Field evaluation of tolerance to Tobacco streak virus in sunflower germplasm,and observations of seasonal disease spread

Strong statistical evidence was found for differences in tolerance to natural infections of Tobacco streak virus (TSV) in sunflower hybrids. Data from 470 plots involving 23 different sunflower hybrids tested in multiple trials over 5 years in Australia were analysed. Using a Bayesian Hierarchical Logistic Regression (BHLR) model for analysis provided: (a) a rigorous method for investigating the relative effects of hybrid, seasonal rainfall and proximity to inoculum source on the incidence of severe TSV disease; (b) a natural method for estimating the probability distributions of disease incidence in different hybrids under historical rainfall conditions; and (c) a method for undertaking all pairwise comparisons of disease incidence between hybrids while controlling the familywise error rate without any drastic reduction in statistical power. The tolerance identified in field trials was effective against the main TSV strain associated with disease outbreaks, TSV‐parthenium. Glasshouse tests indicate this tolerance to also be effective against the other TSV strain found in central Queensland, TSV‐crownbeard. The use of tolerant germplasm is critical to minimise the risk of TSV epidemics in sunflower in this region. We found strong statistical evidence that rainfall during the early growing months of March and April had a negative effect on the incidence of severe infection with greatly reduced disease incidence in years that had high rainfall during this period.     

Transmission of Three Strains of Tobacco Streak Ilarvirus by Different Thrips Species Using Virus-infected Pollen

When cucumber seedlings were dusted with tobacco streak ilarvirus (TSV)-infected pollen and infested with 5–10 thrips (adults and larvae mixed), Thrips tabaci transmitted all three Australian strains of TSV. In similar work, Microcephalothrips abdominalis transmitted both and Frankliniella schultzei one strain, respectively, of two TSV strains tested. Transmission of the Ageratum strain (TSV-Ag) infecting pollen of Ageratum houstonianum was very efficient (100%) by all three thrips species. However, transmission rates of only 0–28% were achieved using the Ajuga strain (TSV-A) and the strawberry strain (TSVS) in pollen of other hosts. A fourth thrips species, T. parvispinus , transmitted TSV-Ag from infected tomato pollen to Chenopodium amaranticolor seedlings. There was, therefore, little or no vector specificity in the thrips transmission of the three strains of TSV, but factors associated with the virus-infected pollen affected the efficiency of transmission. This is the first report of F. schultzei and T. parvispinus as vectors of TSV.     

Properties of a new strain of tobacco streak virus from Clematis vitalba (Ranunculaceae)

A distinct strain of tobacco streak virus (TS V/Cle), isolated in Yugoslavia from wild Clematis vitalba showing chlorotic spots or yellow netting of the leaves and from many symptomless shrubs, is described. TSV/Cle was seed transmitted in C. vitalba (70%), and in the experimental hosts Chenopodium quinoa (80%), Nicotiana benthamiana and N. megalosiphon. It was also detected in the pollen of infected C. quinoa. Purified virus preparations mostly contained quasi-spherical particles measuring 24–26 × 28, 26–28 × 28–30 and 28–31 × 32–36 mn, and sedimented in sucrose density gradient and analytical centrifugation as three components with sedimentation coefficients of 76S, 87S and 98S. The virus contained a single polypeptide species of mol. wt of c. 25 000. Unfractionated TSV/Cle preparations contained four RNA species with mol. wts, estimated by gel electrophoresis in agarose, of 1.1 × 10⁶, 0.9 × 10⁶, 0.7 × 10⁶ and 0.3 × 10⁶. In comparative experiments, TSV/Cle differed from four reference strains of TSV (TSV/B, TSV/HF, TSV/RN, and TSV/Ro) in host range and in symptoms induced in some common hosts. In agar gel double diffusion tests it was more closely related to TSV/B and TSV/M (SDI = 5) than to TSV/HF (SDI = 7), TSV/RN (SDI = 7) or TSV/Ro (SDI = 5–8). Immunoelectrophoresis experiments clearly distinguished TSV/Cle from the reference strains. TSV/Cle strain was detected in C. vitalba plants from distant and climatically different regions in Yugoslavia.     

