NEWS & COMMENT References 1 Andersson, M. (1994) Sexual Selection, Princeton University Press 2 Funk, D.H. and Tallamy, D.W. (2000) Courtship role reversal and deceptive signals in the longtailed dance fly, Rhamphomyia longicauda. Anim. Behav. 59, 411–421 3 Trivers, R.L. (1972) Parental investment and sexual selection. In Sexual Selection and the Descent of Man (Cambell, B., ed.), pp. 136–179, Aldaine–Atherton 4 Owens, I.P.F. and Thompson, D.B.A. (1994) Sex differences, sex ratios and sex roles. Proc. R. Soc. London Ser. B 258, 93–99 5 Petrie, M. (1983) Mate choice in role-reversed species. In Mate Choice (Bateson, P., ed.), pp. 167–179, Cambridge University Press

6 Gwynne, D.T. and Simmons, L.W. (1990) Experimental reversal of courtship roles in an insect. Nature 346, 172–174 7 Downes, J.A. (1970) The feeding and mating behaviour of the specialized Empidinae (Diptera); observations of four species of Rhamphomyia in the high Arctic and a general discussion. Can. Entomol. 102, 769–791 8 Cumming, J.M. (1994) Sexual selection and the evolution of dance fly mating systems (Diptera: Empididae; Empidinae). Can. Entomol. 126, 907–920 9 Svensson, B.G. and Peterson, E. (1988) Nonrandom mating in the dance fly Empis borealis: the importance of male choice. Ethology 79, 307–316

Wolbachia trends

T

he First International Wolbachia Congress, devoted to this fascinating endocellular bacteria (Box 1), was held in June at the Orthodox Academy of Crete*. Because Wolbachia is present in a wide variety of arthropods and nematodes, and because of the diversity of its effects, this conference brought together researchers from various and complementary fields. This plurality allowed a broad spectrum of questions to be addressed, from molecular to population level. Here, we will focus on results that provide new insights into the distribution and plasticity of Wolbachiainduced phenotypes, and the evolutionary dynamics of Wolbachia–host associations.

Wolbachia-induced phenotypes Although cytoplasmic incompatibility (CI; Box 1) is a widespread phenomenon occurring in a variety of arthropods, other Wolbachia-induced phenotypes have, until now, been described from a more limited range of hosts. In particular, feminization (F; Box 1) and parthenogenesis induction (PI; Box 1) were thought to be restricted to isopod crustaceans and Hymenoptera, respectively. Now, such views have to be modified (Table 1). However, it is notable that these phenotypes seem to be nonrandomly distributed and to rely on specific host properties. One such property is that PI occurs only in haplo–diploid species, where arrhenotokous parthenogenesis (male development from unfertilized haploid eggs) is observed in the absence of *First International Wolbachia Congress, Kolymbari, Greece, 7–12 June 2000.

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infection. This is consistent with the idea that the ability to develop from unfertilized eggs is necessary for PI. One might also propose that PI is facilitated in these groups by the purging of recessive deleterious alleles in haploid males, thus the complete homozygosity caused by PI is not seriously detrimental. Another property is that, in isopods, Wolbachia induces F by preventing the maturation of the androgenic gland. It is unlikely that the same process, relying on a specific sex-determination system, occurs in Lepidoptera. However, it is worth noting that in isopods, as well as Lepidoptera, the males are homogametic (ZZ), thus suggesting that F mechanisms might be linked to this property. Indeed, in isopods, Wolbachia action on the androgenic gland might not be (evolutionarily speaking) the primary cause of F. Interestingly, it appears that more than one phenotype can occur in a given group, as previously observed in Hymenoptera3,5. For example, CI (Ref. 6) and PI are observed in Tetranychid mites, whereas male killing (MK; Box 1), CI and F occur in Lepidoptera. The fact that different Wolbachia strains have different effects in closely related hosts suggests that the determinant of the phenotype is more a function of the symbiont than of the host. In isopods, when CI-inducing Wolbachia are artificially transferred from their host to species feminized by their natural Wolbachia strain, CI is still observed (T. Rigaud, pers. commun.), which is consistent with such a view. Conversely, Tetsuhiko Sasaki (University of Tokyo, Japan) reported a case of crossspecies transfer in Lepidoptera, which induces a phenotype shift from F to MK.

