Two different rickettsial bacteria invading Volvox carteri

BACKGROUND

Bacteria of the family Rickettsiaceae are principally associated with arthropods. Recently, endosymbionts of the Rickettsiaceae have been found in non-phagotrophic cells of the volvocalean green algae Carteria cerasiformis, Pleodorina japonica, and Volvox carteri. Such endosymbionts were present in only C. cerasiformis strain NIES-425 and V. carteri strain UTEX 2180, of various strains of Carteria and V. carteri examined, suggesting that rickettsial endosymbionts may have been transmitted to only a few algal strains very recently. However, in preliminary work, we detected a sequence similar to that of a rickettsial gene in the nuclear genome of V. carteri strain EVE.

METHODOLOGY/PRINCIPAL FINDINGS

Here we explored the origin of the rickettsial gene-like sequences in the endosymbiont-lacking V. carteri strain EVE, by performing comparative analyses on 13 strains of V. carteri. By reference to our ongoing genomic sequence of rickettsial endosymbionts in C. cerasiformis strain NIES-425 cells, we confirmed that an approximately 9-kbp DNA sequence encompassing a region similar to that of four rickettsial genes was present in the nuclear genome of V. carteri strain EVE. Phylogenetic analyses, and comparisons of the synteny of rickettsial gene-like sequences from various strains of V. carteri, indicated that the rickettsial gene-like sequences in the nuclear genome of V. carteri strain EVE were closely related to rickettsial gene sequences of P. japonica, rather than those of V. carteri strain UTEX 2180.

CONCLUSION/SIGNIFICANCE

At least two different rickettsial organisms may have invaded the V. carteri lineage, one of which may be the direct ancestor of the endosymbiont of V. carteri strain UTEX 2180, whereas the other may be closely related to the endosymbiont of P. japonica. Endosymbiotic gene transfer from the latter rickettsial organism may have occurred in an ancestor of V. carteri. Thus, the rickettsiae may be widely associated with V. carteri, and likely have often been lost during host evolution.

'Candidatus Megaira polyxenophila' gen. nov., sp. nov.: considerations on evolutionary history, host range and shift of early divergent rickettsiae

"Neglected Rickettsiaceae" (i.e. those harboured by non-hematophagous eukaryotic hosts) display greater phylogenetic variability and more widespread dispersal than pathogenic ones; yet, the knowledge about their actual host range and host shift mechanism is scarce. The present work reports the characterization following the full-cycle rRNA approach (SSU rRNA sequence, specific in situ hybridization, and ultrastructure) of a novel rickettsial bacterium, herewith proposed as 'Candidatus Megaira polyxenophila' gen. nov., sp. nov. We found it in association with four different free-living ciliates (Diophrys oligothrix, Euplotes octocarinatus, Paramecium caudatum, and Spirostomum sp., all belonging to Alveolata, Ciliophora); furthermore it was recently observed as intracellular occurring in Carteria cerasiformis and Pleodorina japonica (Chlorophyceae, Chlorophyta). Phylogenetic analyses demonstrated the belonging of the candidate new genus to the family Rickettsiaceae (Alphaproteobacteria, Rickettsiales) as a sister group of the genus Rickettsia. In situ observations revealed the ability of the candidate new species to colonize either nuclear or cytoplasmic compartments, depending on the host organism. The presence of the same bacterial species within different, evolutionary distant, hosts indicates that 'Candidatus Megaira polyxenophila' recently underwent several distinct host shifts, thus suggesting the existence of horizontal transmission pathways. We consider these findings as indicative of an unexpected spread of rickettsial infections in aquatic communities, possibly by means of trophic interactions, and hence propose a new interpretation of the origin and phylogenetic diversification of rickettsial bacteria.

Molecular identification of rickettsial endosymbionts in the non-phagotrophic volvocalean green algae

Background

The order Rickettsiales comprises Gram-negative obligate intracellular bacteria (also called rickettsias) that are mainly associated with arthropod hosts. This group is medically important because it contains human-pathogenic species that cause dangerous diseases. Until now, there has been no report of non-phagotrophic photosynthetic eukaryotes, such as green plants, harboring rickettsias.

Methodology/Principal Findings

We examined the bacterial endosymbionts of two freshwater volvocalean green algae: unicellular Carteria cerasiformis and colonial Pleodorina japonica. Epifluorescence microscopy using 4′-6-deamidino-2-phenylindole staining revealed the presence of endosymbionts in all C. cerasiformis NIES-425 cells, and demonstrated a positive correlation between host cell size and the number of endosymbionts. Strains both containing and lacking endosymbionts of C. cerasiformis (NIES-425 and NIES-424) showed a >10-fold increase in cell number and typical sigmoid growth curves over 192 h. A phylogenetic analysis of 16 S ribosomal (r)RNA gene sequences from the endosymbionts of C. cerasiformis and P. japonica demonstrated that they formed a robust clade (hydra group) with endosymbionts of various non-arthropod hosts within the family Rickettsiaceae. There were significantly fewer differences in the 16 S rRNA sequences of the rickettsiacean endosymbionts between C. cerasiformis and P. japonica than in the chloroplast 16 S rRNA or 18 S rRNA of the host volvocalean cells. Fluorescence in situ hybridization demonstrated the existence of the rickettsiacean endosymbionts in the cytoplasm of two volvocalean species.

Conclusions/Significance

The rickettsiacean endosymbionts are likely not harmful to their volvocalean hosts and may have been recently transmitted from other non-arthropod organisms. Because rickettsias are the closest relatives of mitochondria, incipient stages of mitochondrial endosymbiosis may be deduced using both strains with and without C. cerasiformis endosymbionts.

Cellular tropism, population dynamics, host range and taxonomic status of an aphid secondary symbiont, SMLS (Sitobion miscanthi L type symbiont)

SMLS (Sitobion miscanthi L type symbiont) is a newly reported aphid secondary symbiont. Phylogenetic evidence from molecular markers indicates that SMLS belongs to the Rickettsiaceae and has a sibling relationship with Orientia tsutsugamushi. A comparative analysis of coxA nucleotide sequences further supports recognition of SMLS as a new genus in the Rickettsiaceae. In situ hybridization reveals that SMLS is housed in both sheath cells and secondary bacteriocytes and it is also detected in aphid hemolymph. The population dynamics of SMLS differ from those of Buchnera aphidicola and titer levels of SMLS increase in older aphids. A survey of 13 other aphids reveals that SMLS only occurs in wheat-associated species.

