unconstrained, environment. Such studies have been carried out most commonly and most productively with organisms having short generation times and that are easily maintained in the lab. As a result, we now have large bodies of data regarding evolutionary responses in bacteria, yeast, nematodes, Drosophila and mice (Hoffmann and Parsons, 1993
; Travisano et al., 1995; Rose et al., 1996; Garland et al., 2002; Bennett, 2003; Kliman et al., 2003; Munoz and Riddle, 2003; Riehle et al., 2003).
In recent years, a number of reviews have pointed out some of the drawbacks associated with selection studies (Gibbs, 1999; Harshman and Hoffmann, 2000; Rose et al., 2005). These include unintended selection criteria, inappropriate controls, variability in replicate populations, and disparate results under ostensibly identical selection regimes.
It can be argued, however, that the observation that lab selection results in a complexity and variability of responses represents an important insight into the processes and mechanisms of evolution (e.g., Garland, 2003; Bradley and Folk, 2004). We are moving out of a period of naiveté in which we felt that strict control of the environment during selection could lead to canalized responses. In fact, the diversity of responses under conditions where the experimentalists have done their utmost to control the conditions of selection, tells a great deal about the power and influence of chance and genetic diversity on evolutionary outcomes.
In many ways, it is evolutionary biologists who have been most disappointed in the results obtained in some selection studies. It has been difficult to come up with firm and repeatable evolutionary outcomes. This has made it difficult to formulate general rules regarding evolutionary trajectories. Even more disappointing has been the failure of laboratory populations to respond, as they move along their evolutionary trajectories, in ways that mimic the characteristics of wild populations thought to have undergone similar selection pressures. This has led to an appropriate level of caution in interpreting the results of laboratory selection, but has also led to the feeling that laboratory selection may not be valuable for understanding responses of natural populations.
For physiologists, the populations obtained through selection have been far less disappointing and have in fact provided a wealth of experimental material (Garland, 2003; Bennett, 2003). We will review in this article the outcome of selection studies carried out in our laboratory, and that of Michael Rose, using Drosophila melanogaster. We restrict ourselves to this single species and a single selection paradigm, namely selection for enhanced desiccation resistance. Despite these limitations in species and selection protocol, it will be seen that the populations produced in these studies provide a wealth of material for physiological studies in a variety of fields. The populations are the products of evolution under conditions of natural laboratory selection. (The five ‘D’ populations have been selected for enhanced desiccation resistance for >200 generations. Each D population is paired with a single control population [C], which has not been subjected to selection for desiccation resistance. Each C–D pair was derived from a single ancestral population, which differs from the ancestor of all other C–D pairs.) The selected D populations are viable and show enhanced fitness under the conditions of selection (i.e., desiccation). The unique physiological characteristics of these populations can be exploited to better understand physiological mechanisms and control processes. Equally importantly, insights can be gained concerning: (1) the processes by which physiological systems evolve, both individually and in concert with other physiological systems and (2) the contribution of evolved traits to overall organismal function and homeostasis. We hope these examples will provide insights for students of comparative and evolutionary physiology and will serve to highlight the value of selection studies in analyzing physiological mechanisms and in studying the evolution of physiological processes.
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REDUCED LIPID CONTENT (AND FECUNDITY?) |
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The tradeoffs between life-history traits are hypothesized as central and limiting to the evolution of such traits (Stearns, 1992
; Roff, 1992). Physiological constraints that underlie most life-history tradeoffs remain unclear (e.g., Rose and Bradley, 1998) and are presumably difficult to demonstrate (Roff, 1992). We propose that our lab-selected, fruit fly populations comprise a system that may lead to a clearer understanding of how a physiological constraint (i.e., reduced lipid stores) shapes a life-history trait (i.e., fecundity). The manipulation of fecundity through the utility of prescribed lab-selection regimes has been achieved in various populations of Drosophila melanogaster (Rose, 1984; Luckinbill et al., 1984; Partridge et al., 1999; Srgo and Partridge, 1999) and has allowed researchers to examine the evolution of fecundity and mechanisms that limit it. By investigating the evolution of fecundity in our lab-selected flies, a better understanding of the physiological constraints upon fecundity in drosophilids is highly feasible.
