Host preferences of Palaearctic Culicoides biting midges: implications for transmission of orbiviruses.

Feeding success depends on host availability, host defensive reactions and host preferences. Host choice is a critical determinant of the intensity at which pathogens are transmitted. The aim of the current study was to describe host preferences of Palaearctic Culicoides species (Diptera: Ceratopogonidae) Latreille using traps baited with the five different host species of poultry, horse, cattle, sheep and goat. Collections were carried out nightly in July and August 2009 in western France with three replicates of a 5 × 5 randomized Latin square (five sites, five hosts). Moreover, an ultraviolet (UV) light/suction trap was operated during host-baited collections to correlate Culicoides biting rates and UV light/suction trap catches. A total of 660 Culicoides belonging to 12 species, but comprised mainly of Culicoides scoticus Downes and Kettle, Culicoides dewulfi Goetghebuer and Culicoides obsoletus Meigen, were collected on animal baits. Abundance was highest for the horse, which accounted for 95% of all Culicoides caught, representing 10 species. The horse, the largest bait, was the most attractive host, even when abundance data were corrected by weight, body surface or Kleiber's scaling factor. Culicoides obsoletus was the only dominant species attracted by birds. Both C. scoticus and C. dewulfi were collected mainly from the upper body of the horse. Finally, the quantification of host preferences allows for discussion of implications for the transmission of Culicoides-borne pathogens such as bluetongue virus.

Effect of phenotypic variation in Xenorhabdus nematophila on its mutualistic relationship with the entomopathogenic nematode Steinernema carpocapsae.

The entomopathogenic nematode Steinernema carpocapsae is mutualistically associated with the bacterium Xenorhabdus nematophila. Infective Juveniles (IJs) transport X. nematophila cells that provide them with good conditions to reproduce within the insect. In the laboratory, long term stationary-phase culture conditions sometimes lead X. nematophila's variant 1 cells, which were previously isolated from the worms, to spontaneously and irreversibly change into a new phenotypic variant (variant 2). In this paper, we tested the ability of each phenotypic variant to (i) be transmitted by IJs, (ii) to optimize the worm's fitness within the insect, and (iii) to counteract the effect of closely related antagonistic bacteria previously shown as being able to totally prevent S. carpocapsae's reproduction within the insect. We found that IJs did associate with cells of both phenotypes but that the variant 2 cells were preferentially retained by the nematodes when both variants were present in the insect. Both phenotypic variants led to the same fitness of S. carpocapsae in insects not infected by antagonistic bacteria. In insects infected by antagonistic bacteria, both variants were able to provide protection to S. carpocapsae. Nevertheless, this protection depended on the phenotypic variant and the antagonistic bacteria that were co-injected into the insect. Further analysis conduced in vitro showed that this variability could be partly linked to the sensitivity of each antagonistic bacterium to xenorhabdicin, produced by X. nematophila.

When mutualists are pathogens: an experimental study of the symbioses between Steinernema (entomopathogenic nematodes) and Xenorhabdus (bacteria).

In this paper, we investigate the level of specialization of the symbiotic association between an entomopathogenic nematode (Steinernema carpocapsae) and its mutualistic native bacterium (Xenorhabdus nematophila). We made experimental combinations on an insect host where nematodes were associated with non-native symbionts belonging to the same species as the native symbiont, to the same genus or even to a different genus of bacteria. All non-native strains are mutualistically associated with congeneric entomopathogenic nematode species in nature. We show that some of the non-native bacterial strains are pathogenic for S. carpocapsae. When the phylogenetic relationships between the bacterial strains was evaluated, we found a clear negative correlation between the effect a bacterium has on nematode fitness and its phylogenetic distance to the native bacteria of this nematode. Moreover, only symbionts that were phylogenetically closely related to the native bacterial strain were transmitted. These results suggest that co-evolution between the partners has led to a high level of specialization in this mutualism, which effectively prevents horizontal transmission. The pathogenicity of some non-native bacterial strains against S. carpocapsae could result from the incapacity of the nematode to resist specific virulence factors produced by these bacteria.

Mouse biodiversity in the genomic era.

Comparative genomics has developed by comparison of distantly related genomes, for which the link between the reported evolutionary changes and species development/physiology/ecology is not obvious. It is argued that the mouse (genus Mus) is an optimal model for microevolutionary genomics in vertebrates. This is because the mouse genome sequence, physical and genetic map have been completed, because mouse genetics, morpho-anatomy, pathology, behavior and ecology are well-studied, and because the Mus genus is a diverse, well- documented taxon, allowing comparative studies at the level of individual, population, subspecies, and species. The potential of the interaction between mouse genome and mouse biodiversity is illustrated by recent studies of speciation in the house mouse Mus musculus, and studies about the evolution of isochores, the peculiar pattern of GC-content variation across mammalian genomes.