The host range of Tobacco streak virus in India and transmission by thrips

Tobacco streak virus (TSV) recently caused an epidemic in peanut (= groundnut, Arachis hypogaea) crops in Andhra Pradesh, India. In the epidemic area TSV occurred in many widely distributed weeds of which Parthenium hysterophorus probably plays a major role in its spread by thrips. Three thrips species, Megalurothrips usitatus, Frankliniella schultzei and Scirtothrips dorsalis were vectors in the presence of infected pollen. Of crop species, Helianthus annuus (sunflower) and Tagetes patula (marigold) could act as sources of inoculum. In limited tests, the virus was not seed‐transmitted in the peanut cultivar JL‐24 or in the sunflower hybrids KBSH‐41, ‐42, ‐44, and ‐50, MSFH‐17 and ZSH‐976. Strategies adopted to reduce the incidence of TSV are discussed.     

Additional notes on Anisopteromalus sp. (Hymenoptera: Pteromalidae), the sibling species of a parasitic wasp of stored‐product pests,Anisopteromalus calandrae (Howard): A new alternative host,an eye color mutant and DNA barcodes

A parasitic wasp of stored‐product pests, Anisopteromalus calandrae (Howard), is known to have a sibling species with a different chromosome number. Here, we report establishment and characterization of an eye color mutant in this sibling species. The phenotype of the mutant is red eye in adults, and crossing experiments revealed that the mutant phenotype is recessive to wild type (brown eye color). We also report DNA barcode sequences (a partial sequence of mitochondrial cytochrome‐c oxidase subunit I) of A. calandrae and the sibling species to enable accurate identification of these morphologically similar species. Analyses of our laboratory strains showed that 12.6% of the analyzed sequences (82 of 652 bp) differed between the two species. Finally, we note that the seed beetle Callosobruchus chinensis (Linné) (family Bruchidae), host of our laboratory strains of the sibling species, is a new record of alternative host at the family‐level for the wasp (known hosts: Anobiidae (natural host) and Curculionidae (alternative host)).     

Begomoviruses Associated with Leaf Curl Disease of Tomato in Java,Indonesia*

We report that several begomoviruses are associated with tomato leaf curl disease in Java, Indonesia. Tomato plants with leaf curl symptoms were collected from Bandung (west Java), Purwokerto (central Java), Magelang (central Java) and Malang (east Java) of Indonesia, the major tomato‐growing areas of the country. Viruses were detected using the polymerase chain reaction (PCR), with universal primers for the genus Begomovirus. PCR‐amplified fragments were cloned and sequenced. Based on sequence comparisons and phylogenetic analyses, the viruses were divided into three groups. With respect to amino acid (aa) identities of the N‐terminal halves of the coat proteins compared in this study, group I was most closely related to Ageratum yellow vein virus (AYVV) (97%), Ageratum yellow vein China virus‐[Hn2] (AYVCNV‐[Hn2]) (96%) and Ageratum yellow vein virus‐[Taiwan] (AYVV‐[Tai]) (95%), and ageratum‐infecting begomovirus from Java (99%). Group II had high sequence identity with a tentative species of tomato leaf curl Java virus (ToLCJAV) (96% aa) for the CP. Group III was most closely related to a proposed species of Pepper yellow leaf curl Indonesia virus (PepYLCIDV) (90% aa identity) by its partial CP sequence.     

