Heredity 84 (2000) 1±8 Received 6 September 1999, accepted 29 November 1999

Short Review The evolutionary ecology of resistance to parasitoids by Drosophila

M. D. E. FELLOWES * & H. C. J. GODFRAY à NERC Centre for Population Biology and àDepartment of Biology, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK

Parasitoids are the most important natural enemies of many now ®rm evidence that resistance is costly to Drosophila, and insect species. Larvae of many Drosophila species can defend the nature of this cost is discussed, and the possibility that it themselves against attack by parasitoids through a cellular may involve a reduction in metabolic rate considered. Com- immune response called encapsulation. The paper reviews parative data on encapsulation and metabolic rates across recent studies of the evolutionary biology and ecological seven Drosophila species provides support for this hypothesis. genetics of resistance in Drosophila, concentrating on Finally, the possible population and community ecological D. melanogaster. The physiological basis of encapsulation, consequences of evolution in the levels of host resistance are and the genes known to interfere with resistance are brie¯y examined. summarized. Evidence for within- and between-population genetic variation in resistance from isofemale line, arti®cial Keywords: Drosophila melanogaster, encapsulation, life- selection and classical genetic studies are reviewed. There is history trade-o€s, metabolic rate, parasitoid, resistance

development (idiobiont parasitoids) or continues development Introduction (koinobiont parasitoids) after attack (Godfray, 1994). Among All organisms face attack by pathogens, parasites and pred- the most common parasitoids of European D. melanogaster ators. Understanding how adaptive changes in resistance and are koinobiont larval endoparasitoids of the genera Asobara virulence in¯uence the interactions between victim and natural (Braconidae: Alysiinae) and Leptopilina (Cynipoidea: Eucoil- enemy is a major challenge for population biologists. Critical idae), but they are also attacked during the pupal stage by to this task is an understanding of the proximate mechanisms idiobiont ectoparasitoids such as the pteromalid, Pachycrepoi- through which resistance and virulence operate. While verteb- deus vindemiae (Boule treau, 1986; Carton et al., 1986). Levels rate immunity and resistance have been studied intensively of larval parasitoid attack su€ered by D. melanogaster in for many years, the equivalent ®eld of invertebrate immunity natural populations vary both spatially and temporally. In has lagged behind. Nevertheless, some important advances southern England, attack rates average 6±8%, but can reach have been achieved in recent years, in part due to the ever- maxima of more than 50%, while in some Swiss populations increasing importance of Drosophila melanogaster as a model up to 100% of larvae can be attacked on occasion (Carton system. The most important natural enemies of Drosophila et al., 1986). Not all of the larvae that are attacked will die, in ®eld populations are a suite of parasitoid wasps that attack as some larvae can mount a successful immune response to the larvae or pupae. Here we review recent studies of the the parasitoid egg. evolutionary biology of Drosophila±parasitoid interactions, and look forward to the integration of ecological, genetic and The physiological battleground physiological approaches to the coevolution of resistance and virulence in this system. Drosophila melanogaster larvae that successfully defend them- Female D. melanogaster lay eggs on many fermenting selves against attack by koinobiont endoparasitoids employ a substrates, and after hatching the larvae feed on yeasts and cellular immune response known as encapsulation (Salt, 1970). bacteria. It is to these substrates that their parasitoids are This immune reaction is thought to act independently of attracted. Most parasitoids are small wasps whose larvae the humoral response mounted against invading pathogens develop on (ectoparasitoids) or in (endoparasitoids) other (Coustau et al., 1996; Nicolas et al., 1996). Blood cells insects, and less frequently other arthropods (reviewed in (haemocytes) circulating in the haemolymph are central to Godfray, 1994). Successful parasitoid attack results in the the immune response to parasitoid attack and are produced by death of the host. The parasitic Hymenoptera have two broad stem cells in the insect haematopeic organ, the larval lymph life-history strategies, de®ned by whether the host ceases glands (Shrestha & Gate€, 1982). Cellular encapsulation in Drosophila has three main stages. First, the parasitoid egg is recognized as nonself. Just how this *Correspondence. E-mail: [email protected] occurs is poorly characterized at present, but it is likely that

