Protection of Pea Aphids Associated with Coinfecting Bacterial Symbionts Persists During Superparasitism by a Braconid Wasp.

Bacterial endosymbionts that associate facultatively with insect herbivores can influence insect fitness and trophic interactions. The pea aphid, Acyrthosiphon pisum, can be protected from parasitism by the braconid wasp Aphidius ervi when harbouring particular symbiotic bacteria, with specific endosymbiont coinfections providing almost complete protection. However, studies often quantify aphid mummification with no control over parasitoid oviposition per aphid; thus, if mummy production fails or is low, the causes are often unclear. Here, we show that the high level of protection associated with the coinfecting endosymbionts Hamiltonella defensa and X-type is maintained even when pea aphids are superparasitised. This contrasts strongly with the protection provided by H. defensa alone, which has been shown by others to be overcome by superparasitism. By dissecting aphids exposed to two parasitoid attacks, we reveal that A. ervi deposits eggs equally freely in endosymbiont-infected and uninfected nymphs, and lack of mummification in endosymbiont-protected nymphs arises from failure of the wasp eggs to hatch or emerging larvae to develop.

What are the implications of variation in root hair length on tolerance to phosphorus deficiency in combination with water stress in barley (Hordeum vulgare)?

BACKGROUND AND AIMS: Phosphorus commonly limits crop yield and is frequently applied as fertilizer; however, supplies of quality rock phosphate for fertilizer production are diminishing. Plants have evolved many mechanisms to increase their P-fertilizer use efficiency, and an understanding of these traits could result in improved long-term sustainability of agriculture. Here a mutant population is utilized to assess the impact of root hair length on P acquisition and yield under P-deficient conditions alone or when combined with drought. METHODS: Mutants with various root hair phenotypes were grown in the glasshouse in pots filled with soil representing sufficient and deficient P treatments and, in one experiment, a range of water availability was also imposed. Plants were variously harvested at 7 d, 8 weeks and 14 weeks, and variables including root hair length, rhizosheath weight, biomass, P accumulation and yield were measured. KEY RESULTS: The results confirmed the robustness of the root hair phenotypes in soils and their relationship to rhizosheath production. The data demonstrated that root hair length is important for shoot P accumulation and biomass, while only the presence of root hairs is critical for yield. Root hair presence was also critical for tolerance to extreme combined P deficit and drought stress, with genotypes with no root hairs suffering extreme growth retardation in comparison with those with root hairs. CONCLUSIONS: The results suggest that although root hair length is not important for maintaining yield, the presence of root hairs is implicit to sustainable yield of barley under P-deficient conditions and when combined with extreme drought. Root hairs are a trait that should be maintained in future germplasm.

How conserved are the bacterial communities associated with aphids? A detailed assessment of the Brevicoryne brassicae (Hemiptera: Aphididae) using 16S rDNA.

Aphids harbor a community of bacteria that include obligate and facultative endosymbionts belonging to the Enterobacteriaceae along with opportunistic, commensal, or pathogenic bacteria. This study represents the first detailed analysis of the identity and diversity of the bacterial community associated with the cabbage aphid, Brevicoryne brassicae (L.). 16S rDNA sequence analysis revealed that the community of bacteria associated with B. brassicae was diverse, with at least four different bacterial community types detected among aphid lines, collected from widely dispersed sites in Northern Britain. The bacterial sequence types isolated from B. brassicae showed little similarity to any bacterial endosymbionts characterized in insects; instead, they were closely related to free-living extracellular bacterial species that have been isolated from the aphid gut or that are known to be present in the environment, suggesting that they are opportunistic bacteria transmitted between the aphid gut and the environment. To quantify variation in bacterial community between aphid lines, which was driven largely by differences in the proportions of two dominant bacterial orders, the Pseudomonales and the Enterobacteriales, we developed a novel real-time (Taqman) qPCR assay. By improving our knowledge of aphid microbial ecology, and providing novel molecular tools to examine the presence and function of the microbial community, this study forms the basis of further research to explore the influence of the extracellular bacterial community on aphid fitness, pest status, and susceptibility to control by natural enemies.

Microsatellite frequency and size variation in the parthenogenetic parasitic wasp Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae).