Molecular evidence for host-adapted races of the fungal endophyte Epichloë bromicola after presumed host shifts

Abstract.— Host shifts of plant‐feeding insects and parasites promote adaptational changes that may result in the formation of host races, an assumed intermediate stage in sympatric speciation. Here, we report on genetically differentiated and host‐adapted races of the fungal endophyte Epichloë bromicola, which presumably emerged after a shift from the grass Bromus erectus to other Bromus hosts. Fungi of the genus Epichloë (Ascomycota) and related anamorphs of Neotyphodium are widespread endophytes of cool‐season grasses. Sexually reproducing strains sterilize the host by formation of external fruiting structures (stromata), whereas asexual strains are asymptomatic and transmitted via seeds. In E. bromicola, strains infecting B. erectus are sexual, and strains from two woodland species, B. benekenii and B. ramosus, are asexual and seed transmitted. Analyses of amplified fragment length polymorphism fingerprinting and of intron sequences of the tub2 and tef1 genes of 26 isolates from the three Bromus hosts collected at natural sites in Switzerland and nearby France demonstrated that isolates are genetically differentiated according to their host, indicating that E. bromicola does not form a single, randomly mating population. Phylogenetic analyses of sequence data did not unambiguously resolve the exact origin of asexual E. bromicola strains, but it is likely they arose from within sexual populations on B. erectus. Incongruence of trees derived from different genes may have resulted from recombination at some time in the recent history of host strains. Reciprocal inoculations of host plant seedlings showed that asexual isolates from B. benekenii and B. ramosus were incapable of infecting B. erectus, whereas the sexual isolates from B. erectus retained the assumed ancestral trait of broad compatibility with Bromus host seedlings. Because all isolates were interfertile in experimental crosses, asexual strains may not be considered independent biological species. We suggest that isolates infecting B. benekenii and B. ramosus represent long‐standing host races or incipient species that emerged after host shifts and that may evolve through host‐mediated reproductive isolation toward independent species.     

Host range evaluation of the leaf-feeding beetle,Zygogramma bicolorata and the stem-boring weevil,Listronotus setosipennis demonstrates their suitability for biological control of the invasive weed,Parthenium hysterophorus in Ethiopia

The invasive weed, parthenium (Parthenium hysterophorus L.) (Asteraceae: tribe Heliantheae), damages agriculture, adversely impacts biodiversity and is hazardous to human and animal health in Ethiopia. The host range of two natural enemies, a leaf-feeding beetle, Zygogramma bicolorata (Coleoptera: Chrysomelidae) and a stem-boring weevil, Listronotus setosipennis (Coleoptera: Curculionidae) was evaluated for biological control of the weed in Ethiopia. The specificity of Z. bicolorata and L. setosipennis was assessed against 29 and 31 non-target plant species, respectively. The host range of Z. bicolorata and L. setosipennis was first assessed using no-choice tests to examine their oviposition and feeding response on non-target plants. Although oviposition by Z. bicolorata occurred on six non-target species in four Asteraceae species in no-choice tests, it was significantly lower than on parthenium and no larvae developed. Zygogramma bicolorata nibbled the leaves of one of the five niger seed (Guizotia abyssinica L. – an oil seed crop closely related to parthenium) cultivars tested, but feeding and oviposition were significantly less than on parthenium. Furthermore, choice tests indicated that Z. bicolorata did not oviposit nor feed on G. abyssinica when parthenium was present. In no-choice tests, L. setosipennis did not oviposit on any of the non-target species assessed. Mean oviposition on parthenium was 39.0?±?3.4 eggs per plant whereas no eggs were laid on any of the 31 species tested. Based on these and other host range tests, permission was obtained to field release Z. bicolorata and L. setosipennis in Ethiopia.     