0169-5347/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.

10 Svensson, B.G. et al. (1989) Why do males of the dance fly Empis borealis refuse to mate? The importance of female age and size. J. Insect. Behav. 2, 387–395 11 Semple, S. and McComb, K. (1996) Behavioural deception. Trends Ecol. Evol. 1, 434–437 12 Simmons, L.W. and Ritchie, M.G. (1996) Symmetry in the songs of crickets. Proc. R. Soc. London Ser. B 263, 305–311 13 Ryan, M.J. (1990) Sexual selection, sensory systems and sensory exploitation. Oxf. Surv. Evol. Biol. 7, 157–195 14 Holland, B. and Rice, W.R. (1998) Chase-away sexual selection: antagonistic seduction versus resistance. Evolution 52, 1–7

This puzzling result shows that host effects act not only quantitatively, as previously shown7, but also qualitatively. The answer to the question of whether symbiont or host has control over the expressed phenotype seems most safely given as ‘it depends...’. Sasaki’s study also demonstrates that a single Wolbachia strain has the ability to induce two phenotypes in two different backgrounds. Greg Hurst (University College London, UK) showed that this is also true in a single background. In Drosophila bifasciata, one Wolbachia strain combines MK and CI (Ref. 8). Freeland and McCabe9 previously modelled the invasion dynamics of double infections by MK- and CI-inducing bacteria. An important conclusion was that CI could hitchhike along with MK, thus invasion by CI was possible starting from lower frequencies than those predicted for CI alone. Similar conclusions can be drawn for this CI1MK-inducing strain in D. bifasciata, suggesting that such a bacterium possesses unusually high invasion abilities. This result is important for predicting the dynamics of Wolbachia invasion and loss. We now turn to consider these dynamics.

Long- or short-term passengers? Wolbachia, with their ability to alter host reproduction, have been appropriately termed ‘influential passengers’3. But, how long do these passengers remain on board? How stable are Wolbachia–host associations? Several communications have provided interesting new insights with regard to this question. In arthropods, new Wolbachia infections occur within species through horizontal transfer (HT), as shown by the lack of congruence between host and symbiont phylogenies3. Several new cases of HT, inferred from phylogenetic data, were presented at the meeting, thus

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NEWS & COMMENT confirming that, at least on an evolutionary timescale, HTs are not that rare. However, such an approach is weakened by the possibility of recombination between Wolbachia strains. Indeed, Frank Jiggins (University College, London) and Jack Werren (University of Rochester, NY, USA, pers. commun.) showed independently, and through different ap-proaches, that recombination does occur. Experimental demonstrations of HT are therefore required. Richard Stouthamer (University of Wageningen, The Netherlands) presented such data, showing that when infected and uninfected parasitoids of the same species infest the same egg, Wolbachia transmission from infected to uninfected parasitoid larvae can occur at frequencies higher than 30% (Ref. 10). These results suggest that even on the scale of a wasp’s lifetime, HTs are not that rare. When investigating the evolution of Wolbachia– host interactions, it becomes obvious that vertical and horizontal transmissions must be considered. Here, metapopulation models could provide a useful framework: arthropods could be seen as a huge metapopulation comprising numerous sub- populations (species), with the whole question being to understand the patterns of extinction (loss) and colonization (gains). Let us now consider extinction rates. Focusing on CI in Hymenoptera, Fabrice Vavre (University of Lyon, France) suggested, using a modelling approach, that the evolution of CI within a host might selectively lead to the loss of Wolbachia5. More generally, because strong CI levels are not selected for3, when Wolbachia is present in a species, it might, through simple genetic drift11,12, lose its ability to induce CI and finally be lost. Similar outcomes might be possible for MK Wolbachia. Recently, Randerson et al.13 showed that if MK-resistant genes occur, the neutralized strain can be replaced by a new one, even if this strain induces weaker effects. This process might lead to the low equilibrium frequencies commonly observed in nature14 – such low frequencies will increase the probability of symbiont loss through random events. Michael and Tamsin Majerus (University of Cambridge, UK) reported the existence of a MK resistance gene (a male rescue gene) in the Asian ladybird Cheliomenes sexmaculatus, thus reinforcing the plausibility of such a scenario. Gain–loss turnovers might also occur for feminizing Wolbachia. In the isopod Armadillidium vulgare, invasion by another feminizing factor (labelled f) can lead to Wolbachia loss3. Using molecular markers, Didier Bouchon (University of Poitiers, France) found confirmation of such a gain–loss system in isopods. TREE vol. 15, no. 11 November 2000