Selection for enhanced desiccation resistance has resulted in divergent patterns of energy storage between the control (C) and selected (D) females (Fig. 1, based on data from Chippindale et al., 1998; Folk et al., 2001). The total amount of dry mass does not differ between the D and C flies, yet the D females contain, on average, 
80% more carbohydrate than the C females (Folk et al., 2001); while the C females contain 50% more lipid (Chippindale et al., 1998). (It is important to note that, despite this significant difference in lipid content between C and D flies, cuticular lipids do not differ quantitatively [Gibbs et al., 1997].) The diets of the C and D populations are identical; both groups consume a carbohydrate-rich food. The major ingredients of the food include bananas, dark and light Karo corn syrup, and barley malt syrup. Insects, in general, have the metabolic capacity to convert dietary carbohydrates to lipids and to store lipids within the fat body. The D and C flies presumably have these metabolic capacities, yet it appears that the relevant metabolic pathways are modified such that biosynthesis and storage of carbohydrate, primarily glycogen, is enhanced in the D flies. It has been proposed that a glycogen-rich phenotype increases desiccation resistance due to water of hydration, which binds to glycogen and becomes available for maintaining water balance during periods of desiccation (Gibbs et al., 1997; Chippindale et al., 1998; Folk and Bradley, 2004). In a previous study, we found a strong correlation between carbohydrate content (presumably glycogen) and desiccation resistance within the D populations (Folk et al., 2001). Furthermore, there is evidence that drosophilids preferentially metabolize carbohydrates (presumably glycogen) during extended periods of desiccation (Marron et al., 2003; D.G.F. and T.J.B., unpublished data).
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Associated with an increase in carbohydrate storage is a reduction in lipid stores in the D populations (Chippendale et al., 1998
). The decrease in lipid content may have implications relevant to oogenesis and, hence, fecundity. During oocyte development, yolk is accumulated rapidly and, as a result, the volume of an oocyte may increase up to 100,000-fold in drosophilids (Chapman, 1998). The yolk consists primarily of lipids and proteins and comprises approximately 90% of the oocyte volume. Lipid is stored within the oocyte principally as triacylglycerol and contributes up to 40% of the dry weight of the fully developed oocyte (Kawooya and Law, 1988). Most of the lipids that accumulate within the oocyte are synthesized in the fat body and then transported into the oocytes. Lipids are not only major constituents of the yolk, but are also vital to oocyte desiccation resistance. Drosophila melanogaster oocytes are protected from desiccation by a waxy layer located just beneath the chorionic layer of the eggshell (Papassideri, 1991). Lipids are therefore of paramount importance both in oogenesis and in waterproofing of the oocyte. We hypothesize that a diminution of lipid stores, such as that observed in the desiccation resistant females, may adversely affect egg production. We discuss this idea below.
An average 4-day-old, sexually mature D female contains 122 µg lipid, while an average C female of the same age contains 182 µg (Chippindale et al., 1998). Given that the average dry mass of a Drosophila melanogaster egg is 6 µg (Djawdan et al., 1996) and that approximately 40% of the dry mass of an egg is lipid, we estimate that a single egg contains 2.4 µg of lipid. Given the relatively higher lipid content in the C flies, an average 4-day-old C female has the potential capacity to produce 25 more eggs than an average 4-day-old D female. The first step in examining this hypothesis is to compare egg production between the C and D females, a study we are currently undertaking. Other experiments would involve providing groups of D females with diets supplemented with varying classes of lipids. Following consumption of lipid-supplemented food, the lipid content of the flies and their egg production could be measured to determine if fecundity had increased above levels measured in females fed the regular, carbohydrate-rich diet. Although metabolic processes in the D females appear biased toward carbohydrate biosynthesis and storage, consumption of dietary lipids might allow the females to channel more lipids into oogenesis. Such a finding would provide strong evidence for the hypothesis that oogenesis is constrained by a reduction in lipid biosynthesis and storage. Finally, many of the characters that have evolved within the five D populations, including lipid content, demonstrate considerable between-population variation (Fig. 1); thus, the predicted variance in egg production and in lipid content should allow an estimation of the strength of correlation between these two characters.
