Microbial community structure reveals instability of nutritional symbiosis during the evolutionary radiation of Amblyomma ticks.

Mutualistic interactions with microbes have facilitated the adaptation of major eukaryotic lineages to restricted diet niches. Hence, ticks with their strictly blood-feeding lifestyle are associated with intracellular bacterial symbionts through an essential B vitamin supplementation. In this study, examination of bacterial diversity in 25 tick species of the genus Amblyomma showed that three intracellular bacteria, Coxiella-like endosymbionts (LE), Francisella-LE and Rickettsia, are remarkably common. No other bacterium is as uniformly present in Amblyomma ticks. Almost all Amblyomma species were found to harbour a nutritive obligate symbiont, Coxiella-LE or Francisella-LE, that is able to synthesize B vitamins. However, despite the co-evolved and obligate nature of these mutualistic interactions, the structure of microbiomes does not mirror the Amblyomma phylogeny, with a clear exclusion pattern between Coxiella-LE and Francisella-LE across tick species. Coxiella-LE, but not Francisella-LE, form evolutionarily stable associations with ticks, commonly leading to co-cladogenesis. We further found evidence for symbiont replacements during the radiation of Amblyomma, with recent, and probably ongoing, invasions by Francisella-LE and subsequent replacements of ancestral Coxiella-LE through transient co-infections. Nutritional symbiosis in Amblyomma ticks is thus not a stable evolutionary state, but instead arises from conflicting origins between unrelated but competing symbionts with similar metabolic capabilities.

Progenesis and facultative life cycle abbreviation in trematode parasites: Are there more constraints than costs?

Complex life cycles provide advantages to parasites (longer life span, higher fecundity, etc.), but also represent a series of unlikely events for which many adaptations have evolved (asexual multiplication, host finding mechanisms, etc.). Some parasites use a radical strategy where the definitive host is dropped; life cycle abbreviation is most often achieved through progenesis (i.e. early maturation) and reproduction in the second intermediate host. In many progenetic species, both the typical and abbreviated life cycles are maintained. However, conditions that trigger the adoption of one or the other strategy, and the pros and cons of each parasite life history strategy, are often complex and poorly understood. We used experimental infections with the trematode Coitocaecum parvum in its fish definitive host to test for potential costs of progenesis in terms of lifespan and fecundity. We show that individuals that adopt progenesis in the intermediate host are still able to establish in the definitive host and achieve higher survival and fecundity than conspecifics adopting the typical three-host life cycle. Our results and that of previous studies show that there seems to be few short-term costs associated with progenesis in C. parvum. Potential costs of self-fertilization and inbreeding are often suggested to select for the maintenance of both life-history strategies in species capable of facultative progenesis. We suggest that, at least for our focal species, there are more constraints than costs limiting its adoption. Progenesis and the abbreviated cycle may become the typical life-history strategy while reproduction in the vertebrate definitive host is now a secondary alternative when progenesis is impossible (e.g. limited host resources, etc.). Whether this pattern can be generalized to other progenetic trematodes is unknown and would require further studies.

Long-term pathogenic response to Plasmodium relictum infection in Culex pipiens mosquito.

The transmission of Plasmodium within a vertebrate host population is strongly associated with the life history traits of its vector. Therefore the effect of malaria infection on mosquito fecundity and longevity has traditionally received a lot of attention. Several species of malaria parasites reduce mosquito fecundity, nevertheless almost all of the studies have focused only on the first gonotrophic cycle. Yet, during their lifetime, female mosquitoes go through several gonotrophic cycles, which raises the question of whether they are able to compensate the fecundity costs induced by the parasite. The impact of Plasmodium infection on female longevity is not so clear and has produced conflicting results. Here we measured the impact of Plasmodium relictum on its vector's longevity and fecundity during three consecutive gonotrophic cycles. In accordance with previous studies, we observed a negative impact of Plasmodium infection on mosquito (Culex pipiens) fecundity in the first gonotrophic cycle. Interestingly, despite having taken two subsequent uninfected blood meals, the negative impact of malaria parasite persisted. Nevertheless no impact of infection on mosquito longevity was observed. Our results are not in line with the hypothesis that the reduction of fecundity observed in infected mosquitoes is an adaptive strategy of Plasmodium to increase the longevity of its vector. We discuss the different underlying mechanisms that may explain our results.