Efficiency Of Vertical Transmission Research Articles

Interaction of High Temperature Stress and Wolbachia Infection on the Biological Characteristic of Drosophila melanogaster.

It was reported that temperature affects the distribution of Wolbachia in the host, but only a few papers reported the effect of the interaction between high temperature and Wolbachia on the biological characteristic of the host. Here, we set four treatment Drosophila melanogaster groups: Wolbachia-infected flies in 25 °C (W+M), Wolbachia-infected flies in 31 °C (W+H), Wolbachia-uninfected flies in 25 °C (W-M), Wolbachia-uninfected flies in 31 °C (W-H), and detected the interaction effect of temperature and Wolbachia infection on the biological characteristic of D. melanogaster in F1, F2 and F3 generations. We found that both temperature and Wolbachia infection had significant effects on the development and survival rate of D. melanogaster. High temperature and Wolbachia infection had interaction effect on hatching rate, developmental durations, emergence rate, body weight and body length of F1, F2 and F3 flies, and the interaction effect also existed on oviposition amount of F3 flies, and on pupation rate of F2 and F3 flies. High temperature stress reduced the Wolbachia vertical transmission efficiency between generations. These results indicated that high temperature stress and Wolbachia infection had negative effects on the morphological development of D. melanogaster.

A diversity of endosymbionts across Australian aphids and their persistence in aphid cultures.

There is increasing interest in the use of endosymbionts in pest control, which will benefit from the identification of endosymbionts from potential donor species for transfer to pest species. Here, we screened for endosymbionts in 123 Australian aphid samples across 32 species using 16S DNA metabarcoding. We then developed a qPCR method to validate the metabarcoding data set and to monitor endosymbiont persistence in aphid cultures. Pea aphids (Acyrthosiphon pisum) were frequently coinfected with Rickettsiella and Serratia, and glasshouse potato aphids (Aulacorthum solani) were coinfected with Regiella and Spiroplasma; other secondary endosymbionts detected in samples occurred by themselves. Hamiltonella, Rickettsia and Wolbachia were restricted to a single aphid species, whereas Regiella was found in multiple species. Rickettsiella, Hamiltonella and Serratia were stably maintained in laboratory cultures, although others were lost rapidly. The overall incidence of secondary endosymbionts in Australian samples tended to be lower than recorded from aphids overseas. These results indicate that aphid endosymbionts probably exhibit different levels of infectivity and vertical transmission efficiency across hosts, which may contribute to natural infection patterns. The rapid loss of some endosymbionts in cultures raises questions about factors that maintain them under field conditions, while endosymbionts that persisted in laboratory culture provide candidates for interspecific transfers.

Congenital transmission of Neospora caninum in wild ungulates and foxes.

Neospora caninum is one of the main causes of abortion in cattle. In wildlife, the occurrence and relevance of vertical infection have not been yet clearly evaluated. The aim of this study was to verify the possibility of vertical transmission of N. caninum in three wild species extensively distributed in Europe, namely roe deer Capreolus capreolus, wild boar Sus scrofa and red fox Vulpes vulpes. A total of 190 fetuses (72 wild boars, 67 foxes and 51 roe deer) from 61 females were included in the study. All animals, which were either found dead or culled within selective control plans in North-western Italy, were tested, in parallel by PCR on central nervous system, skeletal muscle and kidney. The efficiency of vertical transmission in the three target species was of 87.5% (95% CI 69.00-95.66).

Mathematical modelling of the interactive dynamics of wild and Microsporidia MB-infected mosquitoes.