Ó 2000 The Genetical Society of Great Britain. 1 2 M. D. E. FELLOWES & H. C. J. GODFRAY receptors on the haemocyte surface are involved (Carton & The resulting rigid melanized capsule leads to the death of Nappi, 1997; Asgari et al., 1998). There is some evidence that the parasitoid, either directly by asphyxiation (Salt, 1970) or lectin (carbohydrate binding proteins or glycoproteins) recog- through the release of cytotoxic compounds such as superoxide nition plays a role as D. melanogaster di€ering in ability to anions (Nappi et al., 1995) or hydroxy radicals (Nappi & Vass, encapsulate the parasitoid Leptopilina heterotoma also di€er 1998) by capsule components. in their ability to bind to lectins (Nappi & Silvers, 1984). This The number of haemocytes circulating in the larval nonself recognition is followed by changes in the haemocyte haemocoel prior to parasitoid attack appears to be impor- cell surface membranes, with previously hidden molecules tant in determining the encapsulation ability of Drosophila becoming exposed on the cell surface. Non-self recognition larvae. Comparative data show that Drosophila species that presumably triggers a signalling pathway and it has been are better at encapsulating the braconid parasitoid Asobara suggested that the phenoloxidase cascade may be involved, tabida have a higher haemocyte count per unit volume of although experimental evidence for this is weak (Nappi et al., haemolymph than those that are less successful at resisting 1991). In the second stage, there is a short-term increase in the attack (Eslin & Pre vost, 1996, 1998). In particular, Eslin & number of circulating haemocytes produced by the lymph Pre vost (1998; Table 1) showed that amongst six members of glands. Additionally, a class of specialized haemocytes termed the melanogaster subgroup there was a strong and signi®cant lamellocytes di€erentiate from the plasmatocytes, the most correlation between resistance and haemocyte counts. A abundant class of haemocytes in the larval haemolymph (Rizki potential criticism of their conclusion is that species are & Rizki, 1992). This proliferation is thought to involve ligand treated as independent data points and no account was binding by the Toll receptor on the haemocyte cell surface, taken of possible phylogenetic correlations. However, using triggering an internal signalling pathway involving cytoplasmic the phylogeny of this group constructed by Je€s et al. (1994) proteins, such as Cactus (Qui et al., 1998). The activated based on Adh gene sequences (Fig. 1), we re-analysed the lamellocytes move to the parasitoid egg, ¯atten, and then data using independent contrasts (Harvey & Pagel, 1991), adhere to the egg and each other, forming a multilayered and con®rmed Eslin & Pre vost's results (F1,4 ˆ 10.6, capsule (Strand & Pech, 1995). This change in adhesiveness is r2 ˆ 0.73, P ˆ 0.03; Fig. 2). thought to occur through the presentation of integrins (trans- Across Europe, D. melanogaster shows substantial geo- membrane proteins) on the lamellocyte surface: these cell graphical variation in its ability to resist attack by two of its adhesion proteins bind to extracellular matrix molecules such most important parasitoids, A. tabida and Leptopilina boulardi as laminins, collagen IV and vitronectin, and can cause the (Kraaijeveld & van Alphen, 1995; reviewed by Kraaijeveld & cells to adhere together (Strand & Pech, 1995). Scavenger Godfray, 1999). Resistance to A. tabida is strongly correlated receptors, cell surface proteins involved in pattern-recognition with latitude, with southerly populations encapsulating a pathways, may also play a role in cell±cell adhesion. During greater proportion of parasitoids than larvae from populations encapsulation, cell surface changes in the lamellocytes result in further to the north. Drosophila melanogaster populations in the exposure of phosphatidylserine (PS) on the outer cellular central and southern Europe are most adept at defending membrane (Asgari et al., 1997). The dSR-CI (Drosophila themselves against L. boulardi, but the pattern of geographical scavenger receptor class C) has several adhesive domains in variation here is less clear. Parasitoid virulence also varies its extracellular region, and since other scavenger receptors geographically: A. tabida strains originating from the south recognize the PS ligand, it has been suggested that dSR-CI are less likely to be encapsulated than those from northern may be involved in cell±cell binding (Pearson, 1996). The third European populations (Kraaijeveld & van Alphen, 1994). In stage involves a further class of haemocytes, the crystal cells. all cases resistance and virulence were estimated by challenging Lysis of these cells releases prophenoloxidase, an inactive ¯ies or wasps from di€erent localities with a single tester strain zymogen of phenoloxidase, which is activated by protealytic of parasitoid or host. Such a procedure would be misleading if cleavage at its amino terminus (Jiang et al., 1998). The there was coevolution between sympatric ¯y and wasp strains, phenoloxidase catalyses a cascade of reactions, resulting in but the available evidence suggests resistance and virulence are the melanization of the capsule surface (Strand & Pech, 1995). graded traits (like running speed or shell thickness) and that

