Symbiont dynamics during the blood meal of Ixodes ricinus nymphs differ according to their sex.

Ticks harbour rich and diverse microbiota and, among the microorganisms associated with them, endosymbionts are the subject of a growing interest due to their crucial role in the biology of their arthropod host. Midichloria mitochondrii is the main endosymbiont of the European tick Ixodes ricinus and is found in abundance in all I. ricinus females, while at a much lower density in males, where it is even absent in 56 % of the individuals. This endosymbiont is also known to increase in numbers after the blood meal of larvae, nymphs or females. Because of this difference in the prevalence of M. mitochondrii between the two sexes, surveying the density of these bacteria in nymphs that will become either females or males could help to understand the behaviour of Midichloria in its arthropod host. To this aim, we have set up an experimental design by building 3 groups of unfed nymphs based on their scutum and hypostome lengths. After engorgement, weighing and moulting of a subset of the nymphs, a significant difference in sex-ratio among the 3 groups was observed. In parallel, Midichloria load in individual nymphs was quantified by qPCR both before and after engorgement. No difference in either body mass or Midichloria load was observed at the unfed stage, but following engorgement, both features were significantly different between each size group. Our results demonstrate that symbiont dynamics during nymphal engorgement is different between the two sexes, resulting in a significantly higher Midichloria load in nymphs that will become females. The consequences of those findings on our understanding of the interplay between the endosymbiont and its arthropod host are discussed.

Widespread host-dependent hybrid unfitness in the pea aphid species complex.

Linking adaptive divergence to hybrid unfitness is necessary to understand the ecological factors contributing to reproductive isolation and speciation. To date, this link has been demonstrated in few model systems, most of which encompass ecotypes that occupy relatively early stages in the speciation process. Here we extend these studies by assessing how host-plant adaptation conditions hybrid fitness in the pea aphid, Acyrthosiphon pisum. We made crosses between and within five pea aphid biotypes adapted to different host plants and representing various stages of divergence within the complex. Performance of F1 hybrids and nonhybrids was assessed on a "universal" host that is favorable to all pea aphid biotypes in laboratory conditions. Although hybrids performed equally well as nonhybrids on the universal host, their performance was much lower than nonhybrids on the natural hosts of their parental populations. Hence, hybrids, rather than being intrinsically deficient, are maladapted to their parents' hosts. Interestingly, the impact of this maladaptation was stronger in certain hybrids from crosses involving the most divergent biotype, suggesting that host-dependent postzygotic isolation has continued to evolve late in divergence. Even though host-independent deficiencies are not excluded, hybrid maladaptation to parental hosts supports the hypothesis of ecological speciation in this complex.