A recent discovery highlighted that mosquitoes infected with Microsporidia MB are unable to transmit the Plasmodium to humans. Microsporidia MB is a symbiont transmitted vertically and horizontally in the mosquito population, and these transmission routes are known to favor the persistence of the parasite in the mosquito population. Despite the dual transmission, data from field experiments reveal a low prevalence of MB-infected mosquitoes in nature. This study proposes a compartmental model to understand the prevalence of MB-infected mosquitoes. The dynamic of the model is obtained through the computation of the basic reproduction number and the analysis of the stability of the MB-free and coexistence equilibria. The model shows that, in spite of the high vertical transmission efficiency of Microsporidia MB, there can still be a low prevalence of MB-infected mosquitoes. Numerical analysis of the model shows that male-to-female horizontal transmission contributes more than female-to-male horizontal transmission to the spread of MB-infected mosquitoes. Moreover, the female-to-male horizontal transmission contributes to the spread of the symbiont only if there are multiple mating occurrences for male mosquitoes. Furthermore, when fixing the efficiencies of vertical transmission, the parameters having the greater influence on the ratio of MB-positive to wild mosquitoes are identified. In addition, by assuming a similar impact of the temperature on wild and MB-infected mosquitoes, our model shows the seasonal fluctuation of MB-infected mosquitoes. This study serves as a reference for further studies, on the release strategies of MB-infected mosquitoes, to avoid overestimating the MB-infection spread.

Epichloë seed transmission efficiency is influenced by plant defense response mechanisms.

Asexual Epichloë are endophytic fungi that form mutualistic symbioses with cool-season grasses, conferring to their hosts protection against biotic and abiotic stresses. Symbioses are maintained between grass generations as hyphae are vertically transmitted from parent to progeny plants through seed. However, endophyte transmission to the seed is an imperfect process where not all seeds become infected. The mechanisms underpinning the varying efficiencies of seed transmission are poorly understood. Host gene expression in response to Epichloë sp. LpTG-3 strain AR37 was examined within inflorescence primordia and ovaries of high and low endophyte transmission genotypes within a single population of perennial ryegrass. A genome-wide association study was conducted to identify population-level single nucleotide polymorphisms (SNPs) and associated genes correlated with vertical transmission efficiency. For low transmitters of AR37, upregulation of perennial ryegrass receptor-like kinases and resistance genes, typically associated with phytopathogen detection, comprised the largest group of differentially expressed genes (DEGs) in both inflorescence primordia and ovaries. DEGs involved in signaling and plant defense responses, such as cell wall modification, secondary metabolism, and reactive oxygen activities were also abundant. Transmission-associated SNPs were associated with genes for which gene ontology analysis identified "response to fungus" as the most significantly enriched term. Moreover, endophyte biomass as measured by quantitative PCR of Epichloë non-ribosomal peptide synthetase genes, was significantly lower in reproductive tissues of low-transmission hosts compared to high-transmission hosts. Endophyte seed-transmission efficiency appears to be influenced primarily by plant defense responses which reduce endophyte colonization of host reproductive tissues.

Vertical Transmission of Sindbis Virus in Culex Mosquitoes.

Simple SummaryThe transmission of vector-borne viruses requires arthropod vectors that are actively seeking for new blood-meal hosts. In regions where vectors do not blood-feed for prolonged periods, the persistence of a virus depends on alternative mechanisms of transmission, such as the transmission of a virus from an infected arthropod mother to her offspring, via so-called vertical transmission. Far less is known about this type of transmission than about transmission between vertebrate hosts, and it is often viewed as rarely occurring. Sindbis virus is one of many mosquito-borne viruses that originates in the tropics and has become introduced and established in temperate regions. In its northern range, the virus must persist through several months of winter when its mosquito vectors are inactive. In this study, we investigated the vertical transmission of Sindbis virus, both experimentally and in the field, and found evidence from the field that it does occur but with conflicting results in the experiments. This new knowledge highlights factors which are necessary for tropical viruses to establish in temperate regions.Vertical transmission (VT) is a phenomenon of vector-borne diseases where a pathogen is transferred from an infected arthropod mother to her offspring. For mosquito-borne flavi- and alphaviruses, VT is commonly viewed as rare; however, both field and experimental studies report on vertical transmission efficiency to a notably varying degree. It is likely that this reflects the different experimental methods used to test vertical transmission efficiency as well as differences between virus–vector combinations. There are very few investigations of the VT of an alphavirus in a Culex vector. Sindbis virus (SINV) is an arthritogenic alphavirus that utilizes Culex species as main vectors both in the summer transmission season and for its persistence over the winter period in northern latitudes. In this study, we investigated the vertical transmission of the SINV in Culex vectors, both in the field and in experimental settings. The detection of SINV RNA in field-collected egg rafts and emerging adults shows that vertical transmission takes place in the field. Experimentally infected females gave rise to adult offspring containing SINV RNA at emergence; however, three to four weeks after emergence none of the offspring contained SINV RNA. This study shows that vertical transmission may be connected to SINV’s ability to persist throughout northern winters and also highlights many aspects of viral replication that need further study.