New findings from an old pathogen: intraerythrocytic bacteria (family Anaplasmatacea) in red-backed salamanders Plethodon cinereus

During a recent study of red-backed salamanders (Plethodon cinereus), we discovered an intraerythrocytic organism typified by violet-staining, intracellular inclusions, consistent with descriptions of Cytamoeba or Aegyptianella (bacteria). Here we characterize its taxonomic status using molecular techniques and ask basic questions about its nature. Blood smears from 102 salamanders were examined from Pennsylvania, New York, and Virginia to determine prevalence, and whole blood from several infected animals was tested using a PCR which targets the 16S rRNA gene of bacteria. Phylogenetic analysis of partial 16S rRNA gene sequence (1201 bp) indicated this organism was in the order Rickettsiales and is likely a member of the family Anaplasmatacea. The organism differed from currently described taxa and was clearly differentiated from Aegyptianella pullorum of birds and "Candidatus Hemobacterium ranarum" (formally A. ranarum) of frogs. Of all salamanders, 17 (16.7%) were infected and these were significantly larger (snout-vent length) and had higher body condition scores than uninfected ones, and males were more likely to be infected than females. Erythrocytes affected by the pathogen were 5% larger than unaffected ones, but otherwise similar in morphology. Infected animals tended to have a greater number of circulating white blood cells, based on estimates from smears, indicating a nonspecific response to the pathogen by the innate immune system. Given its phylogenetic position, this pathogen is likely transmitted by an arthropod vector, and the male-biased prevalence strongly implicates trombiculid mites, which also live in leaf litter and affect male salamanders more so than females.

Strawberry disease lesions in rainbow trout from southern Idaho are associated with DNA from a Rickettsia-like organism

Strawberry disease (SD) in the USA is a skin disorder of unknown etiology that occurs in rainbow trout Oncorhynchus mykiss and is characterized by bright red inflammatory lesions. To identify a candidate bacterial agent responsible for SD, we constructed 16S rDNA libraries from 7 SD lesion samples and 2 apparently healthy skin samples from SD-affected fish. A 16S rDNA sequence highly similar to members of the order Rickettsiales was present in 3 lesion libraries at 1%, 32% and 54% prevalence, but this sequence was not found in either healthy tissue library. Based on phylogenetic analysis, this Rickettsia-like organism (RLO) sequence is most closely related to 16S rDNA sequences of bacteria that may form a novel lineage within the Rickettsiales. We used nested PCR assays to screen 25 SD-affected fish for RLO or Flavobacterium psychrophilum DNA. Sixteen lesion samples were positive for the RLO sequence and 4 of the matched healthy samples were positive resulting in a significant association between SD lesions and presence of RLO DNA. While F. psychrophilum is reportedly associated with 'cold water strawberry disease' in the UK, we found no significant association between SD lesions and the presence of F. psychrophilum DNA. The statistical association between SD lesions and presence of RLO DNA is not proof of etiology, but these data suggest that RLO may play a role in SD in southern Idaho, USA.

A robust species tree for the alphaproteobacteria

The branching order and coherence of the alphaproteobacterial orders have not been well established, and not all studies have agreed that mitochondria arose from within the Rickettsiales. A species tree for 72 alphaproteobacteria was produced from a concatenation of alignments for 104 well-behaved protein families. Coherence was upheld for four of the five orders with current standing that were represented here by more than one species. However, the family Hyphomonadaceae was split from the other Rhodobacterales, forming an expanded group with Caulobacterales that also included Parvularcula. The three earliest-branching alphaproteobacterial orders were the Rickettsiales, followed by the Rhodospirillales and then the Sphingomonadales. The principal uncertainty is whether the expanded Caulobacterales group is more closely associated with the Rhodobacterales or the Rhizobiales. The mitochondrial branch was placed within the Rickettsiales as a sister to the combined Anaplasmataceae and Rickettsiaceae, all subtended by the Pelagibacter branch. Pelagibacter genes will serve as useful additions to the bacterial outgroup in future evolutionary studies of mitochondrial genes, including those that have transferred to the eukaryotic nucleus.

Prevalence of Bacterial Agents in Ixodes persulcatus Ticks from the Vologda Province of Russia

The prevalence of rickettsiae, ehrlichiae, and the rickettsia-like endosymbiont called Montezuma relative to that of Borrelia was determined in questing Ixodes persulcatus (I. persulcatus) ticks collected in 2002-2003 from Vologda Province, Russia. Ehrlichia muris, Anaplasma phagocytophilum, Montezuma, and new spotted fever group rickettsiae were detected by polymerase chain reaction (PCR) for the first time in this area. The rickettsiae were all Candidatus Rickettsia tarasevichiae, the furthest west this organism has been detected. After Borrelia, Montezuma was the agent most frequently detected; it may be present throughout the distribution of I. persulcatus in Russia. Ehrlichiae and rickettsiae frequently share the same tick host with Borrelia burgdorferi sensu lato so cotransmission and mixed infections in vertebrate hosts, including humans, may occur.

Wolbachia genomes: revealing the biology of parasitism and mutualism

Wolbachia bacteria are endosymbiotic partners of many animal species, in which they behave as either parasites (in arthropod hosts) or mutualists (in nematode hosts). What biochemistry and biology underpin these diverse lifestyles? The recent complete sequencing of genomes from Wolbachia that infect the arthropod Drosophila melanogaster and the nematode Brugia malayi, together with the partial genome sequencing of three Wolbachia strains found in drosophilids, enables this question to begin to be addressed. Parasitic arthropod Wolbachia are characterized by the presence of phages that carry ankyrin-repeat proteins; these proteins might be exported to the host cell to manipulate reproduction. In nematode Wolbachia, which lack these phages, several biochemical pathways can deliver essential metabolites to the nematode hosts. Nematode Wolbachia might also have a role in modulating the mammalian host immune system but the sequenced Wolbachia genomes lack the genes to synthesize lipopolysaccharide, raising questions about the nature of the inducing molecule. The Wolbachia surface protein might carry out this function.

[Molecular genetic variety of rickettsiae]

In this review modern data on the study of the genome of the representatives of the family Rickettsiaceae and, as follows from the accumulated information, on changes in the nomination and hierarchical place of its pathogenic and avirulent representatives are presented.