The hypothesis that differential lipid biosynthesis and storage forms a functional basis for life-history tradeoffs has been examined in crickets that exist in natural populations as two morphs: flightless and flight-capable (Zhao and Zera, 2002; Zera and Zhao, 2003). The flightless morph has higher rates of ovarian growth and contains greater amounts of phospholipids, purportedly important in oogenesis; while the flight-capable morph has delayed ovarian growth and contains greater amounts of triglyceride, which is used for fuel during flight. The differences in lipid biosynthesis and storage between the two morphs appear to derive from different rates of fatty acid synthesis and the allocation of fatty acids either into triglyceride or into phospholipids. These findings lead to postulations concerning the evolution of lipid biosynthesis and storage in the D populations: 1) Have the activities of the relevant metabolic enzymes (e.g., glycogen synthetase and/or fatty acid synthetase) evolved? 2) Have evolved changes in lipid metabolism impacted ovarian growth and development? 3) Has hormonal regulation of carbohydrate and lipid metabolism evolved? (For example, adipokinetic-hypertrehalosemic hormone [AKH-HTH] controls the breakdown of glycogen into glucose. Modifications in the regulation/secretion of AKH-HTH may result in a greater capacity for storing glycogen.) An explanation of the functional bases for tradeoffs constraining fecundity may lie in the answers to these questions.
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INCREASED HEMOLYMPH VOLUME |
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The D females have
35% greater wet mass than the controls, which is due entirely to an increase in body water content and principally to an increase in hemolymph volume (Fig. 2, based on data from Folk et al., 2001; Folk and Bradley, 2003). An average D female contains six-times the hemolymph volume of an average C female (Fig. 3, based on data from Folk and Bradley, 2003). (Hemolymph was estimated by measuring total wet mass; gently tearing the abdomen and blotting the hemolymph; and then measuring wet mass again. The difference between initial and final wet mass measurements was used to quantify hemolymph.) The elevated hemolymph volume protects the tissues from significant water loss for many hours during desiccation (Folk and Bradley, 2003). In addition to protecting tissues from dehydration, the hemolymph "is the medium through which all the chemical exchanges between the organs are affected, hormones conveyed, food carried from the gut, and waste products to the excretory organs" (Wigglesworth, 1972). The physiological consequences of a dramatic increase in hemolymph volume are predictably multifold.
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Glycogen (an animal starch) and trehalose (a non-reducing disaccharide) are the two most commonly stored carbohydrates in insects (Chapman, 1998
). Trehalose is the major blood sugar in adults of a number of Drosophila species (Kimura et al., 1992) and is a readily available source of energy (Wigglesworth, 1972). Because of the high volume of hemolymph, it follows that the D flies may contain greater quantities of trehalose, and indeed our preliminary data suggest that trehalose content in the hemolymph of hydrated D flies is significantly higher than that in C flies. A source of readily accessible carbohydrates is vital to Drosophila during desiccation, because of the tendency to oxidize carbohydrates when facing desiccation stress (Marron et al., 2003; D.G.F. and T.J.B., unpublished data).
Despite the dramatic increase in hemolymph volume, the hemolymph osmotic pressure in the hydrated D flies (315 mOsm/kg) is only slightly lower than that of hydrated C flies (353 mOsm/kg) (Albers and Bradley, 2004). The absolute amounts of the principle inorganic ions in the hemolymph, namely sodium and chloride, have increased concurrently with hemolymph volume in the D flies (Folk and Bradley, 2003; see below for a discussion of the Na+ and Cl– hemolymph concentrations). In addition to inorganic osmolytes, insect hemolymph contains high levels of free amino acids (Nation, 2002). Amino acids have been implicated in the osmoregulation of insect hemolymph during desiccation, a stress during which hemolymph volume is lost and inorganic ions are excreted (Hadley, 1994). The hemolymph amino acid content of the C and D flies remains to be determined, but amino acids comprise 20% of the total osmotic concentration in the hemolymph of larval Drosophila melanogaster (Pierce et al., 1999). Overall, the hemolymph composition of Drosophila melanogaster and the dynamic compositional changes that occur during water stress remain to be fully elucidated.
In insects, hormones are circulated and carried to their targets by the hemolymph. Hormone titers fluctuate during the life of an insect. These fluctuations are regulated and result from varying rates in hormone production, release, degradation, and removal (Chapman, 1998). Through feedback effects, low (or high) hormonal titers act to stimulate (or inhibit) hormone synthesis. A large increase in hemolymph volume may have interesting implications concerning hormonal action. As hemolymph volume expands in the D flies, physiological adjustments may be made to compensate for the possible dilution of hormones. Modifications may arise in receptor sensitivity (Bennett, 2003), in the feedback loop, or in the sensitivity of cells to hormone-receptor binding. Alternatively, greater amounts of hormone may be synthesized and released.