Context‐dependent variability in the population prevalence and individual fitness effects of plant–fungal symbiosis

Abstract Heritable symbionts are often observed at intermediate prevalence within host populations, despite expectations that positive fitness feedbacks should drive beneficial symbionts to fixation. Intermediate prevalence may reflect neutral dynamics of symbionts with weak fitness effects, transient dynamics of symbionts trending towards fixation (or elimination), or a stable intermediate outcome determined by the balance of fitness effects and failed symbiont transmission. Theory suggests that these outcomes should depend on symbiont‐conferred demographic effects and vertical transmission efficiency, which may both depend on environmental context. We established experimental populations of winter bent grass Agrostis hyemalis across a range of prevalence of the heritable fungal endophyte Epichloë amarillans. Using irrigation, we elevated the precipitation for half of the populations, which we hypothesized would weaken the benefits of symbiosis. Across two annual transitions, we assayed 5,485 individuals to determine prevalence and censused 954 individuals for demographic (survival, flowering, reproduction and recruitment) and vertical transmission data. We used hierarchical Bayesian models to infer long‐run equilibria from short‐term changes in symbiont prevalence and estimated demographic vital rates to link individual‐level effects to population‐level outcomes. We found evidence for all three proposed mechanisms for intermediate symbiont prevalence, but the outcome differed qualitatively across years and precipitation treatments. In the first year, symbionts trended towards fixation under drought conditions but drifted neutrally under elevated precipitation. Fixation likely arose from symbiont‐conferred recruitment benefits outweighing reproductive costs under the drought conditions, while elevated precipitation tempered these effects. In the second transition year, we inferred stable intermediate prevalence across both precipitation treatments, which indicated a balance between symbiont conferred recruitment benefits that allowed low‐prevalence populations to increase and imperfect transmission that caused high‐prevalence populations to decrease. Synthesis. We find support for neutral, transient and stable mechanisms underlying symbiont prevalence, indicating that symbiont prevalence is often pushed and pulled in different directions by the composite outcome of symbiont effects on demographic rates and transmission efficiency, and the way in which these processes respond to environmental context.

Efficient but occasionally imperfect vertical transmission of gut mutualistic protists in a wood-feeding termite.

Although mutualistic associations between animals and microbial symbionts are widespread in nature, the mechanisms that have promoted their evolutionary persistence remain poorly understood. A vertical mode of symbiont transmission (from parents to offspring) is thought to ensure partner fidelity and stabilization, although the efficiency of vertical transmission has rarely been investigated, especially in cases where hosts harbour a diverse microbial community. Here we evaluated vertical transmission rates of cellulolytic gut oxymonad and parabasalid protists in the wood-feeding termite Reticulitermes grassei. We sequenced amplicons of the 18S rRNA gene of protists from 24 colonies of R.grassei collected in two populations. For each colony, the protist community was characterized from the gut of 14 swarming reproductives and from a pool of 10 worker guts. A total of 98 operational taxonomic units belonging to 13 species-level taxa were found. The vertical transmission rate was estimated for each protist present in a colony based on its frequency among the reproductives. The results revealed that transmission rates were high, with an average of 0.897 (±0.164) per protist species. Overall, the protist community did not differ between reproductive sexes, suggesting that both the queen and the king could contribute to the gut microbiota of the offspring. A positive relationship between the transmission rate of protists and their prevalence within populations was also detected. However, transmission rates alone do not explain the prevalence ofprotists. In conclusion, these findings reveal key forces behind a conserved, multispecies mutualism, raising further questions on the roles of horizontal transfer and negative selection in shaping symbiont prevalence.