Outer surface protein polymorphisms linked to host‐spirochete association in Lyme borreliae

Lyme borreliosis is caused by multiple species of the spirochete bacteria Borrelia burgdorferi sensu lato. The spirochetes are transmitted by ticks to vertebrate hosts, including small‐ and medium‐sized mammals, birds, reptiles, and humans. Strain‐to‐strain variation in host‐specific infectivity has been documented, but the molecular basis that drives this differentiation is still unclear. Spirochetes possess the ability to evade host immune responses and colonize host tissues to establish infection in vertebrate hosts. In turn, hosts have developed distinct levels of immune responses when invaded by different species/strains of Lyme borreliae. Similarly, the ability of Lyme borreliae to colonize host tissues varies among different spirochete species/strains. One potential mechanism that drives this strain‐to‐strain variation of immune evasion and colonization is the polymorphic outer surface proteins produced by Lyme borreliae. In this review, we summarize research on strain‐to‐strain variation in host competence and discuss the evidence that supports the role of spirochete‐produced protein polymorphisms in driving this variation in host specialization. Such information will provide greater insights into the adaptive mechanisms driving host and Lyme borreliae association, which will lead to the development of interventions to block pathogen spread and eventually reduce Lyme borreliosis health burden.     

Complete Nucleotide Sequence of Ageratum enation virus and an Alphasatellite Infecting a New Host Glycine max in India

During 2011, leaf crumpling, yellowing and stunting were observed on soya bean (Glycine max) in Himachal Pradesh, India. PCR‐based detection confirmed the presence of a begomovirus. The viral genome was amplified by rolling circle amplification, cloned and sequenced. The complete nucleotide sequence of DNA‐A showed highest nucleotide identity to an isolate of Ageratum enation virus infecting a weed Ageratum conyzoides. In addition, a DNA molecule was found which shared 95% nucleotide identity with an alphasatellite infecting ageratum. Neither beta satellite nor DNA‐B was detected in the infected samples.     

Biological control of parthenium (Parthenium hysterophorus): the Australian experience

ABSTRACT

Parthenium is a Weed of National Significance in Australia. Biological control of parthenium in Australia commenced in 1977 and since then nine insect species and two rust fungi have been introduced and established. Seven of them are widespread, however the time taken for field establishment varied widely between various agents, ranging from one to 15 years. Among them, the stem-galling Epiblema moth, the stem-boring Listronotus weevil, the seed-feeding Smicronyx weevil and the root-feeding Carmenta moth occur in all parthenium-infested areas at high population densities. The leaf-feeding Zygogramma beetle occurs only in central and southern Queensland, and not in northern Queensland. The parthenium summer rust occurs seasonally in central and northern Queensland, while the parthenium winter rust is more widespread in southern Queensland than in central Queensland, but does not occur in north Queensland. The sap-feeding Stobaera planthopper and the leaf-mining Bucculatrix moth established and are widespread, but their damage levels remain very low. The stem-galling Conotrachelus weevil and the stem-boring Platphalonidia moth are believed to be established, but at very low abundance. Biological control has resulted in significant reductions in the abundance and impact of parthenium in Australia. As a result, the area infested with parthenium in central Queensland has declined since the mid-1990s. Due to the absence of many of the effective agents in southern and south-eastern Queensland, agents from central Queensland are being redistributed there. Additionally, based on Australian success, many of these agents have also been introduced into other countries around the world.     

Polymerase chain reaction (PCR) identification of rodent blood meals confirms host sharing by flea vectors of plague

Elucidating feeding relationships between hosts and parasites remains a significant challenge in studies of the ecology of infectious diseases, especially those involving small or cryptic vectors. Black‐tailed prairie dogs (Cynomys ludovicianus) are a species of conservation importance in the North American Great Plains whose populations are extirpated by plague, a flea‐vectored, bacterial disease. Using polymerase chain reaction (PCR) assays, we determined that fleas (Oropsylla hirsuta) associated with prairie dogs feed upon northern grasshopper mice (Onychomys leucogaster), a rodent that has been implicated in the transmission and maintenance of plague in prairie‐dog colonies. Our results definitively show that grasshopper mice not only share fleas with prairie dogs during plague epizootics, but also provide them with blood meals, offering a mechanism by which the pathogen, Yersinia pestis, may be transmitted between host species and maintained between epizootics. The lack of identifiable host DNA in a significant fraction of engorged Oropsylla hirsuta collected from animals (47%) and prairie‐dog burrows (100%) suggests a rapid rate of digestion and feeding that may facilitate disease transmission during epizootics but also complicate efforts to detect feeding on alternative hosts. Combined with other analytical approaches, e.g., stable isotope analysis, molecular genetic techniques can provide novel insights into host‐parasite feeding relationships and improve our understanding of the role of alternative hosts in the transmission and maintenance of disease.     