Table 1 Data used in the comparative analyses (from 1Eslin & Pre vost, 1998; 2Berrigan & Hoang, 1999; 3Orr & Irving, 1997). The data on adult metabolic rate and body mass are mean values from both sexes

Drosophila 1Encapsulation 1Total haemocyte 2Metabolic rate 2Body mass )1 )3 species rate (%) count (ml CO2 h (10 )) (mg) mauritiana 20 7996 2.87 0.83 melanogaster 5 7937 3.98 0.92 pseudoobscura3 0 No data 4.24 1.28 sechellia 0 5030 3.78 0.77 simulans 80 43279 2.91 1.03 teissieri 65 33750 2.57 0.71 yakuba 25 14405 2.44 0.59

Ó The Genetical Society of Great Britain, Heredity, 84, 1±8. DROSOPHILA RESISTANCE TO PARASITOIDS 3

defence. It has long been known that a substance injected by the female parasitoid at oviposition (lamellolysin) hinders the functioning of the lamellocytes. This has been shown to be a proteinaceous `virus-like particle' (VLP) which is released from the egg chorion after oviposition, and interferes with the lamellocyte's microtubule cytoskeleton, preventing capsule formation (Rizki & Rizki, 1990a; Dupas et al., 1996). Other non-Drosophila parasitoids inject viruses (rather than VLPs) containing DNA (polydnaviruses: poly disperse DNA viruses) into the host, the viral gene products attacking the host immune system in several di€erent ways (Edson et al., 1981). The viral genome is integrated mainly within that of the parasitoid, although extra-chromosomal segments are also found (Shelby & Webb, 1999). Virions are assembled within the female calyx cells and infect host haemocytes and fat body cells (Shelby & Webb, 1999). The cysteine-rich proteins encoded by these viruses are ®rst glycoslyated, and then bind to haemocytes, where one of their roles is to change the behaviour of the haemocyte surface lectin-binding sites, preventing haemocyte Fig. 1 The phylogeny of Drosophila species used in the adhesion (Theopold & Schmidt, 1997). Reducing the dosage of comparative analyses (after Je€s et al., 1994). polydnavirus reduces the titre of these proteins by a commen- surate amount, and this in turn reduces their ecacy as a counter-defence against the host encapsulation response (Webb & Cui, 1998). This may explain why repeated oviposition by L. boulardi results in an increase in successful host immune response (Vass & Nappi, 1998a).

