Available online at www.sciencedirect.com Wolbachia: more than just a bug in insects genitals Aggeliki Saridaki and Kostas Bourtzis Research on the intracellular bacterial symbiont Wolbachia has grown on many levels, providing interesting insights on various aspects of the microbe���s biology. Although data from fully sequenced genomes of different Wolbachia strains and from experimental studies of host���microbe interactions continue to arise, most of the molecular mechanisms employed by Wolbachia to manipulate the host cytoplasmic machinery and to ensure vertical transmission are yet to be discovered. Apart from the well-established role of Wolbachia in triggering reproductive alterations, a new fascinating aspect is emerging, related to the ecological benefits that the symbiont provides to the host. The mutualistic relationship of Wolbachia strains with disease vectors remains among the top research priorities with new insights having an impact on putative anti-filarial strategies. Address Department of Environmental and Natural Resources Management, University of Ioannina, 2, Seferi st., 30100 Agrinio, Greece Corresponding author: Bourtzis, Kostas (kbourtz@uoi.gr) Current Opinion in Microbiology 2010, 13:67���72 This review comes from a themed issue on Host���microbe interactions: bacteria Edited by C Erec Stebbins and David O���Callaghan Available online 23rd December 2009 1369-5274/$ ��� see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.mib.2009.11.005 Introduction Wolbachia is a maternally transmitted intracellular sym- biont belonging to the a-proteobacteria. Wolbachia is mainly localized in the reproductive tissues of arthropods and it is responsible for the induction of a number of reproductive alterations including feminization, parthe- nogenesis, male-killing and cytoplasmic incompatibility (CI). Apart from reproductive parasitism, Wolbachia also participates in mutualistic relationships with nematode hosts [1 ]. The widespread distribution of Wolbachia as well as the manipulation of host���s reproductive system places this symbiont among the most promising targets for disease/pest control [2]. The aim of this review is to present the most recent advances in this field. Wolbachia distribution, diversity and evolution Wolbachia was first discovered in the gonads of the mos- quito Culex pipientis in 1924. Since then, Wolbachia has been detected in many host tissues (Figure 1). Wolbachia has evolved several strategies to ensure vertical trans- mission through the manipulation of host reproductive system. These strategies include feminization, partheno- genesis, male killing and cytoplasmic incompatibility. All the above phenotypes, commonly referred to as ���repro- ductive parasitism���, increase the frequency of infected females in the host population. PCR-based screening methods revealed the widespread distribution of the symbiont in diverse hosts, covering numerous taxa of arthropods and filarial nematodes. Several studies estimate the levels of infected insect species to be up to 70% worldwide, emphasizing the ���pandemic��� nature of Wolbachia [1 ,3]. Recently, the associ- ation of Wolbachia with Radopholus similis, a plant parasitic nematode, was also reported, broading the host range even more[4].Thegenus Wolbachia istaxonomicallysubdivided into different supergroups on the basis of phylogenetic inference of several genes including 16S rDNA, wsp, ftsZ, and others.Recent studiesusingmultiple gene sequencing extended the number of Wolbachia supergroups up to eleven (namely A���K) [5���7]. Despite the significant number of phylogenetic and taxo- nomic studies, the evolutionary root of Wolbachia is still subject of scientific debate. The examination of Wolbachia strains found in nematodes and arthropods led to the preliminary conclusion that the root of the Wolbachia clade lies between the nematode and arthropod symbionts [8] however, current phylogenetic analysis methods are prone to long-branch attraction artifacts and therefore, the defi- nite evolutionary relationships of the Wolbachia parasites and mutualists remain unresolved [5]. Wolbachia���host interactions Interesting aspects of the host���symbiont genomic inter- play have been recently revealed. Lateral gene transfer is well established between Wolbachia and hosts in both insect and nematode species, although the direction of the transfer is in some cases unclear [9���13]. It is also worth noting that an aphid gene acquired via lateral gene transfer from a bacterial symbiont closely related to Wolbachia has been found to functionally compensate the absence of its homolog from the primary symbiont Buchnera [14]. Recently, genetic exchange between Wol- bachia and other symbionts has also been reported Darby et al. describe the presence of a Wolbachia-like surface protein in Arsenophonus nasoniae genome [15]. Maternally inherited Wolbachia is also an excellent can- didate for inducing epigenetic alterations to the host www.sciencedirect.com Current Opinion in Microbiology 2010, 13:67���72

genome. In the leafhopper Zyginidia pullula, the feminiz- ing strain of Wolbachia was found to alter the host���s genomic imprinting by methylation, influencing the expression of genes involved in sex differentiation and development [16]. Similar mechanisms may operate in other Wolbachia-induced phenotypes as well, although this remains to be proved. Evidence exists that Wolbachia manipulates host anti- oxidantsystemsinamannerthatisbeneficialtoitssurvival. The stable symbiotic interaction with Wolbachia in an Aedes albopictus cell line involves reactive oxygen species (ROS) generation and induction of antioxidant enzymes [17]. In isopods, the localization of Wolbachia within hemocytes clearly affects the hemocyte density and the degree of septicaemia in the haemolymph. The effect of the symbiont on the host���s immunocompetence and survival significantly varied within thesame population,depending on the Wolbachia strain infecting the host [18]. Microarray analysis of Wolbachia-infected and uninfected cells revealed a number of differentially expressed genes involved in reproduction, immunity, and heat stress response. Among these, the angiotensin converting enzyme (Ance) emerged as a candidate CI mediator [19]. Moreover, the role of an insulin/IGF-like signalling enhancer has also been recently proposed for Wolbachia, although mechanism and biological significance of such an effect remain unclear [20]. Undoubtedly, the most extensively studied but still eva- sive Wolbachia-associated phenotype is cytoplasmic incompatibility. The study of CI led to the assumption that there are at least two distinct functions involved in CI, the ���modification��� and the ���rescue��� function. When the female lacks the ���rescue��� function, the ���modification��� of the male results in embryonic lethality. Later studies revealed more of the complexity of the phenomenon, unraveling the multiple rescue determinants that a given Wolbachia strain can carry [21]. Although the exact mechanism of CI remains unclear, the incompatibility phenotype is associated with an asyn- chrony in the development of the male and female pronuclei (reviewed in [22 ]). Subsequent studies extended this knowledge to prove that CI produces a delay in recruitment of the histone H3.3/H4 complex to the male pronucleus, which normally occurs immediately after protamine removal this may be the cause of the later observed mitotic condensation and segregation defects [23 ]. The way Wolbachia promotes these defects is obscure, given the fact that the presence of the symbiont is not required in individual spermatocytes or spermatids for sperm modification in some systems [24]. A possible functional link between different Wolbachia- induced phenotypes emerged with the observation that suppression of the male-killing phenotype can lead to the instantaneous induction of CI in the butterfly Hypolimnas bolina [25]. The ability of a given Wolbachia strain to cause multiple reproductive phenotypes in different hosts has also been reported [26]. The picture of host population dynamics and ecology has been additionally complicated by the report that Wolbachia-associated sex ratio distortion was found altered over short time scales [27]. 68 Host���microbe interactions: bacteria Figure 1 Wolbachia distribution in insect tissues and effects on host biology. Current Opinion in Microbiology 2010, 13:67���72 www.sciencedirect.com