EFFECTS OF RELATIVE HUMIDITY AND TYPE OF CONTAINER ON THE GROWTH OF F1 HYBRID ANNUALS IN CONTROLLED ENVIRONMENTS

The effects of three levels of relative humidity (40%, 65%, and 90%) and two types of containers (clay and plastic) on the seedling growth of three F₁ hybrid annuals were determined after 14 days of controlled-environment treatment. Forty percent relative humidity was severely limiting to the seedling growth of ‘Blue Blazer’ ageratum (Ageratum houstonianum Mill.), ‘Pink Cascade’ petunia (Petunia hybrida Vilm.), and ‘Double Eagle’ marigold (Tagetes erecta L.). Raising the relative humidity to 65% resulted in striking increases in fresh weight, dry weight, and leaf area, especially when clay containers were used. Height of the main shoot was increased significantly at 65% relative humidity but node number was influenced only slightly. Increasing the relative humidity further to 90% had no significant effect on fresh weight, dry weight, or percent dry weight for any of the three species, in either container. Leaf area was increased significantly at 90% only in ageratum seedlings grown in clay pots. Each species responded differently to the type of container used. The fresh weight and dry weight of petunia seedlings were significantly greater in plastic pots at every level of humidity while those of marigold seedlings were unaffected by the type of container used. Ageratum seedlings, on the other hand, had significantly greater fresh weights and dry weights in clay pots only at 90% relative humidity.     

Indirect positive effects of a parasitic plant on host pollination and seed dispersal

Parasitic plants often have a strong fitness‐impact on their plant hosts through increased host mortality and reduced or complete suppression of reproduction. Tristerix corymbosus (Loranthaceae) is a hemiparasitic mistletoe that infects a wide range of host species along its distribution range. Among such species, Rhaphithamnus spinosus (Verbenaceae) is a frequent host with a flowering and fruiting season partially synchronized with mistletoe reproductive phenology. As parasitized hosts have, in principle, a larger flower display and fruit crop size than non‐parasitized hosts, we examined whether host and parasite reproductive synchrony make infected hosts more attractive for pollinators and seed dispersers than uninfected hosts. Our results showed that pollinator visit rates did not differ between parasitized and non‐parasitized hosts. Conversely, seed rain was higher in parasitized than non‐parasitized individuals. The number of seeds fallen under non‐parasitized plants was spatially associated with crop size, while parasitized plants did not show such association. Finally, the number of seedlings of R. spinosus was significantly larger near parasitized than non‐parasitized hosts. Our results suggest that the presence of the mistletoe might be responsible of the higher reproductive success showed by the parasitized fraction of R. spinosus. This effect, however, seems to be related to seed dispersal processes rather than pollination effects.     

Host switching in cowbird brood parasites: how often does it occur?

Avian obligate brood parasites lay their eggs in nests of host species, which provide all parental care. Brood parasites may be host specialists, if they use one or a few host species, or host generalists, if they parasitize many hosts. Within the latter, strains of host‐specific females might coexist. Although females preferentially parasitize one host, they may occasionally successfully parasitize the nest of another species. These host switching events allow the colonization of new hosts and the expansion of brood parasites into new areas. In this study, we analyse host switching in two parasitic cowbirds, the specialist screaming cowbird (Molothrus rufoaxillaris) and the generalist shiny cowbird (M. bonariensis), and compare the frequency of host switches between these species with different parasitism strategies. Contrary to expected, host switches did not occur more frequently in the generalist than in the specialist brood parasite. We also found that migration between hosts was asymmetrical in most cases and host switches towards one host were more recurrent than backwards, thus differing among hosts within the same species. This might depend on a combination of factors including the rate at which females lay eggs in nests of alternative hosts, fledging success of the chicks in this new host and their subsequent success in parasitizing it.     