Genetics Studies of the genetic basis of resistance in Drosophila (and, though less studied, of virulence in its parasitoids) have had two main goals. The ®rst is to identify genes whose function is essential for successful resistance. Screening of sensitive mutants and the identi®cation of the genes responsible o€ers a valuable tool for probing the physiological basis of resistance. The second aim is to understand the ecological genetics of adaptation, to determine the extent and nature of the additive genetic variation for resistance in natural populations of ¯ies, and the genetic basis of di€erences between populations with varying abilities to encapsulate parasitoids. These two goals Fig. 2 The relationship between independent contrasts of total overlap to a certain degree. haemocyte count (mean of attacked larvae in Eslin & Pre vost, A large number of loci that directly or indirectly in¯uence 1998) and encapsulation ability. Data were transformed encapsulation have now been catalogued (see Flybase at http:// [log (+1)] before contrasts were calculated and the regression 10 ¯ybase.harvard.edu). Some of the more important include is constrained to run through the origin. Black cells (tyrosinase gene associated with phenoloxidase structure; Rizki & Rizki, 1990b); lozenge (a transcription increased likelihood of success against one strain imparts an factor important in crystal cell formation; Rizki & Rizki, 1981; advantage against all (Kraaijeveld & Godfray, 1999). Rizki et al., 1985); serpent (required for haemocyte develop- Our understanding of the physiological basis of parasitoid ment; Rehorn et al., 1996); foraging (a protein kinase that virulence is still fragmentary although it is already evident that determines larval foraging behaviour, which in turn in¯uences di€erent parasitoid groups utilize a wide variety of stratagems to parasitoid attack; Osborne et al., 1997); hopscotch (a Janus counter the host's immune response. Amongst the main kinase required for haemocyte proliferation; Mathey-Prevot & Drosophila parasitoids, Asobara tabida appears to adopt a Perrimon, 1998) and Ribosomal protein S6 (regulatory role in passive means of avoiding encapsulation. The chorion of the egg haematopoieisis; Watson et al., 1996). Mutations at many of is ®brous and causes it to adhere to host tissue such as the host's these genes are associated with the occurrence of melanotic fat body. As the host larva moves, the eggs become further pseudotumours. These can be benign or malignant (Wright, embedded, hiding the egg from the host's immune response 1987), and occur when capsule-like cell masses form around (Kraaijeveld & van Alphen, 1994; Eslin et al., 1996). In contrast, the tissues or organs of larval ¯ies. Pseudotumours are L. boulardi and L. heterotoma have a more active means of presumably associated with a breakdown of self/nonself