[Detection of a new species of Rickettsiae in the ticks of Ixodes persulcatus in Russia]

Genetic analyses (polymerase chain reaction and nucleotide sequence test) have been used to describe a new species of Rickettsiae in the ticks Ipersukarus in Russia. This species is named as Rzckettsia tarasevichiae after Academician of the Russian Academy of Medical Sciences Irma Vladimirovna Tarasevich, Head of the Laboratory of Rickettsial Ecology, N.F.Gamaleya Research Institute of Epidemiology and Microbiology. A total of 317 Ipersukatus imagoes were collected in the regions of the Urals, Western Siberia, Eastern Siberia, and Far East in the spring of 2001 and 2002. The authors detected R. tarasevichiae in 25.5 +/- 2.4% (87/317) of the examined I. persulcarus ticks, from 3.8% in the Novosibirsk Region (Western Siberia) to 56.8% in the Primorye Territory (the Far East). This is the first detection of the Rickettsiae in the ticks of the genus Ixodes in Russia. Conceivably, the spread of this Rickettsia species coincides with the area of I. persulcatus that inhabits the biotopes of southern taiga and mixed forests in Eurasia from the western frontier of Russia to the Far East. The nucleotide sequences of the fragments of gene 165 of pRNA and citrate synthctase of this species of Rickettsiae have been declared at the GenBank under numbers AF503168 and AF503167, respectively. Its most phylogenetically close species of Rickettsiae R. canadenss (69% homology in terms of the gene of citrate synthetase and 98% homology in terms of gene 165 of pRNA) which is responsible for cases of acute cerebral vasculitis in Northern America. This requires further studies of a possible role of R. tarasevichiae in the pattern of tick-borne infections recorded in the area of I. persulcatus.

Multiple displacement amplification in combination with high-fidelity PCR improves detection of bacteria from single females or eggs of Metaseiulus occidentalis (Nesbitt) (Acari: Phytoseiidae)

Amplifying microbial DNA by the polymerase chain reaction (PCR) from single phytoseiid mites has been difficult, perhaps due to the low titer of bacteria and to interference by the relatively larger amounts of mite genomic DNA. In this paper we evaluate the efficiency of standard and high-fidelity PCR protocols subsequent to amplification of the whole genome by a multiple displacement amplification (MDA) procedure developed by Dean et al. DNA from the phytoseiid Phytoseiulus persimilis (Athias-Henriot) was tested because it lacks a Cytophaga-like organism (CLO) and we could add known amounts of a plasmid containing a cloned 16S rRNA gene fragment from a CLO from Metaseiulus occidentalis (Nesbitt). P. persimilis genomic DNA was mixed with the serially diluted plasmid and amplified using MDA followed by either standard or high-fidelity PCR. MDA followed by high-fidelity PCR was most efficient and successfully amplified an expected 1.5-kb band from as little as 0.01fg of the plasmid, which is equivalent to about 1 copy. MDA followed by high-fidelity PCR also consistently amplified Wolbachia- or CLO-specific products from naturally infected single females or eggs of M. occidentalis, which will allow detailed studies of infection frequency and transmission of several microorganisms associated with this predatory mite.

History of the ADP/ATP-translocase-encoding gene, a parasitism gene transferred from a Chlamydiales ancestor to plants 1 billion years ago

Nonmitochondrial ADP/ATP translocase is an energy parasite enzyme. Its encoding gene, tlc, is found only in Rickettsiales, Chlamydiales, and plant and alga plastids. We demonstrate the presence of tlc in Parachlamydia acanthamoebae. This gene shares more similarity with the tlc1 gene of Chlamydiaceae and the tlc of plant and alga plastids than with the tlc2 gene of Chlamydiaceae. Phylogenetic analysis, including all other tlc homologs found in GenBank, showed that tlc was duplicated in a Chlamydiales ancestor before the appearance of multicellular eukaryotes. A time scale, calibrated with seven independent time points obtained from fossil estimates and from the 16S rRNA molecular clock, was congruent with the molecular clock provided by tlc. Plant and alga plastids acquired tlc approximately when Parachlamydiaceae and Chlamydiaceae diverged, at the eucaryotic radiation time, ca. 1 billion years ago.

A survey for spotted fever group rickettsiae and ehrlichiae in Amblyomma variegatum from St. Kitts and Nevis

Eighty-nine Amblyomma variegatum ticks were collected from the islands of St. Kitts and Nevis in the Caribbean and preserved in 70% ethanol or local rum. After being washed in sterile water, their DNA was extracted and analyzed by a polymerase chain reaction (PCR) for DNA of spotted fever group rickettsiae and ehrlichiae. None of the tested ticks was positive in a PCR assay using the primers 16S EHRD and 16S EHRR for the 16S rRNA gene of Ehrlichia spp.. Forty-one percent of the A. variegatum (36 of 89 of which 34 [47%] of 72 were adult males, 2 (13%) of 16 were adult females, and 0 (0%) of 1 were nymphs) were positive in a PCR assay using the primer pair 190-70 and 190-701 for the outer membrane protein A (ompA) gene of spotted fever group rickettsiae. All PCR amplification products obtained had 100% sequence homology with Rickettsia africae, the agent of African tick-bite fever.

Detection of Neorickettsia (Ehrlichia) risticii in tissues of mice experimentally infected with cercariae of trematodes by in situ hybridization

Neorickettsia (Ehrlichia) risticii was demonstrated to occur in cercariae developing in Juga yrekaensis snails by experimental transmission, genetic detection and histopathology. Cercariae were isolated from the digestive glands of snails collected in a fresh stream water area of Siskiyou County, CA, and inoculated into CF1 mice. Mice developed clinical signs, splenomegaly and histopathologic abnormalities. The agent was maintained by serial passages of whole blood in CF1 mice. A 527-bp product of the 16S rRNA gene of N. risticii was serially detected by nested PCR in blood, feces, salivary gland, suprarenal gland, spleen, intestine and bone marrow of inoculated mice. N. risticii DNA was detected by in situ hybridization with DIG-labeled probe in PCR-positive salivary gland, intestine and spleen tissue sections of experimental mice on day 30 after inoculation. Infection in mice was established when cercariae were inoculated by either IP or SC routes but not established following intraoral route. N. risticii was detected by PCR in spleen, intestine and bone marrow even after 73 days post-inoculation whereas blood from the same animals became negative at 58 days. N. risticii was observed by in situ hybridization in salivary gland, spleen and intestine of mice infected by IP or SC inoculation. This ISH protocol should aid investigations on the host range of the Neorickettsiosis and pathogenesis of neorickettiosis in vector, animal or human.

Comparison of chickpea rhizobia isolates from diverse Portuguese natural populations based on symbiotic effectiveness and DNA fingerprint

AIMS

To test the hypothesis that differences in chickpea yields obtained in four distinct Portuguese regions (Beja, Elvas-Casas Velhas, Elvas-Estação Nacional de Melhoramento de Plantas (ENMP) and Evora) could be due to variation between the natural rhizobia populations.