Box 1. Wolbachia, their hosts and their effects Wolbachia are an endocellular bacteria infecting arthropods and nematodes. More than 70% of arthropod species might be infected, as suggested by the highly sensitive detection method used by Ayyamperumal Jeyaprakash and Marjorie Hoy (University of Florida, Gainesville, USA)1. Wolbachia is (mainly) vertically transmitted through maternal cytoplasm. Given the dependence of symbiont reproduction on that of the host, such a transmission mode is expected to be associated with mutualism: symbionts profit from an intracellular lifestyle and provide benefits to their host. Such a relationship is observed between Wolbachia and nematodes, especially in pathogenic filarial nematodes2, and is a major cause of morbidity throughout the tropics. In this respect, research on Wolbachia offers serious opportunities for medical applications. Conversely, Wolbachia in arthropods do not, strictly speaking, benefit their host, but they do induce various phenotypes (listed below) affecting host reproduction. These phenotypes share the common property that they favour the spread of infected cytoplasm within uninfected populations. Feminization (F): male offspring from infected females are turned into functional females, thus optimizing cytoplasm transmission. Male killing (MK): male offspring from infected females tend to die, which, in some circumstances, might benefit their sisters. Parthenogenesis induction (PI): in haplo–diploid species, Wolbachia allow development of females from unfertilized eggs through chromosome duplication, thus optimizing cytoplasm transmission. Cytoplasmic incompatibility (CI): when infected males mate with uninfected females or with females infected by a different Wolbachia variant, embryonic mortality is observed. Thus, uninfected cytoplasmic lines are selected against by the presence of infected males in a positive frequency-dependent manner.

Different conclusions can be drawn for PI. Yuval Gottlieb (University of Chicago, IL, USA) showed that PI can lead to the alteration of sexual traits because these are no longer subject to selection. In such cases, when sexual reproduction becomes impossible, the presence of Wolbachia is indispensable to host reproduction. A sort of ‘imposed’ mutualistic relationship occurs, probably leading to stable associations over time. Similarly, in nematodes, where Wolbachia is necessary for host fertility2, Wolbachia–host associations are probably stable; this is consistent with the fact that, in this case, Wolbachia and host phylogenies are congruent [Maurizio Casiraghi, Instituto di Patologia Generale Veterinaria (IPGV), Milano, Italy]15. Thus, it appears that, depending on the induced phenotype, Wolbachia–host associations might or might not have evolutionary stability.

Stable associations might lead to deep host–symbiont integration through co-evolutionary processes. Analysing complete genome sequences of the aphid endosymbiont Buchnera, Hajime Ishikawa (University of Tokyo) and Siv Andersson (University of Uppsala, Sweden) illustrated the fate of such longterm associations at the genomic level. As an example, they showed that Buchnera harbours genes involved in essential amino acid synthesis but has lost anabolic capacity with respect to nonessential amino acids. Going further in the past, Charles Kurland (University of Uppsala) illustrated how mitochondrial and nuclear genomes are precisely co-adapted. Could it be, as suggested by Chris Bazinet (St. John’s University, NY, USA), that looking at mitochondrial behaviour within germ cells, we can find traces of ancestral properties possibly