Ó The Genetical Society of Great Britain, Heredity, 84, 1±8. 4 M. D. E. FELLOWES & H. C. J. GODFRAY recognition mechanisms or the presence of aberrant cells. Orr & Irving (1997) investigated the genetic basis for Some Black cells mutants (located at 55A1±55A4 on the geographical di€erences in D. melanogaster resistance to D. melanogaster cytological map) are unable to melanize the A. tabida. Using classical Drosophila genetic techniques, they haemolytic capsules formed around L. boulardi eggs, whereas showed that the majority of the variation amongst three other mutant alleles result in the formation of melanotic populations of ¯ies could be attributed to alleles segregating at masses in the crystal cells (Rizki & Rizki, 1990b). The wizard a single locus, with the resistance allele completely dominant mutation has a more direct in¯uence on haematopoieisis, as over the susceptible allele. The results of crossing experiments the melanized phenotype is expressed in the lymph glands, using larval phenotypic markers allowed them to locate the crystal cells and larval head (Rodriguez et al., 1996). While resistance gene near the centromere region of the second mutations at many of these loci produce similar melanotic chromosome. Orr & Irving (1997) propose the dopa decar- pseudotumours or capsules, the genes themselves have a boxylase gene cluster as a strong candidate for the locus they multitude of roles. For example, the Toll/Cactus signal have identi®ed. The 18 loci in this gene cluster have important transduction pathway regulates larval haemocyte prolifera- roles in the catalysis of the decarboxylation of dopa to tion, and mutations at these sites result in an overabundance of dopamine, necessary for cuticle sclerotization, and mutations melanized haemocytes (Qiu et al., 1998), whereas larvae in 11 of these loci result in the production of melanotic bearing a mutant form of domino have melanized lymph tumours (Wright, 1996). Additionally, mutations at this site glands, and as a result do not produce haemocytes at all have been shown to compromise D. melanogaster encapsula- (Braun et al., 1998). Such mutants are proving central to tion ability (Nappi et al., 1992). understanding the molecular and developmental genetics of Given that we are interested in understanding how changes resistance (Mathey-Prevot & Perrimon, 1998; the ®rst goal in host resistance may be an adaptive response to parasitoid identi®ed above), but tell us rather little about the nature of attack, we not only need to be aware of the genetics under- genetic variation for resistance in natural populations (the pinning the trait, but also whether there is enough heritable genetics of adaptation; the second goal noted above). variation in resistance within host populations for natural An important technique for investigating the genes involved selection to act upon. There is now ample evidence for quite in encapsulation is the study of isofemale lines. Isofemale lines substantial additive genetic variation in resistance in are formed by taking females that have been inseminated once, D. melanogaster populations. Most of this evidence comes allowing them to lay eggs, and then inbreeding the descendants from experiments in which ¯ies have been arti®cially selected until most loci are homozygous. The genetic variation revealed for increased resistance (see Kraaijeveld et al., 1998 for a by a comparison of isofemale lines derived from a single review of the older literature). Hughes & Sokolowski (1996) set population is an amalgam of additive, dominance and epistatic up population cages of D. melanogaster with or without components, and hence does not provide direct information A. tabida (the ¯y populations were kept isolated within cages, about the ability of the population to respond to increased but parasitoids were supplemented from other sources). They parasitoid selection pressures. Carton & Nappi (1997) reported found that higher resistance evolved in cages where the wasp that the di€erence between two isofemale lines di€ering in was present and where mean rates of parasitism averaged 7% susceptibility to L. boulardi was due to a single locus, with the over 19 generations. Larvae from cages containing parasitoids resistant allele completely dominant to the susceptible one. were almost twice (39% vs. 23%) as likely to encapsulate Chromosome substitution experiments allowed the locus to be successfully a parasitoid egg as those from the control lines located at the 55D±55F interval of the cytological map, on the (see also below). Very strong arti®cial selection for increased long arm of the second chromosome. Further examination of resistance can be obtained by parasitizing a large number of these lines con®rmed this result (Hita et al., 1999), and the larvae and breeding only from those individuals that have authors suggest that this locus is involved speci®cally in successfully encapsulated a parasitoid egg. Using this tech- L. boulardi recognition. Intriguingly, this is close to the location nique, Kraaijeveld & Godfray (1997) obtained an increase in of the gene overgrown haematopeic organs 55DE. Mutants at survival against A. tabida from 5% to 50% in ®ve generations, this site produce di€use lymph glands, which suggests that this while Fellowes et al. (1998a) obtained an increase in survival gene may in¯uence haemocyte production (Torok et al., 1993). against L. boulardi from 0.4% to 45% over the same period. In A similar experiment was performed by Benassi et al. (1998), each case the response to selection was similar across four who investigated the genetic basis of D. melanogaster resistance replicate lines. Narrow sense heritability for resistance was to A. tabida. They also found that di€erences in resistance calculated using a model for threshold traits (which must be amongst a set of isofemale lines appeared to be governed by a treated with some caution as it assumes a quantitative genetic single locus, with the resistant allele showing complete domi- background to the trait; Falconer & Mackay, 1996), giving an nance over the susceptible allele. This gene does not appear to be estimate of about 0.25 for resistance against either parasitoid. the same as that implicated in L. boulardi resistance (Carton & As was discussed above, isofemale line studies have found Nappi, 1997). The isofemale lines used to determine the genetic genes that are speci®cally involved in resistance against nature of resistance to L. boulardi were also assayed for their A. tabida and L. boulardi. Studies of geographical variation ability to encapsulate A. tabida. Both lines resistant and in resistance, and of arti®cially selected lines, also have a susceptible to L. boulardi were very successful at encapsulating bearing upon this question. First, no correlation was found A. tabida, which shows that the susceptible strains are not between resistance to A. tabida and resistance to L. boulardi in a immune-de®cient in general (Vass et al., 1993). comparison of a large number of geographical separated