METHODS AND RESULTS

Estimation of the size of the different rhizobial populations showed that Elvas-ENMP population was the largest one. Elvas-ENMP population also revealed a higher proportion of isolates carrying more than one plasmid. Assessment of genetic diversity of the native rhizobia populations by a DNA fingerprinting PCR method, here designated as DAPD (Direct Amplified Polymorphic DNA), showed a higher degree of variation in Elvas-ENMP and Beja populations. The symbiotic effectiveness (SE) of 39 isolates was determined and ranged 13-34%. Statistical analysis showed that SE was negatively correlated with plasmid number of the isolate.

CONCLUSIONS

The largest indigenous rhizobia population was found in Elvas-ENMP. DAPD pattern and plasmid profile analysis both suggested a higher genetic diversity among the populations of Elvas-ENMP and Beja. No relationship was found between SE of the isolates and their origin site.

SIGNIFICANCE AND IMPACT OF STUDY

The large native population, rather than the symbiotic performance of individual rhizobia, could contribute to the higher chickpea yields obtained in Elvas-ENMP.

Molecular analysis of Neorickettsia risticii in adult aquatic insects in Pennsylvania, in horses infected by ingestion of insects, and isolated in cell culture

Upon ingestion of adult aquatic insects, horses developed clinical signs of Potomac horse fever, and Neorickettsia risticii was isolated from the blood. 16S rRNA and 51-kDa antigen gene sequences from blood, isolates, and caddis flies fed to the horses were identical, proving oral transmission of N. risticii from caddis flies to horses.

Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila

The genera Anaplasma, Ehrlichia, Cowdria, Neorickettsia and Wolbachia encompass a group of obligate intracellular bacteria that reside in vacuoles of eukaryotic cells and were previously placed in taxa based upon morphological, ecological, epidemiological and clinical characteristics. Recent genetic analyses of 16S rRNA genes, groESL and surface protein genes have indicated that the existing taxa designations are flawed. All 16S rRNA gene and groESL sequences deposited in GenBank prior to 2000 and selected sequences deposited thereafter were aligned and phylogenetic trees and bootstrap values were calculated using the neighbour-joining method and compared with trees generated with maximum-probability, maximum-likelihood, majority-rule consensus and parsimony methods. Supported by bootstrap probabilities of at least 54%, 16S rRNA gene comparisons consistently clustered to yield four distinct clades characterized roughly as Anaplasma (including the Ehrlichia phagocytophila group, Ehrlichia platys and Ehrlichia bovis) with a minimum of 96.1% similarity, Ehrlichia (including Cowdria ruminantium) with a minimum of 97.7% similarity, Wolbachia with a minimum of 95.6% similarity and Neorickettsia (including Ehrlichia sennetsu and Ehrlichia risticii) with a minimum of 94.9% similarity. Maximum similarity between clades ranged from 87.1 to 94.9%. Insufficient differences existed among E. phagocytophila, Ehrlichia equi and the human granulocytic ehrlichiosis (HGE) agent to support separate species designations, and this group was at least 98.2% similar to any Anaplasma species. These 16S rRNA gene analyses are strongly supported by similar groESL clades, as well as biological and antigenic characteristics. It is proposed that all members of the tribes Ehrlichieae and Wolbachieae be transferred to the family Anaplasmataceae and that the tribe structure of the family Rickettsiaceae be eliminated. The genus Anaplasma should be emended to include Anaplasma (Ehrlichia) phagocytophila comb. nov. (which also encompasses the former E. equi and the HGE agent), Anaplasma (Ehrlichia) bovis comb. nov. and Anaplasma (Ehrlichia) platys comb. nov., the genus Ehrlichia should be emended to include Ehrlichia (Cowdria) ruminantium comb. nov. and the genus Neorickettsia should be emended to include Neorickettsia (Ehrlichia) risticii comb. nov. and Neorickettsia (Ehrlichia) sennetsu comb. nov.

Rickettsiaceae, rickettsia-like endosymbionts, and the origin of mitochondria

Accumulating evolutionary data point to a monophyletic origin of mitochondria from the order Rickettsiales. This large group of obligate intracellular alpha-Proteobacteria includes the family Rickettsiaceae and several rickettsia-like endosymbionts (RLEs). Detailed phylogenetic analysis of small subunit (SSU) rRNA and chaperonin 60 (Cpn60) sequences testify to polyphyly of the Rickettsiales, and consistently indicate a sisterhood of Rickettsiaceae and mitochondria that excludes RLEs. Thus RLEs are considered as the nearest extant relatives of an extinct last common ancestor of mitochondria and rickettsiae. Phylogenetic inferences prompt the following assumptions. (1) Mitochondrial origin has been predisposed by the long-term endosymbiotic relationship between rickettsia-like bacteria and proto-eukaryotes, in which many endosymbiont genes have been lost while some indispensable genes have been transferred to the host genome. (2) The obligate dependence of rickettsiae upon a eukaryotic host rests on the import of proteins encoded by these transferred genes. The nature of a proto-eukaryotic cell still remains elusive. The divergence of Rickettsiaceae and mitochondria based on Cpn60, and the evolutionary history of two aminoacyl-tRNA synthetases favor the hypothesis that it was a chimera created by fusion of an archaebacterium and a eubacterium not long before an endosymbiotic event. These and other, mostly biochemical data suggest that all the mitochondrion-related organelles, i.e., both aerobically and anaerobically respiring mitochondria and hydrogenosomes, have originated from the same RLE, while hydrogenosomal energy metabolism may have a separate origin resulting from a eubacterial fusion partner.

Mitochondrial DNA polymorphism, sex ratio distorters and population genetics in the isopod Armadillidium vulgare

Two maternally inherited sex ratio distorters (SRD) impose female-biased sex ratios on the wood louse Armadillidium vulgare by feminizing putative males. These SRD are (i) an intracytoplasmic bacterium of the genus Wolbachia, and (ii) another non-Mendelian element of unknown nature: the f element. Mitochondrial DNA variation was investigated in A. vulgare field populations to trace the evolution of host-SRD relationships and to investigate the effect of SRD on host cytoplasmic polymorphism. The Wolbachia endosymbionts showed no polymorphism in their ITS2 sequence and were associated with two closely related mitochondrial types. This situation probably reflects a single infection event followed by a slight differentiation of mitochondria. There was no association between the f element and a given mitochondrial type, which may confirm the fact that this element can be partially paternally transmitted. The spreading of a maternally inherited SRD in a population should reduce the mitochondrial diversity by a hitchhiking process. In A. vulgare, however, a within-population mtDNA polymorphism was often found, because of the deficient spread of Wolbachia and the partial paternal inheritance of the f element. The analysis of molecular variance indicated that A. vulgare populations are genetically structured, but without isolation by distance.

