Statistical analysis of the individual variability of 1D protein profiles as a tool in ecology: an application to parasitoid venom

Understanding the forces that shape eco-evolutionary patterns often requires linking phenotypes to genotypes, allowing characterization of these patterns at the molecular level. DNA-based markers are less informative in this aim compared to markers associated with gene expression and, more specifically, with protein quantities. The characterization of eco-evolutionary patterns also usually requires the analysis of large sample sizes to accurately estimate interindividual variability. However, the methods used to characterize and compare protein samples are generally expensive and time-consuming, which constrains the size of the produced data sets to few individuals. We present here a method that estimates the interindividual variability of protein quantities based on a global, semi-automatic analysis of 1D electrophoretic profiles, opening the way to rapid analysis and comparison of hundreds of individuals. The main original features of the method are the in silico normalization of sample protein quantities using pictures of electrophoresis gels at different staining levels, as well as a new method of analysis of electrophoretic profiles based on a median profile. We demonstrate that this method can accurately discriminate between species and between geographically distant or close populations, based on interindividual variation in venom protein profiles from three endoparasitoid wasps of two different genera (Psyttalia concolor, Psyttalia lounsburyi and Leptopilina boulardi). Finally, we discuss the experimental designs that would benefit from the use of this method.

The role of melanization and cytotoxic by-products in the cellular immune responses of Drosophila against parasitic wasps

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

Variation of Leptopilina boulardi success in Drosophila hosts: what is inside the black box?

Interactions between Drosophila hosts and parasitoid wasps are among the few examples in which occurrence of intraspecific variation of parasite success has been studied in natural populations. Such variations can originate from three categories of factors: environmental, host and parasitoid factors. Under controlled laboratory conditions, it is possible to focus on the two last categories, and, using specific reference lines, to analyze their respective importance. Parasitoid and host contributions to variations in parasite success have largely been studied in terms of evolutionary and mechanistic aspects in two Drosophila parasitoids, Asobara tabida and, in more details, in Leptopilina boulardi. This chapter focuses on the physiological and molecular aspects of L. boulardi interactions with two Drosophila host species, while most of the evolutionary hypotheses and models are presented in Chapter 11 of Dupas et al.

Genetic interactions between the parasitoid wasp Leptopilina boulardi and its Drosophila hosts

Coevolutionary arms races between hosts and parasites would not occur without genetic variation for traits involved in the outcome of parasitism. Genetic variations in resistance and virulence have only rarely been described in pairwise host-parasitoid interactions and have never been analysed in multi-species interactions, in contrast to well-characterized plant-pathogen interactions. This paper reports genetic variation in resistance of Drosophila yakuba to the parasitoid wasp Leptopilina boulardi. The genetic basis and geographic distribution of resistance is analysed. On the basis of these and previous findings, we demonstrate that there are different resistance patterns to the parasitoid species L. boulardi in D. melanogaster and D. yakuba, as well as different specificity levels in the parasitoid species, suggesting complex ecological interactions in the field. This first description of resistance-virulence genetic interactions between a parasitoid and its two host species provides empirical data showing that multi-species interactions may greatly influence coevolutionary processes.

Haemocyte changes in D. Melanogaster in response to long gland components of the parasitoid wasp Leptopilina boulardi: a Rho-GAP protein as an important factor

The hymenopteran wasp Leptopilina boulardi (Figitidae) is a larval solitary parasitoid of Drosophila larvae of the melanogaster sub-group. The factors used by parasitoid females to prevent encapsulation of their eggs by the host are localized in the female long gland and reservoir. We report here the physiological effects of these factors on host haemocytes using in vivo injection experiments. The total number of haemocytes, the number of plasmatocytes and the number of crystal cells were not modified by injection of long gland extracts. In contrast, long gland extracts either from virulent or avirulent strains had a significant effect on the lamellocyte number. Compared to the Ringer control, the avirulent long gland products induced an increase of the lamellocyte number while virulent extracts induced a drastic decrease together with an alteration of the morphology of these cells. Interestingly, changes in the lamellocyte morphology were also observed following injection of the P4 protein, a major component of L. boulardi female long glands that displays a strong immune suppressive effect on Drosophila larvae. The implication of the P4 protein in suppressing the host cellular immunity is discussed in correlation with its predicted molecular function as a Rho-GAP protein.

