Factors driving Wolbachia prevalence in native and invasive populations of Drosophila suzukii

Understanding the factors that drive the prevalence of endosymbionts in natural populations is a central goal of evolutionary ecology. The success of maternally transmitted symbionts can be explained by three parameters: reproductive manipulations of the host, vertical transmission rates and effects on host fitness. Those parameters can be modulated by both environmental and genetic factors. The Wolbachia strain wSuz, which infects the invasive pest species Drosophila suzukii, is a canonical example of maternally transmitted symbiont with low to intermediate prevalence in host populations and weak reproductive manipulation. For Wolbachia strains with such properties, the mechanisms inherent to the infection dynamics remain poorly understood. We investigated infection frequencies and wSuz intra-strain polymorphism in 24 natural populations of D. suzukii from both native (China and Japan) and invasive areas (Europe and America). Infection frequencies in populations from China were on average significantly higher than those in populations from invasive areas. Based on the study of an insertion sequence site and a genomic rearrangement polymorphism in wSuz genome, we could identify three wSuz variants (i.e. European, American and Asian) corresponding to the initial sample location. More in-depth investigations indicated that the European variant was also present in some Asian populations and that one population from Europe included the European and American variants. Additional analyses based on whole-genome sequencing of 70 D. suzukii population samples showed lower Wolbachia genomic diversity in invasive populations (America and Europe) than in native (Japan, Korea and China) populations, consistent with a bottleneck in invasive populations. Finally, we analyzed two types of factors that could affect Wolbachia infection frequencies in natural populations: climatic variation and Wolbachia-induced cytoplasmic incompatibility. We found that variations in the strength of cytoplasmic incompatibilities or in monthly temperatures were not sufficient to fully explain observed prevalence pattern. Overall, our results show that, despite reduced intra-strain genomic diversity, both population prevalence and phenotypic effects on host reproduction can vary greatly among Wolbachia variants, suggesting complex interactions with host genetic background and environmental factors

Comparative analysis of phenolic profiles of ovipositional fluid of Rhinusa pilosa (Mecinini, Curculionidae) and its host plant Linaria vulgaris (Plantaginaceae)

Rhinusa pilosa (Gyllenhal) is a highly specific weevil that induces stem galls on the common toadflax Linaria vulgaris Mill. females oviposit the eggs near the apex of a growing shoot. The act of oviposition is accompanied by secretion of an ovipositional fluid, which is considered to be cecidogen, directly involved in gall induction. The remains of cecidogenic fluid were collected from the surface of the oviposition point on the stem. We performed a comparative analysis of the phenolics extracted from cecidogen, the stem and galls of L. vulgaris and adult and larva of R. pilosa by HPLC-DAD. One compound with A (max) at 273, 332 nm (R (t) 30.65 min) was exclusively found in the methanol extract of cecidogen. To further characterize the cecidogen and stem phenolic profiles, we used UHPLC coupled with an OrbiTrap mass analyzer. Among 49 phenolic compounds extracted from both the ovipositional fluid and the plant, protocatechuic acid and two phenolic glycosides were exclusively found in cecidogen: diosmetin-O-acetylrutinoside and an unidentified compound. The unknown compound produced an MS2 base peak at 387 and 327 and 267 m/z base peaks at MS3 and MS4 fragmentation, respectively, and had the molecular formula C32H31O18. The plausible role of phenolic compounds in the induction of gall formation on L. vulgaris is discussed

The importance of cryptic species and subspecific populations in classic biological control of weeds: a North American perspective

Classical biological control of weeds depends on finding agents that are highly host-specific. This requires not only correctly understanding the identity of the target plant, sometimes to subspecific levels, in order to find suitable agents, but also identifying agents that are sufficiently specific to be safe and effective. Behavioral experiments and molecular genetic tools have revealed that some arthropod species previously thought to be polyphagous really consist of multiple cryptic species, host races or biotypes, some of which are more host-specific than others. Whereas true species are reproductively isolated, individuals from subspecific populations may potentially interbreed with those of other populations if they should encounter them. Furthermore, biotypes may consist of individuals sharing a genotype that is not fixed within a monophyletic group, and thus may not be evolutionarily stable. This raises the question of how such populations should be classified, and how to confirm the identity of live arthropods before releasing them as classical biological control agents. The existence of host races or cryptic species may greatly increase the number of prospective biological control agents available. However, it may also create new challenges for governmental regulation. These issues are discussed using pertinent examples, mainly from North America