Moreau, C. S. | Search bioRxiv

bioRxiv2017

Evidences for convergent evolution of host parasitic manipulation in response to environmental conditions

Environmental conditions exert strong selection on animal behavior. We tested the hypothesis that the altered behavior of hosts due to parasitic manipulation is also subject to selection imposed by changes in environmental conditions over time. Our model system is ants manipulated by parasitic fungi to bite onto vegetation. We analyzed the correlation between forest type (tropical vs. temperate) and biting substrate (leaf vs. twigs), the time required for the fungi to reach reproductive maturity, and the phylogenetic relationship among specimens from tropical and temperate forests in different parts of the globe. We show that the fungal development in temperate forest is longer than the period of time leaves are present and the ants are manipulated to bite twigs. When biting twigs, 90% of the we examined dead ants had their legs wrapped around twigs, which appears to provide better attachment to the plant. Ancestral state character reconstruction suggests that the leaf biting is the ancestral trait and that twig biting is a convergent trait in temperate regions of the globe. These three lines of evidence suggest that changes in environmental conditions have shaped the manipulative behavior of the host by its parasite.

evolutionary biology↗

bioRxiv2017

Mutualistic acacia-ants show that specialized bacteria are not required for the evolution of herbivory

Acacia-ant mutualists in the genus Pseudomyrmex nest obligately in acacia plants and, through stable isotope analysis, we show that they are among the strictest of herbivores, feeding exclusively from their hosts. The diets of herbivorous insects such as these are often enriched by obligate bacterial endosymbionts through nitrogen recycling and even gaseous di-nitrogen fixation. We, therefore, examine the bacterial communities associated with mutualistic acacia-ants, comparing them with related non-mutualists in order to determine whether they host bacterial partners likely to contribute to the enrichment of their diets. However, despite their low trophic position, we find no evidence for bacteria-assisted nutrition in either adults or larvae. These acacia-ants do not host any species- or clade-specific bacteria, though several lineages of acetic acid bacteria present across social insects do differ in abundance between mutualists and non-mutualists, likely in response to the sugar-rich diets of their hosts. In addition, two novel lineages of Actinomycetales inhabit both mutualistic and non-mutualistic Pseudomyrmex and differ in abundance between the juveniles of these groups, potentially serving as defensive symbionts. Metagenomic sequencing of these taxa reveal substantial capacity for the production of defensive chemicals. Overall, we find little evidence for nutrition-associated bacteria in these strictly herbivorous ants, showing that bacteria are not as essential to animal nutrition as is often hypothesized.

ecology↗