We have shown that accumulation of the "extra" hemolymph volume in the D flies does not occur until early adulthood; hence, hormones controlling development and molting in juveniles are not faced with possible dilution. However, hormones that regulate physiological homeostasis in adults may be diluted, up to six-fold. In insects, hormones that control carbohydrate metabolism, namely the adipokinetic/hypertrehalosemic family of hormones (AKH/HTH), are secreted by the corpora cardiaca and control the release of glucose, as well as the biosynthesis of trehalose, from glycogen stores (Gade, 1990; Nijhout, 1994). While some insects have multiple AKH/HTHs, Drosophila melanogaster has only one AKH (Siegert, 1999). It has been proposed that the secretion of AKH is regulated by carbohydrate concentration in the hemolymph. Is the corpora cardiaca in the D flies capable of synthesizing and secreting six-times the ‘normal’ amount of AKH to compensate for the six-fold increase in hemolymph, or has some other adjustment been made? AKH receptors, located on the surface of fat body cells, trigger increased synthesis of cAMP, which serves to activate phosphorylase b kinase, leading to phosphorylase activation, and the subsequent release of glucose from glycogen. Is the density of AKH receptors increased in the D flies in response to the dilution of AKH? Has hormone affinity been modified? Or are these modifications unnecessary because the only consequence of such a large blood volume is that hormone action is merely delayed?
The enlarged blood volume of the D flies is a unique fruit fly phenotype and is a rich source of biological material for the study of the evolution of hormonal regulation and homeostasis in insects. It is interesting to note that, while progress has been made in elucidating the chemistry and gene sequence of AKH/ HTHs, much remains to be known concerning the chemistry of the receptors, the receptor-hormone kinetics, and hormone function (Gade et al., 1997). The exploration of these issues in Drosophila melanogaster, a species in which genetic and molecular techniques are readily available, should be quite productive.
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REGULATION OF SODIUM |
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The class Insecta originated in the terrestrial environment. In this environment potassium, magnesium, calcium, chloride and bicarbonate are readily available, particularly for insects that feed on detritus or plant material. Sodium, by contrast, can be quite difficult to obtain for detritivores and herbivores in the terrestrial environment (Turunen, 1985
; Nation, 2002). As a result, insects have evolved hemolymph ion ratios that are low in sodium and quite distinct from those of marine organisms and vertebrates. The cationic component of the hemolymph is frequently replaced by potassium and free amino acids such as glutamate, proline, lysine and glycine (Sutcliffe, 1963; Florkin and Jeuniaux, 1974; Mullins, 1985; Gillott, 1991). A second consequence of insects' decreased reliance on sodium is their reliance on other ions to serve as the osmotic driving force for fluid secretion. The vertebrates, with their predominance of sodium chloride in the extracellular fluid, require active secretion of sodium for the production of fluid (e.g., in the salivary glands, pancreas, etc.). By contrast, salivary and primary urinary secretions in the insects are rich in potassium and low in sodium.
Interest in the capacities of insect cells and epithelia to transport sodium has lead to intense investigations of the mechanisms for sodium transport in the larvae of saline-water insects and in adults of bloodsucking insects (see e.g., Bradley and Phillips, 1975, 1977; O'Donnell and Maddrell, 1984; Beyenbach et al., 2000). In more recent years, as the techniques of cell biology and molecular biology have been directed at the questions of ion transport in insects, it has become apparent that the transport of cations in the Malpighian tubules and gut of insects is driven by a vacuolar hydrogen ATPase that energizes the apical plasma membrane through the active transport of hydronium ions (Schweikl et al., 1989). The large electrochemical gradient that results is then utilized to transport cations into the lumen through the actions of a K+/nH+ or Na+/ nH+ exchanger (Wieczorek et al., 1991). This process has now been implicated in the function of Malpighian tubules, rectum and midgut of a variety of insects (reviewed by Harvey et al., 1998).