Table 1. Distribution of Wolbachia-induced feminization and parthenogenesis Phenotype

Host

Refs

Parthenogenesis

Hymenoptera Thrips: Franklinothrips vespiformis (Insecta: Thysanoptera) Springtail: Folsomia candida (Collembola: Isotomidae) Mites: Bryobia sp. (Acari: Tetranychidae)

3,4 Hiroaki Noda (NISES, Ibaraki, Japan)a

Isopod crustaceans Moths: Ostrinia furnacalis and Ostrinia scapulalis (Lepidoptera: Crambidae)

3,4 Daisuke Kageyama (University of Tokyo, Tokyo, Japan)a

Feminization

aContact

Tom Vanderckhove (University of Ghent, Belgium)a Andrew Weeks (University of Amsterdam, The Netherlands)a

details of those who presented these results at the First International Wolbachia Congress.

439

NEWS & COMMENT associated with reproductive parasitism? And, could it be that Wolbachia are the mitochondria of the third millennium? Among other approaches, projects to sequence the Wolbachia genome, outlined by Scott O’Neill (Yale University, New Haven, USA) and the host of the conference Kostas Bourtzis (IMBB-FORTH, Heraklion, Crete, Greece), hopefully achieved by the next meeting (July 2002, Kolymbari, Greece), will undoubtedly tell us more. Acknowledgements Thanks to K. Bourtzis, A. Hoffmann, S. O’Neill, R. Stouthamer and J. Werren for organizing the meeting, and to A. Atlan, K. Bourtzis, G. Hurst, M. Majerus, D. Poinsot, T. Rigaud and F. Vavre for helpful comments on this article. Sylvain Charlat Hervé Merçot Institut Jacques Monod, Laboratoire Dynamique du Génome et Evolution, 2 Place Jussieu, 75251 Paris Cedex 05, France ([email protected]; [email protected])

References 1 Jeyaprakash, A. and Hoy, M.A. Long PCR improves Wolbachia DNA amplification: wsp sequences found in 76% of sixty-three arthropod species. Insect Mol. Biol. (in press) 2 Hoerauf, A. et al. (1999) Tetracycline therapy targets intracellular bacteria in the filarial nematode Litomosoides sigmodontis and results in filarial infertility. J. Clin. Invest. 103, 11–18 3 O’Neill, S.L. et al. (1997) Influential Passengers: Inherited Microorganisms and Arthropod Reproduction, Oxford University Press 4 Stouthamer, R. et al. (1999) Wolbachia pipientis: microbial manipulator of arthropod reproduction. Annu. Rev. Microbiol. 53, 71–102 5 Vavre, F. et al. (2000) Evidence for female mortality in Wolbachia-mediated cytoplasmic incompatibility in haplodiploid insects: epidemiologic and evolutionary consequences. Evolution 54, 191–200 6 Breeuwer, J.A.J. (1997) Wolbachia and cytoplasmic incompatibility in the spider mites Tetranychus urticae and T. turkestani. Heredity 79, 41–47 7 Poinsot, D. et al. (1998) Wolbachia transfer from Drosophila melanogaster into D. simulans: host effect and cytoplasmic incompatibility relationships. Genetics 150, 227–237

Take care when studying parenting behaviour

B

ehavioural ecologists get excited by conflicts and there is hardly a better place to look for them than inside a bird’s nest or behind the lips of a mouthbrooding cichlid. The need for parental care sets the stage for intersexual conflict over how much care each parent should provide. Parent–offspring conflict arises over how much total care is given and, finally, conflict occurs among siblings over the allocation of care. A recent workshop on conflicts and cooperation in parental care* sought to make sense of this battleground. The aim of the workshop was to promote cooperation between researchers using different means to understand parenting behaviour. Studying parental care is challenging, not only because of the multitude of conflicts but also because care interacts with other aspects of the ecology of a species – most notably its mating system. John McNamara (University of Bristol, UK) argued that models that relate certainty of paternity to paternal care (for example)

*Cooperation and Conflict in Parental Care: Integrating Empirical and Theoretical Approaches, Bernried, Germany, 4 – 6 August 2000.