Polydnavirus genome: integrated vs. free virus

Polydnaviruses are unique because of their obligatory association with thousands of parasitoid wasp species from the braconid and ichneumonid families of hymenopterans. PDVs are injected into the parasitized hosts and are essential for parasitism success. However, polydnaviruses are also unique because of their genome composed of multiple dsDNA segments. Cytological evidence has recently confirmed the results of genetic and molecular analyses indicating that PDV segments were integrated in the wasp genome. Moreover a phylogenetic study performed using the age of available fossils to calibrate the molecular clock indicated that the polydnaviruses harboured by braconid wasps have resided within the wasp genome for approximately 70 million years. In the absence of horizontal transmission, the evolution of the PDV genomes has been driven exclusively by the reproductive success they have offered the wasps. The consequences of this particular selection pressure can be observed in the gene content of certain PDV genomes from which increasing sequence data are available. Molecular mechanisms already identified could be involved in the acquisition and loss of genes by the PDV genomes and lead us to speculate on the definition of the virus genome.

Polydnavirus replication: the EP1 segment of the parasitoid wasp Cotesia congregata is amplified within a larger precursor molecule

Polydnaviruses are unique viruses: they are essential for successful parasitism by tens of thousands of species of parasitoid wasps. These viruses are obligatorily associated with the wasps and are injected into the host during oviposition. Molecular analyses have shown that each virus sequence in the segmented polydnavirus genome is present in the wasp DNA in two forms: a circular form found in the virus particles and an integrated form found in the wasp chromosomes. Recent studies performed on polydnaviruses from braconid wasps suggested that the circular forms were excised from the chromosome. The different forms of the EP1 circle of Cotesia congregata polydnavirus during the pupal-adult development of the parasitoid wasp were analysed. Unexpectedly, an off-size fragment formerly used to diagnose the integration of the EP1 sequence into wasp genomic DNA was found to be amplified in female wasps undergoing virus replication. The EP1 sequence is amplified within a larger molecule comprising at least two virus segments. The amplified molecule is different from the EP1 chromosomally integrated form and is not encapsidated into virus particles. These findings shed light on a new step towards EP1 circle production: the amplification of virus sequences preceding individual circle excision.

Genetic localization of a Drosophila melanogaster resistance gene to a parasitoid wasp and physical mapping of the region

Drosophila melanogaster larvae usually react against eggs of the parasitoid wasp Leptopilina boulardi by surrounding them with a multicellular melanotic capsule. The genetic determinism of this response has been studied previously using susceptible (non-capsule-forming) and resistant (capsule-forming) strains. The results suggest that differences in their encapsulation response involve a single gene, resistance to Leptopilina boulardi (Rlb), with two alleles, the resistant one being dominant. Rlb confers specific protection against Leptopilina boulardi and is thus probably involved in parasitoid recognition. Recent studies have localized this gene on the right arm of the second chromosome and our aim was to precisely determine its genetic and molecular location. Using strains bearing deletions, we demonstrated that resistance to Leptopilina boulardi is conferred by the 55C; 55F3 region and that the 55E2-E6; F3 region is particularly involved. A physical map of the 55C; 56A region was then constructed, based on a set of overlapping cosmid and P1 phage clones. Using single and double digests, cross hybridization of restriction fragments, and location of genetically mapped genes and STSs, a complete, five-enzyme restriction map of this 830-kb region was obtained.

A sex-specific number of germ cells in embryonic gonads of Drosophila

Male first instar larvae possess more germ cells in their gonads than female larvae of the same stage. To determine the earliest time point of sexual dimorphism in germ cell number, we have counted the germ cells of sexed embryos at different developmental stages. We found no difference in germ cell number of male and female embryos at the blastoderm and early gastrulation stage, or when germ cells are about to exit the midgut pocket. We find, however, that males have significantly more germ cells than females as soon as the germ cells are near the places where the gonads are formed and in all later stages. Our results show that germ cells are subject to a sex-specific control mechanism that regulates the number of germ cells already in embryos.

Identification of two distinct amplifications of the esterase B locus in Culex pipiens (L.) mosquitoes from Mediterranean countries

Two new highly active esterases were detected by starch electrophoretic studies in Culex pipiens mosquitoes from the area of Montpellier (France) and from Cyprus. We demonstrate here that both the French and the Cyprus esterases B are overproduced due to amplification of the coding gene. The production of the esterase B is approximately 50- and 500-fold higher in mosquitoes from France and Cyprus, respectively, than in susceptible insects, whereas the number of gene copies is about 25 and 250. Differences of about 7- and 95-fold were also found in the degree of chlorpyrifos resistance. RFLP comparison of the amplified region containing the esterase B gene revealed large differences between French and Cyprus mosquitoes. It thus appears that two distinct haplotypes with an esterase B gene coding an enzyme with identical electrophoretic mobility have been amplified. We therefore named the haplotypes in mosquitoes from France and Cyprus B4 and B5, respectively. The estimated genetic distance between these two haplotypes is not smaller than those observed in all pair comparisons of other known esterase B haplotypes. These results are discussed in the context of amplification phenomena.