Can maternally inherited endosymbionts adapt to a novel host? Direct costs of Spiroplasma infection, but not vertical transmission efficiency, evolve rapidly after horizontal transfer into D. melanogaster.

Maternally inherited symbionts are common in arthropods and many have important roles in host adaptation. The observation that specific symbiont lineages infect distantly related host species implies new interactions are commonly established by lateral transfer events. However, studies have shown that symbionts often perform poorly in novel hosts. We hypothesized selection on the symbiont may be sufficiently rapid that poor performance in a novel host environment is rapidly ameliorated, permitting symbiont maintenance. Here, we test this prediction for a Spiroplasma strain transinfected into the novel host Drosophila melanogaster. In the generations immediately following transinfection, the symbiont had low transmission efficiency to offspring and imposed severe fitness costs on its host. We observed that effects on host fitness evolved rapidly, being undetectable after 17 generations in the novel host, whereas vertical transmission efficiency was poorly responsive over this period. Our results suggest that long-term symbiosis may more readily be established in cases where symbionts perform poorly in just one aspect of symbiosis.

A hyperparasite affects the population dynamics of a wild plant pathogen.

Assessing the impact of natural enemies of plant and animal pathogens on their host's population dynamics is needed to determine the role of hyperparasites in affecting disease dynamics, and their potential for use in efficient control strategies of pathogens. Here, we focus on the long-term study describing metapopulation dynamics of an obligate pathogen, the powdery mildew (Podosphaera plantaginis) naturally infecting its wild host plant (Plantago lanceolata) in the fragmented landscape of the Åland archipelago (southwest Finland). Regionally, the pathogen persists through a balance of extinctions and colonizations, yet factors affecting extinction rates remain poorly understood. Mycoparasites of the genus Ampelomyces appear as good candidates for testing the role of a hyperparasite, i.e. a parasite of other parasites, in the regulation of their fungal hosts' population dynamics. For this purpose, we first designed a quantitative PCR assay for detection of Ampelomyces spp. in field-collected samples. This newly developed molecular test was then applied to a large-scale sampling within the Åland archipelago, revealing that Ampelomyces is a widespread hyperparasite in this system, with high variability in prevalence among populations. We found that the hyperparasite was more common on leaves where multiple powdery mildew strains coexist, a pattern that may be attributed to differential exposure. Moreover, the prevalence of Ampelomyces at the plant level negatively affected the overwinter survival of its fungal host. We conclude that this hyperparasite may likely impact on its host population dynamics and argue for increased focus on the role of hyperparasites in disease dynamics.