Fecundity-Longevity Trade-Off, Vertical Transmission, and Evolution of Virulence in Sterilizing Pathogens.

Sterilizing pathogens are common, yet studies focused on how such pathogens respond adaptively to fecundity reductions caused in their hosts are rare. Here we assume that the infected hosts, as a result of redistributing energy resources saved by reduced fecundity, have increased longevity and focus on exploring the consequences of such a fecundity-longevity trade-off on sterility virulence evolution in the pathogens. We find that the trade-off itself cannot prevent the evolution of full sterilization. Therefore, we allow for vertical transmission and reveal that the fecundity-longevity trade-off strongly determines the threshold efficiency of vertical transmission above which partial host sterilization evolves. Partial sterilization may appear as an intermediate level of sterility virulence or as a stable dimorphism at which avirulent and highly virulent strains coexist. The fecundity-longevity trade-off significantly contributes to determining the actual outcome, in many cases countering predictions made in the absence of this trade-off. It is known that in well-mixed populations, partial sterilization may evolve in pathogens under a combination of horizontal and vertical transmission. Our study highlights that this is independent of the form of horizontal transmission and the type of density dependence in host demography and that the fecundity-longevity trade-off is an important player in sterility virulence evolution.

Notice of Retraction: Research and Application of Multi-Node Communication and Energy Consumption Prediction Control in Underwater Acoustic Network

The capacity analysis of the existing underwater acoustic network mainly refers to the analysis method of a terrestrial wireless network. Network capacity is affected by many factors, such as node distribution, transmission delay, transmission strategy, and link layer characteristics. Aiming at the challenges of the capacity of the underwater acoustic network, a closed solution for one-hop transmission of any node based on the 3-D stochastic underwater acoustic network under the protocol model is derived, the closed solution takes into account the fact that the vertical transmission efficiency of the underwater acoustic signal is more effective than the horizontal transmission efficiency, it is in line with the channel characteristics of underwater acoustic communication, and with different network parameters, there is an optimal communication radiation range of nodes to maximize the throughput of the network. Aiming at the problem that some relay nodes in the underwater acoustic network run out of energy prematurely due to overuse, a model predictive control method based on maximum algebra is proposed, which can predict the information generation rate of nodes periodically and estimate the life value of nodes at different routing time points according to the current residual energy of nodes.

Basic Properties and Qualitative Dynamics of a Vector-Borne Disease Model with Vector Stages and Vertical Transmission

This work systematically discusses basic properties and qualitative dynamics of vector-borne disease models, particularly those with vertical transmission in the vector population. Examples of disease include Dengue and Rift Valley fever which are endemic in Sub-Saharan Africa, and understanding of the dynamics underlying their transmission is central for providing critical informative indicators useful for guiding control strategies. Of particular interest is the applicability and derivation of relevant population and epidemic thresholds and their relationships with vertical infection. This study demonstrates how the failure of R0 derived using the next-generation method compounds itself when varying vertical transmission efficiency, and it shows that the host type reproductive number gives the correct R0. Further, novel relationships between the host type reproductive number, vertical infection, and ratio of female mosquitoes to host are established and discussed. Analytical results of the model with vector stages show that the quantities Q0, Q0v, and R0c, which represent the vector colonization threshold, the average number of female mosquitoes produced by a single infected mosquito, and effective reproductive number, respectively, provide threshold conditions that determine the establishment of the vector population and invasion of the disease. Numerical simulations are also conducted to confirm and extend the analytical results. The findings imply that while vertical infection increases the size of an epidemic, it reduces its duration, and control efforts aimed at reducing the critical thresholds Q0, Q0v, and R0c to below unity are viable control strategies.