PATTERNS IN HOST RANGE FOR TWO STRAINS OF AMOEBOPHRYA (DINOPHYTA) INFECTING THECATE DINOFLAGELLATES: AMOEBOPHYRA SPP. EX ALEXANDRIUM AFFINE AND EX GONYAULAX POLYGRAMMA1

The endoparasitic dinoflagellate Amoebophrya ceratii (Koeppen) Cachon uses a number of its free‐living relatives as hosts and may represent a species complex composed of several host‐specific parasites. Two thecate host–parasite systems [Amoebophrya spp. ex Alexandrium affine (Inoue and Fukuyo) Balech and ex Gonyaulax polygramma Stein], were used to test the hypothesis that two strains of Amoebophrya have a high degree of host specificity. To test this hypothesis, a series of cross‐infection experiments were conducted, with 10 thecate and three athecate dinoflagellate species as potential hosts. Surprisingly, the two strains of Amoebophrya lacked host specificity and had wider host ranges than previously recognized. Among the host species tested, Amoebophrya sp. ex Alexandrium affine was capable of infecting only species of genus Alexandrium (Alexandrium affine, Alexandrium catenella, and Alexandrium tamarense), while the parasite from Gonyaulax polygramma infected species covering five genera (Alexandrium, Gonyaulax, Prorocentrum, Heterocapsa, and Scripsiella). In the context of previous reports, these results suggest that host specificity of Amoebophrya strains varies from extremely species‐specific to rather unspecific, with specificity being stronger for strains isolated from athecate hosts. Information on host specificity of Amoebophrya strains provided here will be helpful in assessing the possibility of using these parasites as biological control agents for harmful algal blooms, as well as in defining species of Amoebophrya in the future.     

Host hybridization as a potential mechanism of lateral symbiont transfer in deep‐sea vesicomyid clams

Deep‐sea vesicomyid clams live in mutualistic symbiosis with chemosynthetic bacteria that are inherited through the maternal germ line. On evolutionary timescales, strictly vertical transmission should lead to cospeciation of host mitochondrial and symbiont lineages; nonetheless, examples of incongruent phylogenies have been reported, suggesting that symbionts are occasionally horizontally transmitted between host species. The current paradigm for vesicomyid clams holds that direct transfers cause host shifts or mixtures of symbionts. An alternative hypothesis suggests that hybridization between host species might explain symbiont transfers. Two clam species, Archivesica gigas and Phreagena soyoae, frequently co‐occur at deep‐sea hydrocarbon seeps in the eastern Pacific Ocean. Although the two species typically host gammaproteobacterial symbiont lineages marked by divergent 16S rRNA phylotypes, we identified a number of clams with the A. gigas mitotype that hosted symbionts with the P. soyoae phylotype. Demographic inference models based on genome‐wide SNP data and three Sanger sequenced gene markers provided evidence that A. gigas and P. soyoae hybridized in the past, supporting the hypothesis that hybridization might be a viable mechanism of interspecific symbiont transfer. These findings provide new perspectives on the evolution of vertically transmitted symbionts and their hosts in deep‐sea chemosynthetic environments.     