Phylogeny of the arthropod endosymbiont Wolbachia based on the wsp gene

Bacteria of the genus Wolbachia (Rickettsiae) are widespread in arthropods and can induce cytoplasmic incompatibility (CI), thelytoky (T) or feminization (F) in their host. Recent research on the wsp gene of mainly CI inducing Wolbachia has shown that this gene evolves at a much faster rate than previously sequenced genes such as 16S or ftsZ. As a result this gene appears to be very useful in subdividing the Wolbachia and twelve groups have been distinguished to date. Here we extend the Wolbachia wsp data set with fifteen T-Wolbachia, one F-Wolbachia and three other CI-Wolbachia strains. The results showed: (i) the addition of seven groups; (ii) no relation between host phenotype and Wolbachia phylogenetic position; and (iii) possible horizontal Wolbachia transfer between the moth Ephestia kuehniella and its parasitoid Trichogramma spp.

Gene organization of the dnaA region of Wolbachia

The dnaA region of Wolbachia, an intracellular bacterial parasite of insects, is unique. A glnA cognate was found upstream of the dnaA gene, while neither of the two open reading frames detected downstream of dnaA has any homologue in the database. This unusual gene arrangement may reflect requirements associated with the unique ecological niche this agent occupies.

The first detection of the insertion sequence ISW1 in the intracellular reproductive parasite Wolbachia

Wolbachia are maternally inherited intracellular rickettsia-like bacteria known to infect a wide range of arthropods. They are associated with a number of different reproductive phenotypes in their hosts, such as cytoplasmic incompatibility, parthenogenesis, and feminization. We report on a novel insertion sequence (IS), ISW1, which was identified in the region downstream of groEL of a Wolbachia strain, wTai. The 573-bp-long ISW1 sequence is the first IS element observed in this organism, displays significant similarity to IS200, and lacks terminal inverted repeats. There were more than 20 copies of ISW1 on the chromosome of wTai. Sequence analysis of nine distinct ISW1 copies and their flanking regions showed that the copies were identical and suggested that ISW1 has no preference for its insertion sites. Possible roles of ISW1 in the adaptation of Wolbachia to intracellular environments and in various reproductive alterations caused by this bacterium are discussed.

Evolution of Wolbachia pipientis transmission dynamics in insects

Wolbachia pipientis is an intracellular bacterial parasite of arthropods that enhances its transmission by manipulating host reproduction, most commonly by inducing cytoplasmic incompatibility. The discovery of isolates with modified cytoplasmic incompatibility phenotypes and others with novel virulence properties is an indication of the potential breadth of evolutionary strategies employed by Wolbachia.

Phylogenetic relationships of symbiotic methanogens in diverse termites

Termites harbor symbiotic microorganisms in their gut which emit methane. The phylogeny of the termite methanogens was inferred without cultivation based on nucleotide sequences of PCR-amplified 16S ribosomal RNA genes. Seven methanogen sequences from four termite species were newly isolated, and together with those previously published, these sequences were phylogenetically compared. The termite methanogen sequences were divided into three clusters. Two clusters of sequences, derived from the gut DNA of so-called higher termites, were related to methanogens in the orders Methanosarcinales or Methanomicrobiales. All of the sequences in the case of lower termites were closely related to the genus Methanobrevibacter. However, most of the termite symbionts were found to be distinct from known methanogens. They are not dispersed among diverse methanogen species, but rather formed unique lineages in the phylogenetic trees.

Effects of tetracycline on the filarial worms Brugia pahangi and Dirofilaria immitis and their bacterial endosymbionts Wolbachia

Wolbachia endosymbiotic bacteria have been shown to be widespread among filarial worms and could thus play some role in the biology of these nematodes. Indeed, tetracycline has been shown to inhibit both the development of adult worms from third-stage larvae and the development of the microfilaraemia in jirds infected with Brugia pahangi. The possibility that these effects are related to the bacteriostatic activity of tetracycline on Wolbachia symbionts should be considered. Here we show that tetracycline treatment is very effective in blocking embryo development in two filarial nematodes, B. pahangi and Dirofilaria immitis. Embryo degeneration was documented by TEM, while the inhibition of the transovarial transmission of Wolbachia was documented by PCR. Phylogenetic analysis on the ssrDNA sequence of the Wolbachia of B. pahangi confirms that the phylogeny of the bacterial endosymbionts is consistent with that of the host worms. The possibility that tetracycline inhibition of embryo development in B. pahangi and D. immitis is determined by cytoplasmic incompatibility is discussed.

Spacer 2 region and 5S rDNA variation of Wolbachia strains involved in cytoplasmic incompatibility or sex-ratio distortion in arthropods

Bacteria in the genus Wolbachia are widespread in arthropods and can induce sex-ratio distortion or cytoplasmic incompatibility in their hosts. The phylogeny of Wolbachia has been studied using 16S rDNA and the cell cycle gene ftsZ, but sequence variation of those genes is limited. The spacer 2 region (SR2) was amplified to determine whether this region would improve phylogenetic resolution. The SR2 of Wolbachia is 66 bp long, shows higher variation than ftsZ and has very low homology with closely related bacteria. Due to the small length of SR2 of Wolbachia, little phylogenetic information could be retrieved.

Phylogeny of Wolbachia in filarial nematodes

Intracellular bacteria have been observed in various species of filarial nematodes (family Onchocercidae). The intracellular bacterium of the canine filaria Dirofilaria immitis has been shown to be closely related to Wolbachia, a rickettsia-like micro-organism that is widespread among arthropods. However, the relationships between endosymbionts of different filariae, and between these and the arthropod wolbachiae, appear not to have been studied. To address these issues we have examined ten species of filarial nematodes for the presence of Wolbachia. For nine species, all samples examined were PCR positive using primers specific for the ftsZ gene of Wolbachia. For one species, the examined samples were PCR negative. Sequences of the amplified ftsZ gene fragments of filarial wolbachiae fall into two clusters (C and D), which are distinct from the A and B clusters recognized for arthropod wolbachiae. These four lineages (A-D) are related in a star-like phylogeny, with higher nucleotide divergence observed between C and D wolbachiae than that observed between A and B wolbachiae. In addition, within each of the two lineages of filarial wolbachiae, the phylogeny of the symbionts is consistent with the host phylogeny. Thus, there is no evidence for recent Wolbachia transmission between arthropods and nematodes. Endosymbiont 16S ribosomal DNA sequences from a subset of filarial species support these findings.