The five replicate populations of Drosophila selected for enhanced desiccation resistance have recently been shown to be capable of producing a rapid secretion of sodium-rich urine during desiccation (Folk and Bradley, 2003). As previously discussed, the D populations have a hemolymph volume that is considerably larger than their control populations (Fig. 3). The hemolymph in dipterans contains appreciable sodium, and the hemolymph of hydrated D flies contains 7-fold more sodium than the hydrated control flies (Folk and Bradley, 2003). Despite this increase in sodium content of the hemolymph, the hemolymph sodium concentrations of the D and C flies are not significantly different. (Hemolymph sodium was determined by measuring whole body sodium content of flies and the sodium content in exsanguinated flies. Hemolymph sodium was then estimated as the difference of these two measurements.) During desiccation, water is lost preferentially from the hemolymph. As a result, the sodium concentration of the hemolymph rises. Both the D and C populations remove sodium from the hemolymph and excrete it as a means of regulating hemolymph sodium concentration, but this transport activity is much enhanced in the D flies (Folk and Bradley, 2003). At some point between 8 and 16 hours after the initiation of the desiccation bout, the D flies excrete 13.7 nmol of sodium per fly. (The highest rate of excretion observed in the C flies was 2.6 nmol of sodium per fly between hours 8 and 16 of desiccation.) The exact rate of secretion is not known since the precise time over which this excretion occurs has not been determined. By comparison, larvae of the saline-water mosquito Aedes taeniorhynchus, which have been intensively studied for their sodium-transporting capabilities (Bradley and Phillips, 1975, 1977; Bradley, 1985), excrete 8 nmol/hour when reared in full strength seawater. The amount of sodium handled by the fly is, therefore, within an order of magnitude of that excreted by a saline water mosquito larva, which has a specialized salt gland for handling this ionic species.
In the D populations, the Malpighian tubules are the likely site of secretion of the sodium-rich fluid. The Malpighian tubules of Drosophila have been the subject of considerable physiological analysis, involving electrophysiological, ultrastructural, and molecular techniques (O'Donnell et al., 1996; Wessing et al., 1999). Dow et al. (1997) examined the Malpighian tubules of Drosophila to determine the molecular processes by which sodium is transported across the plasma membranes of these organs. They found that no epithelial Na+ channel (EnaC) genes were expressed in the Malpighian tubules, but genes of the NHE family of sodium exchangers were expressed as sequence tags. The D populations have been shown to transport much more sodium than their controls under very explicit and reproducible environmental conditions (Folk and Bradley, 2003). The Malpighian tubules of flies from these populations are excellent subjects for the further exploration of sodium transport in insects. It would, for example, be of interest to determine if the total fluid transport capacities of the tubules are also enhanced, whether the exchange moieties are expressed differentially, and whether hormonal control of these processes has been modified. At present it is unclear which, if any, of these changes has occurred. By combining availability of flies exhibiting significantly enhanced sodium transport with analyses of ion levels in the hemolymph (Folk and Bradley, 2003), known hormones operating in the tubules (Cabrero et al., 2002), electrophysiological approaches, and molecular analyses, considerable progress in the analysis of cation transport in insects could be achieved.
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LARGE BODY MASS AND THE BIOMECHANICS OF FLIGHT |
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SYNOPSISINTRODUCTIONREDUCED LIPID CONTENT (AND...INCREASED HEMOLYMPH VOLUMEREGULATION OF SODIUMLARGE BODY MASS AND...CONCLUSIONReferences |
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Fly populations selected for enhanced desiccation resistance have responded with a dramatic increase in hemolymph volume, resulting in an increase in total body mass (Fig. 2 and 3). The D flies have
35% increase in mean body mass compared to their controls. As outlined below, this increased mass has significant implications for the most energetic activity in the life of the flies, namely flight.
