Wolbachia in the spittlebug Prosapia ignipectus: Variable infection frequencies, but no apparent effect on host reproductive isolation

Wolbachia infection and genetic diversity of Italian populations of Philaenus spumarius, the main vector of Xylella fastidiosa in Europe

Philaenus spumarius is a cosmopolitan species that has become a major threat to European agriculture being recognized as the main vector of the introduced plant pathogen Xylella fastidiosa, the agent of the “olive quick decline syndrome”, a disease which is devastating olive orchards in southern Italy. Wolbachia are bacterial symbionts of many insects, frequently as reproductive parasites, sometime by establishing mutualistic relationships, able to spread within host populations. Philaenus spumarius harbors Wolbachia, but the role played by this symbiont is unknown and data on the infection prevalence within host populations are limited. Here, the Wolbachia infection rate was analyzed in relation to the geographic distribution and the genetic diversity of the Italian populations of P. spumarius. Analysis of the COI gene sequences revealed a geographically structured distribution of the three main mitochondrial lineages of P. spumarius. Wolbachia was detected in half of the populations sampled in northern Italy where most individuals belonged to the western-Mediterranean lineage. All populations sampled in southern and central Italy, where the individuals of the eastern-Mediterranean lineage were largely prevalent, were uninfected. Individuals of the north-eastern lineage were found only in populations from the Alps in the northernmost part of Italy, at high altitudes. In this area, Wolbachia infection reached the highest prevalence, with no difference between north-eastern and western-Mediterranean lineage. Analysis of molecular diversity of COI sequences suggested no significant effect of Wolbachia on population genetics of P. spumarius. Using the MLST approach, six new Wolbachia sequence types were identified. Using FISH, Wolbachia were observed within the host’s reproductive tissues and salivary glands. Results obtained led us to discuss the role of Wolbachia in P. spumarius, the factors influencing the geographic distribution of the infection, and the exploitation of Wolbachia for the control of the vector insect to reduce the spread of X. fastidiosa.

Temperature effects on cellular host-microbe interactions explain continent-wide endosymbiont prevalence

Endosymbioses influence host physiology, reproduction, and fitness, but these relationships require efficient microbe transmission between host generations to persist. Maternally transmitted Wolbachia are the most common known endosymbionts,¹ but their frequencies vary widely within and among host populations for unknown reasons.^2,3 Here, we integrate genomic, cellular, and phenotypic analyses with mathematical models to provide an unexpectedly simple explanation for global wMel Wolbachia prevalence in Drosophila melanogaster. Cooling temperatures decrease wMel cellular abundance at a key stage of host oogenesis, producing temperature-dependent variation in maternal transmission that plausibly explains latitudinal clines of wMel frequencies on multiple continents. wMel sampled from a temperate climate targets the germline more efficiently in the cold than a recently differentiated tropical variant (∼2,200 years ago), indicative of rapid wMel adaptation to climate. Genomic analyses identify a very narrow list of wMel alleles-most notably, a derived stop codon in the major Wolbachia surface protein WspB-that underlie thermal sensitivity of cellular Wolbachia abundance and covary with temperature globally. Decoupling temperate wMel and host genomes further reduces transmission in the cold, a pattern that is characteristic of host-microbe co-adaptation to a temperate climate. Complex interactions among Wolbachia, hosts, and the environment (GxGxE) mediate wMel cellular abundance and maternal transmission, implicating temperature as a key determinant of Wolbachia spread and equilibrium frequencies, in conjunction with Wolbachia effects on host fitness and reproduction.^4,5 Our results motivate the strategic use of locally selected wMel variants for Wolbachia-based biocontrol efforts, which protect millions of individuals from arboviruses that cause human disease.⁶.

Male Age and Wolbachia Dynamics: Investigating How Fast and Why Bacterial Densities and Cytoplasmic Incompatibility Strengths Vary

Endosymbionts can influence host reproduction and fitness to favor their maternal transmission. For example, endosymbiotic Wolbachia bacteria often cause cytoplasmic incompatibility (CI) that kills uninfected embryos fertilized by Wolbachia-modified sperm. Infected females can rescue CI, providing them a relative fitness advantage. Wolbachia-induced CI strength varies widely and tends to decrease as host males age. Since strong CI drives Wolbachia to high equilibrium frequencies, understanding how fast and why CI strength declines with male age is crucial to explaining age-dependent CI’s influence on Wolbachia prevalence. Here, we investigate if Wolbachia densities and/or CI gene (cif) expression covary with CI-strength variation and explore covariates of age-dependent Wolbachia-density variation in two classic CI systems. wRi CI strength decreases slowly with Drosophila simulans male age (6%/day), but wMel CI strength decreases very rapidly (19%/day), yielding statistically insignificant CI after only 3 days of Drosophila melanogaster adult emergence. Wolbachia densities and cif expression in testes decrease as wRi-infected males age, but both surprisingly increase as wMel-infected males age, and CI strength declines. We then tested if phage lysis, Octomom copy number (which impacts wMel density), or host immune expression covary with age-dependent wMel densities. Only host immune expression correlated with density. Together, our results identify how fast CI strength declines with male age in two model systems and reveal unique relationships between male age, Wolbachia densities, cif expression, and host immunity. We discuss new hypotheses about the basis of age-dependent CI strength and its contributions to Wolbachia prevalence.