Testing the roles of vertical transmission and drought stress in the prevalence of heritable fungal endophytes in annual grass populations.

Beneficial inherited symbionts are expected to reach high prevalence in host populations, yet many are observed at intermediate prevalence. Theory predicts that a balance of fitness benefits and efficiency of vertical transmission may interact to stabilize intermediate prevalence. We established populations of grass hosts (Lolium multiflorum) that varied in prevalence of a heritable fungal endophyte (Epichloё occultans), allowing us to infer long-term equilibria by tracking change in prevalence over one generation. We manipulated an environmental stressor (elevated precipitation), which we hypothesized would reduce the fitness benefits of symbiosis, and altered the efficiency of vertical transmission by replacing endophyte-positive seeds with endophyte-free seeds. Endophytes and elevated precipitation both increased host fitness, but symbiont effects were not stronger in the drier treatment, suggesting that benefits of symbiosis were unrelated to drought tolerance. Reduced transmission suppressed the inferred equilibrium prevalence from 42.6% to 11.7%. However, elevated precipitation did not modify prevalence, consistent with the result that it did not modify fitness benefits. Our results demonstrate that failed transmission can influence the prevalence of heritable microbes and that intermediate prevalence can be a stable equilibrium due to forces that allow symbionts to increase (fitness benefits) but prevent them from reaching fixation (failed transmission).

Characterization of a novel member of genus Iflavirus in Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera, is one of the most important agricultural pests of many economic crops worldwide. Herein, we found a novel single-strand RNA virus by RNA-Seq and Polymerase Chain Reaction (PCR) method in H. armigera named Helicoverpa armigera iflavirus (HaIV), which possessed a genome with 10,017 nucleotides in length and contained a single large open reading frame (ORF) encoding a putative polyprotein of 3021 amino acids with a predicted molecular mass of 344.16kDa and a theoretical isoelectric point (pI) of 6.45. The deduced amino acid sequence showed highest similarity (61.0%) with the protein of Lymantria dispar Iflavirus 1. Phylogenetic analysis with putative RdRp amino acid sequences indicated that the virus clustered with members of the genus Iflavirus. The virus was mainly distributed in the fat body of its host and was found to be capable of both horizontal and vertical transmission. The efficiency of perorally horizontal transmission was dose dependent (100% infection rate with a viral dose of 108copies/μl) while vertical transmission efficiency was found to be relatively low (<28.57%). These results suggest that we have found a novel member of genus Iflavirus in H. armigera.

Predicting Rift Valley Fever Inter-epidemic Activities and Outbreak Patterns: Insights from a Stochastic Host-Vector Model.

Rift Valley fever (RVF) outbreaks are recurrent, occurring at irregular intervals of up to 15 years at least in East Africa. Between outbreaks disease inter-epidemic activities exist and occur at low levels and are maintained by female Aedes mcintoshi mosquitoes which transmit the virus to their eggs leading to disease persistence during unfavourable seasons. Here we formulate and analyse a full stochastic host-vector model with two routes of transmission: vertical and horizontal. By applying branching process theory we establish novel relationships between the basic reproduction number, R0, vertical transmission and the invasion and extinction probabilities. Optimum climatic conditions and presence of mosquitoes have not fully explained the irregular oscillatory behaviour of RVF outbreaks. Using our model without seasonality and applying van Kampen system-size expansion techniques, we provide an analytical expression for the spectrum of stochastic fluctuations, revealing how outbreaks multi-year periodicity varies with the vertical transmission. Our theory predicts complex fluctuations with a dominant period of 1 to 10 years which essentially depends on the efficiency of vertical transmission. Our predictions are then compared to temporal patterns of disease outbreaks in Tanzania, Kenya and South Africa. Our analyses show that interaction between nonlinearity, stochasticity and vertical transmission provides a simple but plausible explanation for the irregular oscillatory nature of RVF outbreaks. Therefore, we argue that while rainfall might be the major determinant for the onset and switch-off of an outbreak, the occurrence of a particular outbreak is also a result of a build up phenomena that is correlated to vertical transmission efficiency.