Ó The Genetical Society of Great Britain, Heredity, 84, 1±8. DROSOPHILA RESISTANCE TO PARASITOIDS 5 populations of D. melanogaster (Kraaijeveld & van Alphen, ism has been shown to occur in many vertebrate taxa (Zuk, 1995; Kraaijeveld & Godfray, 1999). Secondly, lines selected for 1990; Mùller et al., 1998), but we are aware of only one other resistance against A. tabida do not show increased resistance study that has shown di€erences in an insect. Agnew et al. to L. boulardi, whereas lines selected for resistance against (1999) found that microsporidian infection of the mosquito L. boulardi show a large increase in the likelihood of their Culex pipiens is more costly to females than males. Addition- successfully encapsulating A. tabida (Fellowes et al., 1999a). ally, the pupae of larvae that have encapsulated a parasitoid Both sets of lines show elevated resistance to a third parasitoid have relatively thinner puparial walls (Fellowes et al., 1998b), species, Leptopilina heterotoma. Asobara tabida and L. hetero- and this may explain an unexpected indirect cost of encapsu- toma attack a wide range of Drosophila species in fermenting lating a parasitoid. The pupal parasitoid, P. vindemiae, will substrates, whereas L. boulardi is restricted to D. melanogaster preferentially attack hosts that have previously survived attack and its sibling species. Fellowes et al. (1999a) argued that their by a larval parasitoid, possibly because of the reduction in results could be explained by a two-component resistance puparium thickness, which will reduce the handling time model: a generalized increase in encapsulation ability, perhaps required for successful attack (Fellowes et al., 1998b). Given associated with elevated haemocyte densities, that increases that encapsulation is energetically costly and successful protection against many parasitoid species, and a speci®c defence leaves fewer resources to be allocated to other response against the countermeasures of the specialized para- physiologically demanding traits, it is unsurprising that resis- sitoid L. boulardi. Unfortunately, the genes responsible for the tance ability is reduced in D. melanogaster larvae exposed to changes in resistance levels in the arti®cial selection lines have environmental stress. Levels of successful defence falls when not yet been identi®ed, and it is not possible to say whether they larvae are reared at high densities (Wajnberg et al., 1985), are are the same as those identi®ed in the isofemale line studies, or in exposed to insecticides (Delpuech et al., 1996), and are Orr & Irving's study of geographical variation. provided with a diet de®cient in yeast (Vass & Nappi, 1998b).