Drosophila melanogaster has been a valuable model system for the analysis of flight kinematics, aerodynamics and mechanics. For example, Lehmann and Dickinson (1997) examined the limits of flight performance in untethered Drosophila. They found that the insect's peak mechanical power output could generate forces approximating 150% of the animal's weight. Increases in mechanical power output were achieved by increasing stroke amplitude and decreasing stroke frequency. Marden et al. (1997) used a computerized three-dimensional tracking system to quantify the maximum flight velocity of fruit flies during climbing flight. They used this technique to determine if there were changes in flight performance and/or motivation in flies selected for enhanced upwind flight ability. In additional studies, Lehman and Dickinson (2001), found that flies that were achieving a maximum power output exhibited a unique combination of stroke amplitude, stroke frequency and mean force coefficient, a combination they referred to as the insect's "kinematic envelope." Lehman et al. (2000), measured the metabolic cost of flight, respiratory pattern, and rate of respiratory water loss in Drosophila. They found that hovering flight was very close to the maximum cost observed for any flight pattern. Roberts et al. (2003) examined the effects of wing shape on flight performance in Drosophila. Using flies in which developmental abnormalities had been induced through heat shock in the pupal stage, these authors demonstrated that wing shape was a critical factor in the capacity of the flies to resist free fall.
It can be seen therefore that Drosophila is a useful model for examining the relationship between flight kinematics, muscle mechanics and metabolic costs. Flight behavior can be examined in free flight, in tethered animals, or in a virtual-reality flight arena. The D flies have proven to be fully functional and capable of flight in a variety of conditions including fully hydrated, when they have a 35% increase in body weight compared to their controls, and after 24 hours of desiccation, when their body composition resembles that of hydrated, control flies (Folk and Bradley, 2004). These populations offer the opportunity to study the flight kinematics of flies that have increased their body weight in a natural physiological manner, involving a process that has included selection for fitness consequences for numerous generations. This approach is similar to studies conducted on bees at varying temperatures and levels of nectar loading (Heinrich, 1993; Kammer and Heinrich, 1974). The D flies therefore offer interesting examples of high-body weight flies for the examination of flight dynamics, wing kinematics and wing shape. Maximum flight performance has been shown to produce a downward force equal to 1.5 times the weight of the body, so a 35% increase in body mass should have a substantial effect on flight performance (Lehman and Dickinson, 1997). It would be interesting to know whether the increase in body weight caused by selection (over >200 generations) has also led to a change in wing morphology. If no change in wing size has occurred, then wing loading will have increased substantially and this should lead to a substantial change in aerodynamics and kinematics. Alternatively, if wing morphology has changed, it would be very interesting to know which features of the wing have been modified to improve flight performance and compensate for the increase in body weight.
Similarly interesting insights into the energetics of flight are available. Lehmann et al. (2000) demonstrated that mechanical power output during flight can be measured in combination with metabolic costs in Drosophila. Activation of the flight muscles causes a 10-fold increase in metabolic rate in flying Drosophila. Flow-through respirometry provides a sensitive method for determining the metabolic cost of flight in a single insect at different body mass.
It can be seen that numerous techniques are available for the analysis of flight kinematics, muscle performance and metabolic costs in Drosophila. The availability of populations derived by laboratory natural selection provides new experimental material for examining the effects of body size on insect flight behavior and energetics. The capacity for conducting these studies in an insect species that has been the subject of intense genetic and genomic analysis makes these studies all the more attractive.
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CONCLUSION |
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We describe in this paper how laboratory selection experiments involving replicate populations of Drosophila melanogaster have "generate(d) biological novelt(ies)" (Bennett, 2003
), which are powerful tools for the analyses of physiological mechanisms. Through adaptation to the stress of desiccation, the selected populations have diverged from the control populations and show unique physiological changes, including a significant increase in carbohydrate content and decrease in lipid content, as well as a dramatic increase in blood volume, sodium content, and body size. Such changes in physiological characters have not been observed in wild populations of xeric drosophilids (Gibbs and Matzkin, 2001; Marron et al., 2003). We presume that tradeoffs exist for flies in nature that prevent an accumulation of six-times the normal blood volume or of very large amounts of glycogen. Yet we argue that the newly evolved traits comprise a fertile field for the study of insect physiology.
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ACKNOWLEDGMENTS |
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We thank Ted Garland Jr. and John Swallow for organizing the SICB symposium and inviting us to participate. We are grateful for the insightful comments of two anonymous reviewers and to Al Bennett for Bennett (2003)
. Our research and manuscript preparation were supported by University of California, Irvine and the National Science Foundation under Grants IBN-0079501 and IBN-0331571. |
FOOTNOTES |
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1 From the Symposium Selection Experiments as a Tool in Evolutionary and Comparative Physiology: Insights into Complex Traits presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 5–9 January 2004, at New Orleans, Louisiana.
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