CROSS-RESISTANCE FOLLOWING ARTIFICIAL SELECTION FOR INCREASED DEFENSE AGAINST PARASITOIDS IN DROSOPHILA MELANOGASTER

An increase in resistance to one natural enemy may result in no correlated change, a positive correlated change, or a negative correlated change in the ability of the host or prey to resist other natural enemies. The type of specificity is important in understanding the evolutionary response to natural enemies and was studied here in a Drosophila-paxasitoid system. Drosophila melanogaster lines selected for increased larval resistance to the endoparasitoid wasps Asobara tabida or Leptopilina boulardi were exposed to attack by A. tabida, L. boulardi and Leptopilina heterotoma at 15°C, 20°C, and 25°C. In general, encapsulation ability increased with temperature, with the exception of the lines selected against L. boulardi, which showed the opposite trend. Lines selected against L. boulardi showed large increases in resistance against all three parasitoid species, and showed similar levels of defense against A. tabida to the lines selected against that parasitoid. In contrast, lines selected against A. tabida showed a large increase in resistance to A. tabida and generally to L. heterotoma, but displayed only a small change in their ability to survive attack by L. boulardi. Such asymmetries in correlated responses to selection for increased resistance to natural enemies may influence host-parasitoid community structure.

Selection for resistance to a fungal pathogen in Drosophila melanogaster

An artificial selection experiment designed to explore the evolution of resistance to a fungal pathogen, Beauveria bassiana, in Drosophila melanogaster is reported here. The experiment was designed to test whether there is sufficient additive genetic variation in this trait for increased resistance to evolve, and, if so, whether there are correlated responses that might represent a cost to defence. After 15 generations of selection, flies from selected lines did not have higher overall fitness after infection compared with control lines. The response to selection for resistance against this pathogen is thus much weaker than against other species, in particular, parasitoids. There was, however, evidence for increased late-life fecundity in selected lines, which may indicate evolved tolerance of fungal infection. This increase was accompanied by reduced early-life fitness, which may reflect the well-known trade-off between early and late reproduction. In the absence of fungal infection, selected flies had lower fitness than control flies, and the possibility that this is also a trade-off with increased tolerance is explored.

Cost of resistance to parasites in digital organisms

Virtually all organisms are attacked by parasites and are therefore expected to evolve resistance against these natural enemies. Parasite resistance is costly in a wide range of organisms, although the generality of such costs has been questioned, especially when resistance is not based on reallocation of resources. Digital organisms are increasingly used to explore aspects of life in general. In the Tierra system, there is a trade-off between resistance against parasites and competitive ability. Because digital organisms are too simple to store resources, the finding that resistance to parasites is costly in digital organisms suggests that costs of parasite resistance can also occur when resistance is not resource based.

No trade-off between learning ability and parasitoid resistance in Drosophila melanogaster

Learning ability and immunity to parasites are linked at the physiological level in several insect species. The aim of this work was to investigate the relationship between learning and immunity at an evolutionary level. We tested whether selection for improved learning ability in Drosophila melanogaster led to changes in parasitoid resistance as a correlated response. Similarly, we assayed whether selection for better parasitoid resistance led to a change in learning ability. There was no significant difference between selected and control lines in either case; the estimated confidence intervals for the differences indicate that a trade-off relationship is unlikely.

Fitness effects and transmission routes of a microsporidian parasite infecting Drosophila and its parasitoids

A microsporidian infection was discovered in laboratory cultures of Drosophila species. Ultrastructural examination suggested it belonged to the poorly characterized species Tubulinosema kingi, and morphological and sequence data are presented. We explored how T. kingi affected the fitness of Drosophila melanogaster and D. subobscura, as well as the fitness of 2 of their parasitoids, Asobara tabida and Pachycrepoideus vindemiae. In Drosophila, infections caused changes in most of the traits we looked at that were associated with fitness, in particular causing a 34-55% reduction in early-life fecundity. Parasitoid fitness was affected more severely by infection than that of their hosts, with pupal mortality in particular increasing by 75-89%. We investigated the most important routes of transmission for T. kingi in a laboratory setting. Letting Drosophila larvae feed on medium contaminated with spores from infected dead flies resulted in 100% infection. Low levels of transmission (<10%) were found between larvae, and vertically between mothers and their offspring. Parasitoids developing in infected hosts all became infected, but infected adults were neither able to transmit the pathogen to their offspring nor to their offspring's Drosophila host, either directly, or via contamination of the ovipositor or other body parts. A field survey of Drosophila and their parasitoids in southern England revealed no natural infections. We discuss the potential importance of Microsporidia in parasitoid-host interactions, and for those working with Drosophila in the laboratory.