Life-history trade-offs Resistance ability The extensive additive genetic variation in resistance to The costs of the ability to resist parasitoid attack are more parasitoids found in D. melanogaster is most likely to be subtle than the costs of actual defence. Lines selected for maintained by a combination of ¯uctuating selection pres- increased resistance to A. tabida (Kraaijeveld & Godfray, sures, both temporal and spatial, and costs of resistance. The 1997) or L. boulardi (Fellowes et al., 1998a) show a decrease in latter are central to most arguments about evolutionary larval competitive ability under conditions of resource stress. interactions between hosts and parasites, or predators and In both cases the loss of competitive ability is correlated with a prey, and insect parasitoids have proved a valuable model reduction in the feeding rate of the more resistant larvae system for studying these issues. (Fellowes et al., 1999c). There are two main issues in assessing the costs and bene®ts When arti®cial selection was relaxed, the level of resistance of investing in resistance. The ®rst concerns what happens in the lines selected against A. tabida quickly dropped from the after a parasitoid attack: are there costs to the successful maximum of 65% survival to approximately 30% (Kraaijeveld encapsulation of the parasitoid (or, equivalently, what are the et al., 1998). Recall that in the last section we described how bene®ts of resistance)? The second concerns the costs of Hughes & Sokolowski (1996) found that D. melanogaster lines maintaining the resistance machinery, costs that are borne cultured with an A. tabida population that achieved a 7% irrespective of whether the host is attacked or not. By analogy, attack rate had higher resistance levels than those cultured the two types of costs can be thought of as the penalties of without parasitoids. In these experiments (designed to study ®ghting a war (actual defence) and of maintaining a standing the rover-sitter polymorphism), the nonparasitoid treatments army (resistance ability). were cultured at higher population densities. Also, the wild populations from which the base stock was isolated su€ered higher parasitoid attack rates (16%) than the parasitoid Actual defence treatments. These factors suggest an alternative explanation Since successful parasitoid attack will result in death, resis- for their results, not that higher resistance evolved in the tance will always be in the interest of the host, even if survival parasitoid treatments, but that costly resistance decayed faster is associated with a reduction in ®tness in comparison with an in the nonparasitoid treatments. individual that had escaped attack. However, investment in Why should increased resistance be associated with a resistance mechanisms will be less likely to occur if ¯ies that reduction in feeding rate, with a consequent reduction in survive have low reproductive success. Female ¯ies that have competitive ability at high densities? Conceivably, the two survived parasitoid attack as larvae are smaller (both thorax responses could be pleiotropic e€ects of the same gene or genes, and wing length) and less fecund (Carton & David, 1983; perhaps via increased investment in defensive functions, Fellowes et al., 1999b). Males also have smaller thoraxes, but resulting in decreased investment in trophic functions such as there is no signi®cant di€erence in wing length, and males feeding rate. Another possibility is that lower metabolic rates experience only a small reduction in the number of o€spring are selected for in the presence of high risks of parasitism. It has they sire, and then only under certain conditions (Fellowes often been observed that Drosophila and other insects selected et al., 1999b). Males clearly su€er less from resisting attack for tolerance to a variety of stresses evolve a lower metabolic than females. This sexual dimorphism in the costs of parasit- rate (though the response of an individual to increased stress is

Ó The Genetical Society of Great Britain, Heredity, 84, 1±8. 6 M. D. E. FELLOWES & H. C. J. GODFRAY