Nine genomic libraries of the parthenogenetic wasp Venturia canescens were screened for microsatellite loci. In contrast to other Hymenoptera (GT)n and not (CT)n, was the predominant repeat category found with 14 kb and 28 kb genomic DNA between loci, respectively. Mono- and trinucleotide microsatellites were rarer, occurring at frequencies between 231 kb and 589 kb of genome, whilst tetranucleotide repeats are scarce, with (ATTC)n and (CCGG)n loci occurring every 692 kb and 983 kb, respectively, and only one small imperfect (GATA)n locus and no (GACA)n loci were revealed. Over 70% of the dinucleotide, and all the trinucleotide microsatellites were small (less than eleven repeats), whilst 60% to 80% of loci were imperfect. Moreover, very few compound microsatellites and only a single association between different microsatellites were observed.

Single-locus complementary sex determination in Diadegma chrysostictos (Gmelin) (Hymenoptera: Ichneumonidae).

Following the establishment of isofemale lines and subsequent inbreeding, the ichneumonid parasitoid wasp Diadegma chrysostictos (Gmelin) was shown by segregation of polymorphic alloenzyme loci to have single-locus complementary sex determination (sl-CSD). This and the biparental nature of diploid males was confirmed using two independent Mendelian recessive phenotypic markers. The existence of diploid males, sl-CSD, and the abrogation of diploid males following outbreeding was further confirmed by flow cytometry, a potentially general method that is independent of the maternal sex allocation or the need for genetic markers. Estimates of the number of sex alleles in several British populations demonstrated 17-19 alleles in Britain, with a decline toward the northerly limit of the parasitoid's range, varying from 16 in the south of England to 4-5 in central Scotland, in broad agreement with the rate of attainment of a male-biased sex ratio when used to establish en masse laboratory cultures. These data represent the second confirmation of the existence of sl-CSD in the Ichneumonidae (and the first in the Campopleginae subfamily), lending further support to the notion that sl-CSD was the ancestral condition in the Aculeata/Ichneumonoidea clade (Cook 1993a; Periquet et al. 1993).

Horizontal transfer of Wolbachia between phylogenetically distant insect species by a naturally occurring mechanism.

Wolbachia is a genus of alpha-proteobacteria found in obligate intracellular association with a wide variety of arthropods, including an estimated 10-20% of all insect species [1]. Wolbachia represents one of a number of recently identified 'reproductive parasites' [2] which manipulate the reproduction of their hosts in ways that enhance their own transmission [3] [4] [5] [6] [7] [8] [9]. The influence of Wolbachia infection on the dynamics of host populations has focused considerable interest on its possible role in speciation through reproductive isolation [3] [10] [11] and as an agent of biological control [2] [12] [13]. Although Wolbachia normally undergoes vertical transmission through the maternal line of its host population [14], there is compelling evidence from molecular phylogenies that extensive horizontal (intertaxon) transmission must have occurred [1] [9] [15] [16] [17]. Some of the best candidate vectors for the horizontal transmission of Wolbachia are insect parasitoids [15], which comprise around 25% of all insect species and attack arthropods from an enormous range of taxa [18]. In this study, we used both fluorescence microscopy and PCR amplification with Wolbachia-specific primers to show that Wolbachia can be transmitted to a parasitic wasp (Leptopilina boulardi) from its infected host (Drosophila simulans) and subsequently undergo diminishing vertical transmission in this novel host species. These results are, to our knowledge, the first to reveal a natural horizontal transfer route for Wolbachia between phylogenetically distant insect species.

Nuclear magnetic resonance microscopy of the development of the parasitoid wasp Venturia canescens within its host moth Plodia interpunctella.

Nuclear magnetic resonance microscopy was used to image the parasitoid wasp Venturia canescens (Hymenoptera: Ichneumonidae) within larval and pupal instars of its host, the Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae). The images were obtained using gradient-echo and chemical shift selective pulse sequences and clearly showed the location and shapes of the parasitoid as it developed from the L1 larva to a pupal stage within the host. The digestive, nervous, and tracheal systems of the host were identified and changes were observed as the host underwent metamorphosis. Destruction of the host tissues by the parasitoid was visible. It was found that the parasitoid first ate the fat body and digestive system of the host, allowing the host to continue to grow, and only progressed to the vital organs when its own development had neared pupation.

Spectral content and dispersion of hyper-Rayleigh scattering.