Costs of resistance in insect-parasite and insect-parasitoid interactions

Most, if not all, organisms face attack by natural enemies and will be selected to evolve some form of defence. Resistance may have costs as well as its obvious benefits. These costs may be associated with actual defence or with the maintenance of the defensive machinery irrespective of whether a challenge occurs. In this paper, the evidence for costs of resistance in insect-parasite and insect-parasitoid systems is reviewed, with emphasis on two host-parasitoid systems, based on Drosophila melanogaster and pea aphids as hosts. Data from true insect-parasite systems mainly concern the costs of actual defence; evidence for the costs of standing defences is mostly circumstantial. In pea aphids, the costs of standing defences have so far proved elusive. Resistance amongst clones is not correlated with life-time fecundity, whether measured on good or poor quality plants. Successful defence by a D. melanogaster larva results in a reduction in adult size and fecundity and an increased susceptibility to pupal parasitoids. Costs of standing defences are a reduction in larval competitive ability though these costs only become important when food is limited. It is concluded that costs of resistance can play a pivotal role in the evolutionary and population dynamic interactions between hosts and their parasites.

Clonal variation and covariation in aphid resistance to parasitoids and a pathogen

The potential rate of evolution of resistance to natural enemies depends on the genetic variation present in the population and any trade-offs between resistance and other components of fitness. We measured clonal variation and covariation in pea aphids (Acyrthosiphon pisum) for resistance to two parasitoid species (Aphidius ervi and A. eadyi) and a fungal pathogen (Erynia neoaphidis). We found significant clonal variation in resistance to all three natural enemies. We tested the hypothesis that there might be trade-offs (negative covariation) in defensive ability against different natural enemies, but found no evidence for this. All correlations in defensive ability were positive, that between the two parasitoid species significantly so. Defensive ability was not correlated with fecundity. A number of aphid clones were completely resistant to one parasitoid (A. eadyi), but a subset of these failed to reproduce subsequently. We discuss the factors that might maintain clonal variation in natural enemy resistance.

Costs of counterdefenses to host resistance in a parasitoid of Drosophila

The ability of a parasitoid to evolve enhanced counterdefenses against host resistance and its possible costs were studied in a Drosophila-parasitoid system. We reared Asobara tabida (Braconidae, Hymenoptera) exclusively on D. melanogaster to impose artificial selection for improved counterdefenses against cellular encapsulation, the main host defense against parasitism. Controls were reared on D. subobscura, the main host of the population of wasps from which the laboratory culture was derived and a species that never encapsulates parasitoids. We observed improved survival and avoidance of encapsulation in all five selection lines compared to their paired control lines, although there was unexpected variation among pairs. Improved survival was associated with parasitoid eggs becoming embedded in host tissue, where they were protected from circulating haemocytes. There were no differences among lines in average adult size, fat content, egg load, or performance on D. subobscura. However, the duration of the egg stage in selection lines was longer than that of control lines, probably because of reduced nutrient and/or oxygen supply when eggs are embedded in host tissue. We suggest that this delay in hatching reduces the probability of parasitoid survival if another parasitoid egg is laid in the same host (superparasitism or multiparasitism) and hence is a cost of enhanced counterdefenses against host resistance.

Basis of the trade-off between parasitoid resistance and larval competitive ability in Drosophila melanogaster

Drosophila melanogaster can be artificially selected for increased resistance against parasitoid wasps that attack the larvae. Lines selected for greater resistance are poorer larval competitors under conditions of resource scarcity. Here we investigated the mechanistic basis of this apparent trade-off. We found that resistant lines have approximately twice the density of haemocytes (blood cells) than that of controls. Haemocytes are involved in encapsulation, the chief cellular immune defence against parasitoids. We have previously shown that resistant lines feed more slowly than controls and hypothesize that limiting resources are being switched from trophic to defensive functions.

Evolutionary interactions between Drosophila melanogaster and its parasitoid Asobara tabida

Drosophila melanogaster is attacked by parasitoids that develop internally in the larva. They can defend themselves by a cellular immune response (host resistance), although this can be disabled by parasitoid countermeasures (parasitoid virulence). D. melanogaster and its parasitoids are an excellent system in which to study coevolution experimentally. We designed an experiment to compare changes in resistance and virulence in replicate populations of flies and parasitoids maintained together for approximately 10 fly (five parasitoid) generations. The experiment had three treatments each with three replicates: (A) no parasitoids (B) outbred parasitoids (C) partially inbred parasitoids. Host resistance increased in treatments B and C but there was no difference between these treatments. Parasitoid virulence appeared not to change during the experiment. Host larvae in treatments B and C fed at lower rates than those in A, evidence of a trade-off between resistance and larval competitive ability. We found no evidence for local adaptation, as hosts from the different replicates of treatment C performed no differently against parasitoids from the same and other replicates. Also, we found no evidence for the evolution of behavioural traits in the host that could lead to lower probabilities of being attacked. Comparing the evolution of host resistance in these seminatural settings with that in artificial selection experiments provides insight into how the conflicting selection pressures on host resistance interact.