440

should be self-consistent. A self-consistent model takes into account that reduced paternity in one brood not only affects the social father of that brood but also gives paternity to other males; that is, it increases the benefits that males achieve via extra-pair fertilizations. Ignoring such consistency checks is one reason for the confusingly mixed predictions of previous models. However, it is not only theoreticians that should be more careful in their studies of care. It is essential that assumptions of models fit the natural history of the species concerned, when both deriving predictions and testing them. A particularly clear example was provided by Fritz Trillmich (University of Bielefeld, Germany), who showed that many of the assumptions of the theory of adaptive sexratio adjustment were poorly supported in pinnipeds. Further challenges arise when conflicts that might underlie behaviour are not directly measurable in the outcome observed in nature. Geoff Parker (University of Liverpool, UK) showed that parent– offspring conflict can make parents provide less food than would be optimal from either the parents’ or the offspring’s point of view. Yoh Iwasa (Kyushu University,

0169-5347/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.

8 Hurst, G.D.D. et al. Male-killing Wolbachia in Drosophila: a temperature sensitive trait with a threshold density. Genetics (in press) 9 Freeland, S.J. and McCabe, B.K. (1997) Fitness compensation and the evolution of selfish cytoplasmic elements. Heredity 78, 391–402 10 Huigens, M.E. et al. (2000) Infectious parthenogenesis. Nature 405, 178–179 11 Hoffmann, A.A. et al. (1996) Naturally-occurring Wolbachia infection that does not cause cytoplasmic incompatibility. Heredity 76, 1–8 12 Poinsot, D. and Merçot, H. (1999) Wolbachia can rescue from cytoplasmic incompatibility while being unable to induce it. In From Symbiosis to Eukaryotism – Endocytobiology VII (Wagner, E. et al., eds), pp. 221–234, Universities of Geneva and Freiburg in Breisgau 13 Randersson, J.P. et al. (2000) Evolutionary dynamics of male-killers and their hosts. Heredity 84, 152–160 14 Hatcher, M.J. (2000) Persistence of selfish genetic elements: population structure and conflict. Trends Ecol. Evol. 15, 271–277 15 Casiraghi, M. et al. A phylogenetic analysis of filarial nematodes: comparison with the phylogeny of Wolbachia endosymbionts? Parasitology (in press)

Fukuoka, Japan) presented a series of models in which the threat of infanticide by unmated males forced females to allocate paternity among several males. In this case, if the counterstrategy of the female is successful, no infanticide is observed at the evolutionary equilibrium. As Ben Sheldon (University of Oxford, UK) pointed out, key parameters of models, for example parental investment and certainty of paternity, are similarly notoriously hard to measure empirically. Sheldon presented results from an experiment that manipulated the genetic paternity of collared flycatchers, Ficedula albicollis, by temporarily removing the male before his mate laid eggs; he found a positive relationship between realized paternity and male care. Yet, he argued that such experiments do not prove that the response was adaptive – ruling out alternative explanations, such as effects of removal on the male’s testosterone level, is difficult, as is measuring the lifetime fitness effects of any behavioural decision. Demanding as they might be, lifetime fitness calculations can be rewarding. In the Seychelles warbler Acrocephalus seychellensis, studied by Jan Komdeur (University of Groningen, Haren, The Netherlands), sex-ratio adjustment in broods can be related to the lifetime fitness benefits through daughters and sons. This population is now known to show high levels of extra-group paternity and also some reproduction by female helpers. Thus, as Komdeur indicated, genetic

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Wolbachia trends - Cell Press

tailed dance fly, Rhamphomyia longicauda. Anim. Behav. 59, 411–421. 3 Trivers, R.L. (1972) Parental investment and sexual selection. In Sexual Selection and ...

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