We have used a high-intensity tunable picosecond infrared laser source to measure the spectral content of light scattered near the second harmonic for two well-known organic dyes, p-Nitroaniline and Disperse Red 1, at a number of wavelengths in the near infrared. We found that in Disperse red 1 a broad two-photon fluorescence competes with the sharp hyper-Rayleigh peak. Further, dephasing of the virtual excited state leads to elimination of hyper-Rayleigh scattering in favor of fluorescence if the harmonic is in the linear absorption tail. We determined the dispersion of the magnitude of the first hyperpolarizability for both dyes, using a simple referencing technique, and found it to be consistent with that of a two-level dispersion model.

Influence of vegetation on the spatial distribution of Toxorhynchites moctezuma ovipositions in the field.

The influence of the vegetation surrounding black-painted calabash ovitraps on the number of eggs of the mosquito Toxorhynchites moctezuma they attracted was investigated using oviposition data gathered from a seasonal-deciduous forest in Trinidad, West Indies. More eggs were laid into ovitraps situated either within or directly adjacent to trees or bamboo stools than those not associated with trees or bamboo. This result is discussed in terms of the initial oviposition-site searching behavior of female Toxorhynchites.

Influence of humidity and temperature on the diel periodicity of oviposition of Toxorhynchites moctezuma (Diptera: Culicidae) in the field.

The diurnal pattern of oviposition by Toxorhynchites moctezuma (Dyar & Knab), ambient relative humidity, and ambient air temperature were monitored hourly between 0600 and 1800 hours for 46 consecutive d in a tropical rainforest in Trinidad, West Indies. The mean number of eggs per ovitrap per hour was correlated negatively with mean ambient relative humidity and positively with air temperature. Partial correlation coefficients among these variables and contingency table analysis indicated a stronger relationship between the oviposition rate and relative humidity than between oviposition rate and temperature. We suggest that this relationship is further evidence of the ability of female Tx. moctezuma to detect suitable oviposition sites using humidity gradients.

Parasitoid diets: Does superparasitism pay?

The ability of insect parasitoids to discriminate between parasitized and unparasitized hosts is well documented. Despite this, hosts that have been parasitized more than once are frequently found, an occurrence known as superparasitism. Since superparasitism results in interlarval competition, it was generally assumed to be caused by discriminatory 'mistakes'. Recently, theoretical studies have suggested that under certain circumstances superparasitism can be optimal. Superparasitism can thus be viewed as an active foraging strategy rather than as a passive process. Because parasitoids show a direct link between foraging success and reproductive output, they afford important opportunities to test evolutionary models of behaviour.

Oviposition habitat preferences of Toxorhynchites moctezuma mosquitoes in four types of tropical forest in Trinidad.

1. Oviposition of the mosquito Toxorhynchites moctezuma Dyar & Knab was investigated in four types of tropical forest in Trinidad, West Indies, using surrogate and natural ovitraps. Larvae of Tx. moctezuma are obligate predators that might be useful for the biological control of Aedes aegypti (L). 2. Significantly more oviposition occurred in seasonal-deciduous forest than in either montane or evergreen-seasonal forest. 3. Oviposition in surrogate containers (black-painted polystyrene cups, 90 mm diameter) was compared with that occurring in typical natural containers (nutpots of Lecythis zapucajo Aublet). Surrogate ovipots were relatively insensitive indicators of oviposition activity, and would be an inefficient means of harvesting Tx. moctezuma eggs. 4. Implications for the collection, culture and mass release of Tx. moctezuma are discussed.

The functional response of Toxorhynchites rutilus rutilus to changes in the population density of its prey Aedes aegypti.

We present an analysis of the functional response of the predator Toxorhynchites rutilus rutilus (Coquillett) to changes in the density of the larvae of Aedes aegypti (L.) (Diptera: Culicidae). The experiment was replicated for five different ages, and at three different densities of the predator. The data were fitted to Rogers' (1972) random predator equation by non-linear least-squares in order to estimate searching efficiency and handling time for each experimental treatment. The data show that estimated searching efficiencies are highest at intermediate ages of the predator for all predator densities tested. Handling time declines exponentially with increasing predator age. There is a marked interference effect; searching efficiency decreases with increased predator density, and this is most pronounced at intermediate prey ages. Estimated handling times increase with predator density at a rate which declines with increasing predator age.