Heritable symbionts in a world of varying temperature.

Heritable microbes represent an important component of the biology, ecology and evolution of many plants, animals and fungi, acting as both parasites and partners. In this review, we examine how heritable symbiont-host interactions may alter host thermal tolerance, and how the dynamics of these interactions may more generally be altered by thermal environment. Obligate symbionts, those required by their host, are considered to represent a thermally sensitive weak point for their host, associated with accumulation of deleterious mutations. As such, these symbionts may represent an important determinant of host thermal envelope and spatial distribution. We then examine the varied relationship between thermal environment and the frequency of facultative symbionts that provide ecologically contingent benefits or act as parasites. We note that some facultative symbionts directly alter host thermotolerance. We outline how thermal environment will alter the benefits/costs of infection more widely, and additionally modulate vertical transmission efficiency. Multiple patterns are observed, with symbionts being cold sensitive in some species and heat sensitive in others, with varying and non-coincident thresholds at which phenotype and transmission are ablated. Nevertheless, it is clear that studies aiming to predict ecological and evolutionary dynamics of symbiont-host interactions need to examine the interaction across a range of thermal environments. Finally, we discuss the importance of thermal sensitivity in predicting the success/failure of symbionts to spread into novel species following natural/engineered introduction.

Transmission modes and the evolution of feminizing symbionts.

Vertically transmitted symbionts can distort their host's reproduction to increase their own transmission. In Wolbachia and some other symbionts, a particular distortion of this sort is feminization, whereby genetic males, which cannot transmit symbionts, are converted during development into functional females, which do transmit symbionts when they reproduce. In this work, we propose a model to study how feminization intensity (i.e. penetrance) can evolve under different ecological constraints in WZ/ZZ hosts. More specifically, our model incorporates both imperfect vertical and horizontal transmission modes. The model shows that for most parameter values feminizing symbionts drive genetic females to extinction, which in turn favours the evolution of maximum feminization penetrance. Once genetic females are extinct, the actual value of feminization penetrance never depends on the efficiency of vertical transmission. Instead, the model shows that in conditions where the reproductive rate is high at demographic equilibrium, higher feminization levels are favoured. One consequence of this can be, for example, that evolutionarily stable feminization penetrance increases with the host's natural death rate, just as the virulence is predicted to do with the host's natural death rate in classic epidemiological models. Finally, we found that horizontal transmission had no impact on how feminization penetrance evolved when genetic females were extinct. However, horizontal transmission can permit genetic females to coexist with symbionts and, in this case, we demonstrate that the presence of genetic females selects symbionts for lower feminization penetrance.

Male‐killing bacteria as agents of insect pest control

Summary Continual effort is needed to reduce the impact of exotic species in the context of increased globalization. Any innovation in this respect would be an asset. We assess the potential of combining two pest control techniques: the well‐established sterile insect technique (SIT) and a novel male‐killing technique (MKT), which comprises inoculation of a pest population with bacteria that kill the infected male embryos. Population models are developed to assess the efficiency of using the MKT for insect pest control, either alone or together with the SIT. We seek for conditions under which the MKT weakens requirements on the SIT. Regarding the SIT, we consider both non‐heritable and inherited sterility. In both cases, the MKT and SIT benefit one another. The MKT may prevent the SIT from failing when not enough sterilized males are released due to high production costs and/or uncertainty on their mating ability following a high irradiation dose. Conversely, with already established SIT, pest eradication can be achieved after introduction of male‐killing bacteria with lower vertical transmission efficiency than if the MKT was applied alone. For tephritid fruit flies with non‐heritable sterility, maximal impact of the SIT is achieved when the released males are fully sterile. Conversely, for lepidopterans with inherited sterility, maximal impact of the SIT is achieved for intermediate irradiation doses. In both cases, increasing vertical transmission efficiency of male‐killing bacteria benefits the SIT; high enough vertical transmission efficiency allows for pest eradication where the SIT is absent or induces only pest suppression when used alone. Synthesis and applications. While both techniques can suppress or eliminate the pest on their own, combined application of the male‐killing technique and the sterile insect technique substantially increases pest control efficiency. If male‐killing bacteria are already established in the pest, any assessment of the sterile insect technique needs to account for their presence; otherwise, management recommendations could be exaggerated and unnecessarily costly.