in e€ect gambling the costs of resistance against the risk of being discovered by a parasitoid. Such an outcome might explain the inability of D. pseudoobscura to defend itself against any of its parasitoids. Under other conditions an arms race occurs with spiralling levels of costly resistance and virulence, until a point is reached when e€ective resistance is so costly that the host abandons defence, again gambling on not been found. At this point the parasitoid is selected to reduce its level of virulence, and the cycle begins again (Sasaki & Godfray, 1999). This result can explain persistent levels of additive genetic variation for resistance. Whether such models describe host±parasitoid interactions in nature is unclear, especially for D. melanogaster and its parasitoids where, perhaps surprisingly, the population dynamics of the host± parasitoid interaction have been little studied in the ®eld. Rapid evolutionary change in levels of resistance, on what perhaps would traditionally have been thought of as ecological Fig. 3 The relationship between independent contrasts of timescales, may also in¯uence the community structure of metabolic rate (here not corrected for size) and encapsulation interacting species (Fellowes & Kraaijeveld, 1998; Thompson, ability. Data were transformed [log10 (+1)] before contrasts 1998). For example, a D. melanogaster population subject to were calculated and the regression is constrained to run heavy parasitism by the specialist L. boulardi might evolve through the origin. enhanced levels of resistance that would also protect it against A. tabida. Asobara tabida might then not be able to invade, or normally an elevated metabolic rate; Ho€mann & Parsons, might as a consequence of the presence of L. boulardi be 1991). Possibly decreased metabolic rate and a consequent subject to selection for increased virulence. The asymmetric reduction in feeding rates is part of a suite of adaptations to patterns of cross-resistance discussed above means that a prepare the ¯y for the stress of parasitoid attack. D. melanogaster population that had evolved to protect itself We do not yet have data on the metabolic rates of ¯ies from against A. tabida would not be pre-adapted to withstand lines selected for increased resistance, but the idea can be tested L. boulardi attack. Such indirect selection pressures may be by a cross-species comparison of encapsulation and metabolic common features of coevolutionary interactions (Miller & rates. As discussed above, Eslin & Pre vost (1998; Table 1) Travis, 1996), and in many ways can be considered analogous reported the capacity of six members of the melanogaster to apparent competition in ecological interactions (Holt, subgroup to encapsulate A. tabida, and to this list we have 1977). E€ects such as this `apparent coevolution', in addition added D. pseudoobscura which does not encapsulate this to more direct evolutionary and coevolutionary changes, parasitoid species (Kraaijeveld & van Alphen, 1993; Orr & may be signi®cant factors in determining the structure of Irving, 1997). For metabolic rates, we used the data in Berrigan insect-parasitoid and other resource-consumer communities & Hoang (1999) which is based on respiration rates (Table 1). (Fellowes & Kraaijeveld, 1998). We tested the correlation using independent contrasts, again using the phylogeny produced by Je€s et al. (1994; Fig. 1). The Conclusions data allowed six contrasts to be made, and these were sucient to show that encapsulation is negatively associated with adult Parasitoids have provided valuable model systems for investi- metabolic rate, after controlling for mass (partial correlation gating a variety of problems in evolutionary biology, especially beta ˆ ) 0.79, t4 ˆ 5.01, P < 0.01; Fig. 3). Clearly this is no testing theories of adaptive sex ratios and other reproductive more than an exploratory analysis (for example it assumes strategies (Godfray, 1994). Between 1940 and 1960, parasitoids metabolic rates are correlated between the adult and larva) but were also used as a major genetic model system, especially the it does suggest that the relationship between metabolic rate and species Bracon hebetor and Nasonia (then called Mormoniella) resistance is worth further investigation. vitripennis. Very few of the numerous mutant lines isolated during this period survive today. Drosophila and its parasitoids have been used extensively in behavioural and behavioural Ecological consequences of resistance ecological work, and in studies of host±parasitoid physiology, Many hosts and parasitoids exist in tightly coupled ecological but have been used rarely as ecological genetic models. This, interactions, the parasitoid being the major regulatory factor however, is changing, largely spurred by the ever-increasing a€ecting host densities (Hassell, 2000). Parasitoids thus act as value of D. melanogaster as one of the animal models. a major selective force on the host, and at least theoretically Drosophila parasitoids have many advantages as model there is the potential for signi®cant feedback between popu- organisms; in particular, most species can be cultured as easily lation and genetic dynamics (Godfray & Hassell, 1991; Sasaki as their hosts, and their generation times are short, although & Godfray, 1999; Godfray, 2000). For example, one joint about 50% longer than their host. A disadvantage is that the dynamic±genetic model has two main outcomes: under certain genetics of Drosophila parasitoids are as yet poorly under- conditions the host invests nothing in resistance mechanisms, stood, with very few visual or molecular markers available.

Ó The Genetical Society of Great Britain, Heredity, 84, 1±8. DROSOPHILA RESISTANCE TO PARASITOIDS 7

A challenge for biologists interested in the reciprocal ESLIN, P.AND PRE PREVOST VOST, G. 1996. Variation in Drosophila concentration evolutionary interactions between hosts and parasites and of haemocytes associated with di€erent ability to encapsulate predators and prey is to understand the nature of the adaptive Asobara tabida larval parasitoid. J. Insect Physiol., 42, 549±555. , P.AND PRE PREVOST VOST, G. 1998. Hemocyte load and immune resistance genetic changes, at all levels from the ecological to the ESLIN to Asobara tabida are correlated in species of the Drosophila molecular. We suggest that Drosophila and its parasitoids are melanogaster subgroup. J. Insect Physiol., 44, 807±816. likely to prove very valuable for disentangling these issues. FALCONER, D. S.AND MACKAY , T. F. C. 1996. Introduction to Quantitative Genetics, 4th edn. Longman Scienti®c, Harlow. Acknowledgements FELLOWES, M. D. E.AND KRAAIJEVELD , A. R. 1998. 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