Phylogenetic analysis of parthenogenesis-inducing Wolbachia in the genus Aphytis (Hymenoptera: Aphelinidae)

Parthenogenesis-inducing intracellular bacteria of the genus Wolbachia are found in a variety of parasitoid wasp genera. The presence of Wolbachia in the uniparental Aphytis species A. lingnanensis Compere, A. diaspidis (Howard), A. chilensis Howard, and A. chrysomphali (Mercet) was tested using primers specific for the ftsZ gene. The symbiont was detected in all of these species. Wolbachia ftsZ genes that were sequenced from the four hosts show a high degree of similarity. Both the PCR with specific primers for group 'A' and phylogenetic analysis place these Wolbachia in group 'A'. The fact that the tested Aphytis species belong to different phylogenetic groups and harbour what seem to be almost identical Wolbachia, lends credence to the horizontal transmission hypothesis.

Wolbachia in two insect host-parasitoid communities

Wolbachia form a group of intracellular bacteria that alter reproduction in their arthropod hosts. Two major phylogenetic subdivisions (A and B) of Wolbachia occur. Using a polymerase chain reaction assay we surveyed for the A and B group Wolbachia in 82 insect species from two temperate host-parasitoid communities (food webs) and a general collection of Lepidoptera caught at a light trap. One host-parasitoid community was based around leaf-mining Lepidoptera, and the other around Aphids. We found that: (i) 22.0% of insects sampled were infected with Wolbachia; and (ii) the prevalence and type (A or B) of Wolbachia infection differed significantly between communities and taxonomic groups. We obtained DNA sequences from the ftsZ gene for the group B Wolbachia found in six leaf-mining species and one of their parasitoids, as well as four of the Lepidoptera caught by a light trap. Taken together, the results of our survey and phylogenetic analyses of the sequence data suggest that host-parasitoid transfer of Wolbanchia is not the major route through which the species we have examined become infected. In addition, the Wolbachia strains observed in five leaf-mining species from the same genus were not closely related, indicating that transfer between species has not occurred due to a shared feeding niche or cospeciation.

Gene structure, activity and localization of a catalase from intracellular bacteria in Onchocerca volvulus

Within the context of studies on the antioxidant enzymes in Onchocerca volvulus, DNA clones encoding catalase (CAT) were isolated from an O. volvulus adult lambda zapII cDNA library. Analysis of their nucleotide and encoded amino acid sequences revealed that they derive from intracellular bacteria, rather than the O. volvulus nuclear genome. The endobacterial CAT gene was found to lie in a gene cluster, followed by a ferritin gene and an excinuclease gene. The endobacterial CAT gene encodes a functional enzyme capable of detoxifying H2O2, demonstrated by producing an active recombinant protein in an E. coli expression system. The purified 54 kDa protein has CAT activity over a broad pH range, with a specific activity of 103,000 +/- 3000 U mg(-1). The optical spectrum of the endobacterial CAT shows that it is a ferric haem-containing protein with a Soret band at 405 nm. To investigate the phylogeny of the intracellular bacterium in O. volvulus, a segment of the 16S rRNA gene was amplified from total genomic DNA by a polymerase chain reaction using universal eubacterial primers. A phylogenetic analysis of the O. volvulus-derived 16S rRNA sequence revealed that the endobacterium belongs to a distinct Wolbachia clade of the order Rickettsiales. Onchocercomata and biopsies containing different onchocercal species were immunohistochemically stained using polyclonal antibodies raised against the recombinant endobacterial CAT. CAT was detected in the endobacteria in the hypodermis of adult male and female O. volvulus, O. ochengi, O. gibsoni and O. fasciata. The endobacterial enzyme was also detected in onchocercal oocytes and all embryonic stages including intrauterine microfilariae as well as skin microfilariae. O. volvulus thus harbours Wolbachia-like endosymbionts which are transovarially transmitted and show particular affinity for the hypodermal tissues of the lateral chords.

A phylogenetic analysis of the cytochrome b and cytochrome c oxidase I genes supports an origin of mitochondria from within the Rickettsiaceae

We have cloned and sequenced the genes encoding cytochrome b (cob) and cytochrome c oxidase subunit I (cox1) from Rickettsia prowazekii, a member of the alpha-proteobacteria. The phylogenetic analysis supports the hypothesis that mitochondria are derived from the alpha-proteobacteria and more specifically from within the Rickettsiaceae. We have estimated that the common ancestor of mitochondria and Rickettsiaceae dates back to more than 1500 million years ago.

Evidence for widespread Wolbachia infection in isopod crustaceans: molecular identification and host feminization

Wolbachia are maternally inherited, intracellular, alpha proteobacteria that infect a wide range of arthropods. They cause three kinds of reproductive alterations in their hosts: cytoplasmic incompatibility, parthenogenesis and feminization. There have been many studies of the distribution of Wolbachia in arthropods, but very few crustacean species are known to be infected. We investigated the prevalence of Wolbachia in 85 species from five crustacean orders. Twenty-two isopod species were found to carry these bacteria. The bacteria were found mainly in terrestrial species, suggesting that Wolbachia came from a continental environment. The evolutionary relationships between these Wolbachia strains were determined by sequencing bacterial genes and by interspecific transfers. All the bacteria associated with isopods belonged to the Wolbachia B group, based on 16S rDNA sequence data. All the terrestrial isopod symbionts in this group except one formed an independent clade. The results of interspecific transfers show evidence of specialization of Wolbachia symbionts to their isopod hosts. They also suggest that host species plays a more important role than bacterial phylogeny in determining the phenotype induced by Wolbachia infection.

Cloning and characterization of a gene encoding the major surface protein of the bacterial endosymbiont Wolbachia pipientis

The maternally inherited intracellular symbiont Wolbachia pipientis is well known for inducing a variety of reproductive abnormalities in the diverse arthropod hosts it infects. It has been implicated in causing cytoplasmic incompatibility, parthenogenesis, and the feminization of genetic males in different hosts. The molecular mechanisms by which this fastidious intracellular bacterium causes these reproductive and developmental abnormalities have not yet been determined. In this paper, we report on (i) the purification of one of the most abundantly expressed Wolbachia proteins from infected Drosophila eggs and (ii) the subsequent cloning and characterization of the gene (wsp) that encodes it. The functionality of the wsp promoter region was also successfully tested in Escherichia coli. Comparison of sequences of this gene from different strains of Wolbachia revealed a high level of variability. This sequence variation correlated with the ability of certain Wolbachia strains to induce or rescue the cytoplasmic incompatibility phenotype in infected insects. As such, this gene will be a very useful tool for Wolbachia strain typing and phylogenetic analysis, as well as understanding the molecular basis of the interaction of Wolbachia with its host.