Can maternally inherited endosymbionts adapt to a novel host? Direct costs of Spiroplasma infection, but not vertical transmission efficiency, evolve rapidly after horizontal transfer into D. melanogaster.

Maternally inherited symbionts are common in arthropods and many have important roles in host adaptation. The observation that specific symbiont lineages infect distantly related host species implies new interactions are commonly established by lateral transfer events. However, studies have shown that symbionts often perform poorly in novel hosts. We hypothesized selection on the symbiont may be sufficiently rapid that poor performance in a novel host environment is rapidly ameliorated, permitting symbiont maintenance. Here, we test this prediction for a Spiroplasma strain transinfected into the novel host Drosophila melanogaster. In the generations immediately following transinfection, the symbiont had low transmission efficiency to offspring and imposed severe fitness costs on its host. We observed that effects on host fitness evolved rapidly, being undetectable after 17 generations in the novel host, whereas vertical transmission efficiency was poorly responsive over this period. Our results suggest that long-term symbiosis may more readily be established in cases where symbionts perform poorly in just one aspect of symbiosis.

Discovery and identification of a male-killing agent in the Japanese ladybird Propylea japonica (Coleoptera: Coccinellidae)

BackgroundEndosymbionts that manipulate the reproduction of their hosts have been reported widely in invertebrates. One such group of endosymbionts is the male-killers. To date all male-killers reported are bacterial in nature, but comprise a diverse group. Ladybirds have been described as a model system for the study of male-killing, which has been reported in multiple species from widespread geographic locations. Whilst criteria of low egg hatch-rate and female-biased progenic sex ratio have been used to identify female hosts of male-killers, variation in vertical transmission efficiency and host genetic factors may result in variation in these phenotypic indicators of male-killer presence. Molecular identification of bacteria and screening for bacterial presence provide us with a more accurate method than breeding data alone to link the presence of the bacteria to the male-killing phenotype. In addition, by identifying the bacteria responsible we may find evidence for horizontal transfer between endosymbiont hosts and can gain insight into the evolutionary origins of male-killing. Phylogenetic placement of male-killing bacteria will allow us to address the question of whether male-killing is a potential strategy for only some, or all, maternally inherited bacteria. Together, phenotypic and molecular characterisation of male-killers will allow a deeper insight into the interactions between host and endosymbiont, which ultimately may lead to an understanding of how male-killers identify and kill male-hosts.ResultsA male-killer was detected in the Japanese coccinellid, Propylea japonica (Thunberg) a species not previously known to harbour male-killers. Families produced by female P. japonica showed significantly female-biased sex ratios. One female produced only daughters. This male-killer trait was maternally inherited and antibiotic treatment produced a full, heritable cure. Molecular analysis identified Rickettsia to be associated with the trait in this species of ladybird.ConclusionWe conclude that P. japonica is host to a bacterial male-killer that is vertically inherited with variable transmission efficiency. Rickettsia presence correlates with the male-killing trait, but there is some variation in the phenotypic expression of the trait due to interaction with host factors. Phylogenetic analysis using the 16S rRNA and 17 kDa antigen genes suggests there may have been horizontal transfer of Rickettsial male-killers between different ladybird hosts.