Host range, purification and properties of potato virus T

Potato virus T (PVT) infected nine species of tuber-bearing Solanum, most of them symptomlessly, and as a rule was transmitted through the tubers to progeny plants: two genotypes of S. tuberosum ssp. andigena were not infected. The virus was also transmitted by inoculation with sap to 37 other species in eight plant families. Chenopodium amaranticolor is useful as an indicator host, C quinoa as a source of virus for purification, and Phaseolus vulgaris as a local-lesion assay host; the systemic symptoms in Datura stramonium, Nicotiana debneyi and in these three species are useful for diagnosis. Attempts to transmit PVT by aphids failed, but the virus was transmitted through seed to progeny seedlings of four solanaceous species, and from pollen to seed of S. demissum. PVT was purified by clarifying sap with n-butanol or bentonite, followed by precipitation with polyethylene glycol, differential centrifugation and sedimentation in a sucrose density gradient. Purified preparations had an E260/E280 ratio of 1.18 and contained a single infective component with a sedimentation coefficient of 99 S. This component consisted of flexuous filamentous particles of about 640 times 12 nm that showed a characteristic substructure when stained with uranyl acetate. The virus particles contained a single species of infective single-stranded RNA, of molecular weight 2–2 times 106 daltons, and a single species of polypeptide of molecular weight about 27 000 daltons. PVT is serologically related to apple stem grooving virus but not to four other common potato viruses with flexuous filamentous particles. Apple stem grooving virus and PVT cause similar symptoms in several hosts, but also differ somewhat in host range and symptomatology. Apple stem grooving virus did not infect potato, caused additional symptoms in C. quinoa also infected with PVT, and its particles did not show the structural features specific to PVT. The two viruses are considered to be distinct. The cryptogram of PVT is R/1:2–2/(5): E/E: S/C.     

Isolation and Phylogenetic Analysis of <Emphasis Type="Italic">Xylella fastidiosa</Emphasis> from Its Invasive Alternative Host,Porcelain Berry

A strain of Xylella fastidiosa was isolated from an invasive alternative host species, porcelain berry. Its genetic relationship with strains isolated from a native alternative host, wild grape; a nonnative alternative host, mulberry; and other economically important hosts including cultivated grape, peach, plum, oak, maple and oleander was determined by using sequence analysis of the 16S–23S rRNA intergenic spacer region. Our phylogenetic analysis revealed that the porcelain berry strain is most closely related to the wild grape strain. These two strains are more closely related to the oak, peach, and plum strains than to the mulberry and oleander strains. They are separated from the maple and cultivated grape strains. Our data suggest that suppression of porcelain berry, wild grape, and mulberry in the vicinity of susceptible economically important hosts such as oak, peach, and plum may provide an important control measure for diseases caused by X. fastidiosa.     

The mushroom habitat as an ecological arena for global exchange of Wolbachia

Wolbachia infect a variety of arthropod and nematode hosts, but in arthropods, host phylogenetic relationships are usually poor predictors of strain similarity. This suggests that new infections are often established by horizontal transmission. To gain insight into the factors affecting the probability of horizontal transmission among host species, we ask how host phylogeny, geographical distribution and ecology affect patterns of Wolbachia strain similarity. We used multilocus sequence typing (MLST) to characterize Wolbachia strain similarity among dipteran hosts associated with fleshy mushrooms. Wolbachia Supergroup A was more common than Supergroup B in Diptera, and also more common in mycophagous than non‐mycophagous Diptera. Within Supergroup A, host family within Diptera had no effect on strain similarity, and there was no tendency for Wolbachia strains from sympatric host species to be more similar to one another than to strains from hosts in different biogeographical realms. Supergroup A strains differed between mycophagous and non‐mycophagous Diptera more than expected by chance, suggesting that ecological associations can facilitate horizontal transmission of Wolbachia within mycophagous fly communities. For Supergroup B, there were no significant associations between strain similarity and host phylogeny, biogeography, or ecology. We identified only two cases in which closely related hosts carried closely related Wolbachia strains, evidence that Wolbachia‐host co‐speciation or early introgression can occur but may not be a major contributor to overall strain diversity. Our results suggest that horizontal transmission of Wolbachia can be influenced by host ecology, thus leading to partial restriction of Wolbachia strains or strain groups to particular guilds of insects.