Phylogeny and PCR-based classification of Wolbachia strains using wsp gene sequences

Wolbachia are a group of intracellular inherited bacteria that infect a wide range of arthropods. They are associated with a number of different reproductive phenotypes in their hosts, such as cytoplasmic incompatibility, parthenogenesis and feminization. While it is known that the bacterial strains responsible for these different host phenotypes form a single clade within the alpha-Proteobacteria, until now it has not been possible to resolve the evolutionary relationships between different Wolbachia strains. To address this issue we have cloned and sequenced a gene encoding a surface protein of Wolbachia (wsp) from a representative sample of 28 Wolbachia strains. The sequences from this gene were highly variable and could be used to resolve the phylogenetic relationships of different Wolbachia strains. Based on the sequence of the wsp gene from different Wolbachia isolates we propose that the Wolbachia pipientis clade be initially divided into 12 groups. As more sequence information becomes available we expect the number of such groups to increase. In addition, we present a method of Wolbachia classification based on the use of group-specific wsp polymerase chain reaction (PGR) primers which will allow Wolbachia isolates to be typed without the need to clone and sequence individual Wolbachia genes. This system should facilitate future studies investigating the distribution and biology of Wolbachia strains from large samples of different host species.

Endosymbionts of ticks and their relationship to Wolbachia spp. and tick-borne pathogens of humans and animals

The presence, internal distribution, and phylogenetic position of endosymbiotic bacteria from four species of specific-pathogen-free ticks were studied. These included the hard ticks Ixodes scapularis (the black-legged tick), Rhipicephalus sanguineus (the brown dog tick), and Haemaphysalis longicornis and the African soft tick Ornithodoros moubata. PCR assays for bacteria, using two sets of general primers for eubacterial 16S and 23S rRNA genes (rDNAs) and seven sets of specific primers for wolbachial, rickettsial, or Francisella genes, indicated that I. scapularis possessed symbiotic rickettsiae in the ovaries and that the other species harbored eubacteria in both the ovaries and Malpighian tubules. Phylogenetic analysis based on the sequence of 16S rDNA indicated that the symbiont of I. scapularis belonged to the alpha subgroup of proteobacteria and was closely related to the members of the genus Rickettsia. The other species had similar microorganisms in the ovaries and Malpighian tubules, which belonged to the gamma subgroup of proteobacteria, and formed a monophyletic group with the Q-fever pathogen, Coxiella burnetii. O. moubata harbored another symbiont, which formed a monophyletic group with Francisella tularensis and Wolbachia persica, the latter a symbiont previously isolated from Malpighian tubules of the soft tick Argas (Persicargas) arboreus. Thus, the symbionts of these four tick species were not related to the Wolbachia species found in insects. The two symbionts that live in the Malpighian tubules, one closely related to C. burnetii and the other closely related to F. tularensis, appear to be of ancient origin and be widely distributed in ticks.

Reassessment of the taxonomic position of Rickettsiella grylli

We determined the 16S rRNA gene sequence of Rickettsiella grylli, an intracellular parasite of Gryllus bimaculatus and related species of crickets. Phylogenetic inferences made from alignment of this sequence with the sequences of other bacteria demonstrated that R. grylli is most closely related to Coxiella burnetii and Legionella species in the gamma subclass of the phylum Proteobacteria. R. grylli was previously thought to be related to members of the order Rickettsiales, but the representatives of this order have been shown to be members of the alpha 1 subclass of the Proteobacteria. Our results indicate that R. grylli should be removed from the order Rickettsiales.

Occurrence and transmission of facultative endosymbionts in aphids

The occurrence of a secondary bacterial symbiont (PASS) of pea aphid, Acyrthosiphon pisum (Harris), was detected by polymerase chain reaction (PCR) with specific nucleotide primers based on PASS 16S rDNA nucleotide sequences from over 80% (50/57) of clones of pea aphid collected from widely separated locations in California. PASS was also detected by PCR in both red and green phenotypes of rose aphid,Macrosiphum rosae (L.), but not in six other species of aphids examined, including blue alfalfa aphid (A. kondoi Shinji). The nucleotide sequences of the PCR-amplified, partial 16S rDNAs (1060 bp) from pea aphid and rose aphid were identical and 99.9% similar to the published 16S rDNA of PASS. PASS and a recently described new rickettsia of pea aphid (PAR) were transmitted by needle injection of hemolymph from positive pea aphid clones into negative clones and into blue alfalfa aphids. Both PASS and PAR were maintained in the offspring of some of the injected mother aphids via high rate of maternal transmission.

Wolbachia: intracellular manipulators of mite reproduction

Cytoplasmically transmitted Wolbachia (alpha-Proteobacteria) are a group of closely related intracellular microorganisms that alter reproduction in arthropods. They are found in a few isopods and are widespread in insects. Wolbachia are implicated as the cause of parthenogenesis in parasitic wasps, feminization in isopods and reproductive (cytoplasmic) incompatibility in many insects. Here we report on the widespread occurrence of Wolbachia in spider mites and predatory mites based on a PCR assay for a 730 bp fragment of the ftsZ gene with primers that are specific for Wolbachia. An additional PCR, using two primer pairs that amplify a 259 bp region of the ftsZ gene that are diagnostic for the two Wolbachia subdivisions A and B, showed that infected mites only carried type B and not type A Wolbachia. The fact that some species tested negative for Wolbachia does not mean that the entire species is uninfected. We found that natural populations of Tetranychus urticae are polymorphic for the infection. The possible effects of Wolbachia on mite reproduction and post-zygotic reproductive isolation are discussed.

Molecular identification of a Wolbachia endosymbiont in a Tetranychus urticae strain (Acari: Tetranychidae)

Wolbachia, a maternally inherited bacterium, is involved in alterations of arthropod sexuality. Reproductive incompatibilities are often observed in miles, but the existence of this microorganism in their cytoplasm has not yet been demonstrated. We identified the presence of Wolbachia in a strain of the spider mite Tetranychus urticae based on the amplification and sequencing of part of the 16S rDNA and ftsZ genes. In order to establish the phylogenetic relationships between Wolbachia found in T. urticae and in other arthropods, we aligned the resulting sequences with already published ones. For both 16S and ftsZ genes the Wolbachia carried by T. urticae clustered together with Wolbachia found in other arthropods.