Geographic Patterns in the Evolution of Resistance and Virulence in Drosophila and Its Parasitoids

Many insects are attacked by internal parasitoids against which they mount a largely cellular immunological defense. The resistance of a host and the virulence of a parasitoid determine which species survives after parasitism. Drosophila is parasitized by several hymenopterous parasitoids, especially those in the genera Asobara and Leptopilina. Geographic patterns have been found in parasitoid virulence and host resistance, the clearest of which is a cline in Asobara tabida virulence from the north (low) to the south (high) of Europe. Drosophila melanogaster resistance is highest in central-southern Europe and lower elsewhere. We review and interpret these patterns in the light of recent experimental and theoretical studies of the evolution and coevolution of these traits. We find no evidence for genotype-specific virulence and defense, which makes "Red Queen"-type coevolution unlikely. The most important explanation for the patterns is geographic differences in host-parasitoid community structure. Asobara tabida virulence is positively correlated with the resistance of its main hosts, and there is more limited evidence that D. melanogaster resistance is influenced by the virulence of its parasitoids. We critically appraise whether the evidence available so far supports a coevolutionary explanation for the levels of these traits.

Trade-off associated with selection for increased ability to resist parasitoid attack in Drosophila melanogaster

Costs of resistance are widely assumed to be important in the evolution of parasite and pathogen defence in animals, but they have been demonstrated experimentally on very few occasions. Endoparasitoids are insects whose larvae develop inside the bodies of other insects where they defend themselves from attack by their hosts' immune systems (especially cellular encapsulation). Working with Drosophila melanogaster and its endoparasitoid Leptopilina boulardi, we selected for increased resistance in four replicate populations of flies. The percentage of flies surviving attack increased from about 0.5% to between 40% and 50% in five generations, revealing substantial additive genetic variation in resistance in the field population from which our culture was established. In comparison with four control lines, flies from selected lines suffered from lower larval survival under conditions of moderate to severe intraspecific competition.

The coevolution of host resistance and parasitoid virulence

Host-parasitoid interactions are abundant in nature and offer great scope for the study of coevolution. A particularly fertile area is the interaction between internal feeding parasitoids and their hosts. Hosts have evolved a variety of means of combating parasitoids, in particular cellular encapsulation, while parasitoids have evolved a wide range of countermeasures. Studies of the evolution of host resistance and parasitoid virulence are reviewed, with an emphasis on work involving Drosophila and its parasitoids. Genetic variation in both traits has been demonstrated using isofemale line and artificial selection techniques. Recent studies have investigated the fitness costs of maintaining the ability to resist parasitoids, the comparative fitness of flies that have successfully defended themselves against parasitoids, and the degree to which resistance and virulence act against one or more species of host or parasitoid. A number of studies have examined geographical patterns, and sought to look for local adaptation; or have compared the traits across a range of species. Finally, the physiological and genetic basis of change in resistance and virulence is being investigated. While concentrating on Drosophila, the limited amount of work on different systems is reviewed, and other possible areas of coevolution in host-parasitoid interactions are briefly discussed.

Trade-off between parasitoid resistance and larval competitive ability in Drosophila melanogaster

The extent to which an organism is selected to invest in defences against pathogens and parasites depends on the advantages that ensue should infection occur, but also on the costs of maintaining defences in the absence of infection. The presence of heritable variation in resistance suggests that costs exist, but we know very little about the nature or magnitude of these costs in natural populations of animals. A powerful technique for identifying trade-offs between fitness components is the study of correlated responses to artificial selection. We have selected Drosophila melanogaster for improved resistance against an endoparasitoid, Asobara tabida. Endoparasitoids are insects whose larvae develop internally within the body of other insects, eventually killing them, although their hosts can sometimes survive attack by mounting a cellular immune response. We found that reduced larval competitive ability in unparasitized D. melanogaster is a correlated response to artificial selection for improved resistance against A. tabida. The strength of selection for competitive ability and parasitoid resistance is likely to vary temporally and spatially, which may explain the observed heritable variation in resistance.