HERBIVORY MEDIATES GRASS–ENDOPHYTE RELATIONSHIPS: COMMENT

In the recent paper by Koh and Hik (2007), the pattern of Neotyphodium spp. endophyte infection frequency is shown to be related with the gradient of grazing pressure by two native herbivores, pikas (Ochotona collaris) and hoary marmots (Marmota caligata) in Yukon Territory (Canada). The grazing gradient is determined by the behavior of herbivores, being high ‘‘on’’ boulder fields and low ‘‘far’’ from there. The mechanism invoked to explain the pattern of infection frequency is the relative fitness of endophyteinfected and uninfected plants along this gradient. According to the defensive mutualism and optimal defense theories (Clay 1993, Zangerl and Rutledge 1996), infected hosts with alkaloid-based deterrents would be favored where grazing was consistently high, while uninfected plants or infected plants without deterrents would be favored where herbivory was low or absent. To reinforce the hypothesis, a trial was carried out to evaluate pikas selectivity for endophyteinfected and uninfected plants sampled from ‘‘on þ near’’ and ‘‘far’’ from boulder fields. Herbivores were able to discriminate endophyte-infected and uninfected Festuca altaica plants from boulder fields, but they did not discriminate when plants came from meadows. In summary, the authors conclude that grazing pressure and herbivory selectivity is the major factor generating the pattern in endophyte infection frequency. Hence, endophyte infection is viewed as an adaptive trait in the environment with high grazing pressure. In our opinion, Koh and Hik (2007) present little information supporting that differences in endophyte infection frequency and herbivore avoidance of infected grasses were mainly a consequence of the distinct longer term grazing history of the grasses mediated by the deterrence fungus effect. We propose that the effect of herbivores on infection frequency might as well be mediated through selection of phenotypic traits not related to antiherbivory (e.g., compensatory growth ability [Karban and Baldwin 1997, Sullivan et al. 2007]), or through an interaction with other environmental factors that can affect both partners to a different extent (Hill et al. 2005, Rasmussen et al. 2007). A usual observational approach to study broad ecological patterns is to find out simple relationships between the distribution and abundance of organisms and any environmental factor. However, this procedure may result in oversimplifications leading to wrong conclusions (see the case of shared enemies discussed by Holt and Lawton 1994). For the specific case of Neotyphodium endophytes, ecological mechanisms underlying patterns of endophyte infection frequency are quite complex (Faeth 2002). In fact, Koh and Hik (2007) acknowledge that ‘‘variations at the population level in the deterrence ability of infected tillers, frequency of infection, and hyphal densities are thought to be produced by multiple factors (genetic and environmental)’’ among which they cite ‘‘differences in grazing pressure.’’ Different theoretical and modeling approaches have indicated that the endophyte infection frequency in local populations may result from some of the following mechanisms: (1) relative fitness of endophyte-infected and uninfected plants, (2) vertical transmission efficiency of endophyte in infected plants, and (3) migration of infected and uninfected individuals among connected local populations (Clay 1993, Ravel et al. 1997, Saikkonen et al. 2002, Gundel et al. 2008). Here, we discuss each of those mechanisms and confront them with the arguments presented by Koh and Hik (2007). Our aim is to show that relative fitness may or may not be related to herbivory deterrence and that the other two mechanisms have been almost completely neglected. 1. The relative fitness of endophyte-infected and uninfected related plants has been always used to explain natural variations in endophyte infection frequency in grass populations (Clay 1993, Clay and Schardl 2002).— By this mechanism, it is predicted that the endophyte infection frequency tends to be high if endophyte enhances the host plant fitness or, alternatively, the endophyte goes to extinction if it depresses the fitness of the plants (Clay 1993). An ecological trait can be considered adaptive if it has a direct impact on fitness in natural environments. In the case of the endophyte infection, to consider it as adaptive based solely on the observed correlation between infection frequency and grazing level may be problematic. Even when the correlation is observed, it is difficult to determine whether the ecological trait (endophyte infection) per Manuscript received 14 December 2007 ; revised 11 March 2008; accepted 18 April 2008. Corresponding Editor: J. N. Klironomos. 1 IFEVA-Facultad de Agronomia UBA/CONICET, Catedra de Ecologia, Av. San Martin 4453 (C1417DSE) Buenos Aires, Argentina. 2 E-mail: gundel@agro.uba.ar