Characterization of the unlinked 16S rDNA and 23S-5S rRNA operon of Wolbachia pipientis, a prokaryotic parasite of insect gonads

The rRNA-encoding genes (rDNAs) have been cloned and characterized from Wolbachia pipientis (Wp), the gonadial bacteria-like parasite of the mosquito Culex pipiens (Cp) and the moth Ephestia cautella (Ec). In Wp from both insect species the rDNAs are organized in a way which appears to be very unusual. The rRNAs are encoded by two unlinked transcription units, each present in a single copy per genome. One contains the 16S rDNA only, while the other is an operon encoding both the 23S and 5S rDNAs. Each transcription unit contains two putative upstream promoters, and downstream a Rho-independent terminator. The 16S rDNA, as well as the 23S-5S rRNA operon are not linked to any tRNA-encoding sequence and lack the antitermination boxes which are usually present immediately downstream from eubacterial promoters of rDNAs. Wp infecting Ec and Cp are highly similar taking as criteria the rDNAs and their flanking sequences. However, it clearly appears that each insect species harbours a different and specific Wp strain, or even subspecies. Phylogenetic relationships deduced from the complete sequences of their rDNAs undoubtedly confirm that Wp from Cp and Ec belong to the alpha-group of Proteobacteria, and are closely related to the Rickettsia.

Wolbachia pipientis: bacterial density and unidirectional cytoplasmic incompatibility between infected populations of Aedes albopictus

Unidirectional cytoplasmic incompatibility is seen when certain Wolbachia-infected insect populations are crossed. Two hypotheses might explain this phenomenon: superinfections with mutually incompatible strains of Wolbachia producing incompatibility when crossed to individuals infected with only a single bacterial strain or, alternatively, a bacterial dosage model, with differences in Wolbachia densities responsible for the incompatibility. A quantitative PCR assay was set up as a general method to compare Wolbachia densities between populations. Using this assay in unidirectionally incompatible stocks of the mosquito Aedes albopictus, we have determined that densities are significantly higher in Houston than in the Mauritius and Koh Samui stocks. This is consistent with a dosage model for the observed crossing patterns, but does not rule out the possibility that superinfection is the primary cause of the incompatibility.

Hemocytic rickettsia-like organisms in ticks: serologic reactivity with antisera to Ehrlichiae and detection of DNA of agent of human granulocytic ehrlichiosis by PCR

Ixodid ticks were collected from Connecticut, Massachusetts, Missouri, Pennsylvania, Rhode Island, and British Columbia (Canada) during 1991 to 1994 to determine the prevalence of infection with hemocytic (blood cell), rickettsia-like organisms. Hemolymph obtained from these ticks was analyzed by direct and indirect fluorescent antibody (FA) staining methods with dog, horse, or human sera containing antibodies to Ehrlichia canis, Ehrlichia equi, or Rickettsia rickettsii. Of the 693 nymphal and adult Amblyomma americanum, Dermacentor variabilis, Ixodes scapularis, and Ixodes pacificus ticks tested with dog anti-E. canis antiserum, 209 (32.5%) contained hemocytic bacteria. The prevalence of infected ticks varied greatly with species and locale. In parallel tests of duplicate hemolymph preparations from adult I. scapularis ticks, the hemocytic organisms reacted positively with E. canis and/or E. equi antisera, including sera from persons who had granulocytic ehrlichiosis. In separate PCR analyses, DNA of the agent of human granulocytic ehrlichiosis was detected in 59 (50.0%) of 118 adult and in 1 of 2 nymphal I. scapularis ticks tested from Connecticut. There was no evidence of Ehrlichia chaffeensis DNA in these ticks. In indirect FA tests of hemolymph for spotted fever group rickettsiae, the overall prevalence of infection was less than 4%. Specificity tests of antigens and antisera used in these studies revealed no cross-reactivity between E. canis and E. equi or between any of the ehrlichial reagents and those of R. rickettsii. The geographic distribution of hemocytic microorganisms with shared antigens to Ehrlichia species or spotted fever group rickettsiae is widespread.

Wolbachia superinfections and the expression of cytoplasmic incompatibility

Strains of Drosophila simulans from Riverside, California (DSR) and Hawaii (DSH) harbour distinct strains of the cytoplasmic incompatibility microorganism Wolbachia, resulting in the expression of bidirectional incompatibility when crossed. D. simulans lines carrying both of these (superinfected) Wolbachia strains were generated by the transfer of infected DSH cytoplasm into DSR embryos by microinjection. The superinfected flies were unidirectionally incompatible with both DSR and DSH individuals. As a result of this pattern, the superinfected state was observed to replace single infections in laboratory populations. The ability of the superinfection to spread was modulated by the production of singly infected offspring from superinfected mothers: strain segregation was observed under crowded larval rearing conditions. An inverse correlation between the penetrance of the cytoplasmic incompatibility phenotype and the degree of larval crowding was also observed. The findings have implications for the evolution of bidirectionally incompatible strains, and lead to the prediction that superinfections should be relatively common in field populations. Evidence for a natural superinfection in the mosquito Aedes albopictus is discussed. The results also have applied significance for the generation of insect lines capable of driving desirable genes into populations already infected with Wolbachia, thus allowing repeated opportunities for population replacement.

Wolbachia and cytoplasmic incompatibility in mycophagous Drosophila and their relatives

Bacterial symbionts belonging to the genus Wolbachia are associated with postzygotic reproductive incompatibility in a number of insect species. Using polymerase chain reaction (PCR) amplification of bacterial gene sequences, strains from 10 species belonging to the closely related quinaria, testacea and tripunctata groups of Drosophila were screened for the presence of Wolbachia in their reproductive tissues. Those screened included the mycophagous species D. falleni, D. recens, D. phalerata, D. testacea, D. neotestacea, D. orientacea, D. putrida and D. tripunctata, and the nonmycophagous species D. palustris and D. quinaria. Two species, D. recens and D. orientacea, were found to be infected with Wolbachia. Subsequent tests of four additional strains of D. recens found all to be infected with the bacteria. It was established that these bacteria cause partial cytoplasmic incompatibility in D. recens by antibiotic curing followed by crosses between cured and uncured strains. Curing was confirmed by a PCR assay. Although most species of insects shown to be infected with Wolbachia are cosmopolitan and/or have undergone recent range expansion in association with human activity, D. recens and D. orientacea are endemic species with specialized ecological habits. Preliminary molecular phylogenetic analysis indicates that, among the species we examined, D. quinaria is most closely related to D. recens. To determine whether the bacteria are involved in reproductive isolation between these two species, reciprocal crosses were carried out between D. quinaria and both infected and uninfected (cured) strains of D. recens. Although these species did mate with each other, all interspecific crosses failed to yield hybrid progeny, indicating that the bacteria are not responsible for reproductive incompatibility between these species.