Susceptibility of natural hybrids between house mouse subspecies to Sarcocystis muris.

A previous study showed that the susceptibility of hybrids between two Mus musculus musculus and Mus musculus domesticus did not apply to every parasite. The authors proposed that only parasites which exerted enough constraints would induce the selection of poorly compatible systems of resistance in the subspecies. This study completes the previous work. Experimental infections of mice of the two subspecies and their hybrids with the tissue-dwelling protozoan Sarcocystis muris show that hybrids are more susceptible to the coccidian than M. m. musculus and M. m. domesticus individuals. This result demonstrates that the hybrids are not only susceptible to intestinal helminths, and confirms the 'constraint hypothesis'.

Experimental trypanosomiasis of natural hybrids between house mouse subspecies.

This study characterises the extent of the susceptibility to parasites (first demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone.

Comparison between patterns of pinworm infection (Aspiculuris tetraptera) in wild and laboratory strains of mice, Mus musculus.

Sixteen laboratory and 7 wild-derived strains of mice were infected with the pinworm Aspiculuris tetraptera in order to compare their resistance levels estimated by the intestinal parasite loads. It appears that (i) in 4 strains out of 23, females and males harbour different parasite loads; (ii) wild and laboratory mice display a broad range of infection levels when compared independently; (iii) the laboratory strains are more resistant than the wild ones. We suggest that (i) compared to sex, the strain (i.e. genetic) effect is the main parameter which determines the levels of infection; (ii) resistance was selected in laboratory strains during their breeding because of the parasite pressure present in captivity.

Entry and survival of Leishmania amazonensis amastigotes within phagolysosome-like vacuoles that shelter Coxiella burnetii in Chinese hamster ovary cells.

Coxiella burnetii, a rickettsia, and Leishmania amazonensis, a protozoan flagellate, lodge in their host cells within large phagolysosome-like vacuoles. In the present study, C. burnetii-infected Vero or CHO cells were superinfected with L. amazonensis amastigotes to determine if these parasites can home to and survive within heterologous vacuoles. Six hours after superinfection, Leishmania amastigotes were located almost exclusively within large Coxiella-containing vacuoles. Thereafter, the numbers of parasites in the vacuoles increased at the same rate as those in cells infected with L. amazonensis alone. Furthermore, in cultures shifted to 25 degrees C, some of the amastigotes transformed into promastigote-like forms that moved their flagella within the adoptive vacuoles. Thus, L. amazonensis amastigotes not only entered Coxiella vacuoles, most likely by fusion of donor and recipient vacuoles, but temporarily survived, differentiated, and replicated therein. This appears to be the first account of the temporary cohabitation of two living pathogens within the same vacuole in a mammalian cell.

Hybrid vigour against parasites in interspecific crosses between two mice species.

The resistance and susceptibility to the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse Mus musculus, is experimentally analysed using both the F1 from wild-type mice of the two subspecies (M. m. domesticus and M. m. musculus) and the F1 from different laboratory inbred mice. The results show that: (i) the F1 from wild-type mice harbour a lower parasite load than the parental mice, suggesting a phenomenon of hybrid vigour; and (ii) the F1 from inbred mice harbour parasite loads similar to the resistant parent, suggesting that resistance is inherited as a dominant feature in these laboratory mice. This analysis supports the hypothesis that recombinations occurring between the two mouse genomes (i.e. M. m. domesticus and M. m. musculus) are responsible for the hybrid dysgenesis observed in the natural hybrid zone between the two mice subspecies.

Experimental evidence of genetic determinism in high susceptibility to intestinal pinworm infection in mice: a hybrid zone model.

In the hybrid zone of the two mouse subspecies Mus musculus musculus and Mus musculus domesticus, mice with hybrid genotypes harbour, on the average, more helminth parasites (cestodes and nematodes) than mice of the two parental taxa. In order to determine the roles played by genetic parameters in this phenomenon, mice with recombined and parental genotypes were experimentally infected with the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse. The results showed that the high susceptibility of the hybrid zone mice is genetically determined. In addition, this study shows the occurrence of variability among resistant parental populations.

Allozyme variation between laboratory reared and wild populations of Teladorsagia circumcincta.

The technique of allozyme electrophoresis was applied to two laboratory strains (isolated from the center or south-east of France) and wild populations of Teladorsagia circumcincta from the center of France. Five systems out of 13 (GPI, MPI, MDH, LDH, PGM) could be interpreted. By means of a multivariate analysis, it was shown that the laboratory strains were very similar with each other and genetically different from the wild populations. A deficiency of heterozygotes was recorded for each enzyme locus (except for MDH) in all populations studied.