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Aim: Highly host-specific symbionts are very rarely found except with their typical host species. Although switches to new hosts are rare and difficult to detect, a switch to a host phylogenetically distant from the original one (a 'major host switch') could allow diversification of the symbionts onto the new host lineage. The consequences of such major host switches on the diversification of highly host-specific symbionts of animals have rarely been explored. Here, we examine the host specificity of vane-dwelling feather mites, a group that shows strong specificity, together with their host-switching dynamics and the consequences of major host switches for their diversification. Location: Global. Time period: From 1882 to 2015. Major taxa studied: Feather mites and birds. Methods: Using the largest published dataset of feather mite–bird associations, we analysed raw, phylogenetic and geographical host specificity of feather mites. We studied host-switching dynamics by describing the sharing by feather mites of bird species with different phylogenetic distances. For three of the most species-rich feather mite families, we quantified the consequences of major host switches for feather mite diversification. Results: Most feather mite species (84%) inhabit one to three very closely related host species. Assemblages of feather mites on birds do not show a geographical signature, but rather show strong host-driven structuring. The probability that a mite species occurs on two host species decays sharply with host phylogenetic distance, with only one instance of a feather mite species occupying distantly related hosts from different orders. However, results suggest that despite the strong host specificity, a few major host switches triggered the origin of 21% of the species and 38% of the genera of the mite families studied. Main conclusions: We show that feather mites are highly host-specific symbionts, whose assemblages do not show geographical structure, even at a continental scale. We conclude that major host switches are very rare events with strong macroevolutionary consequences for feather mite diversity.

We detected signals of positive selection and parallel evolution in H7N9 avian influenza virus sequences that occurred during the multiple invasions of this virus into humans. These changes likely reflect adaptations of this virus to the new mammalian (human) environment. Abstract Background Avian influenza A virus H7N9 has caused 5 epidemic waves of human infections in China since 2013. Avian influenza A viruses may face strong selection to adapt to novel conditions when establishing themselves in humans. In this study, we sought to determine whether adaptive evolution had occurred in human-isolated H7N9 viruses. Methods We evaluated all available genomes of H7N9 avian influenza A virus. Maximum likelihood trees were separately reconstructed for all 8 genes. Signals of positive selection and convergent evolution were then detected on branches that lead to changes in host tropism (from avian to human). Results We found that 3 genes had significant signals of positive selection (all of them P < .05). In addition, we detected 34 sites having significant signals for parallel evolution in 8 genes (all of them P < .05), including 7 well-known sites (Q591K, E627K, and D701N in PB2 gene; R156K, V202A, and L244Q in HA; and R289K in NA) that play roles in crossing species barriers for avian influenza A viruses. Conclusion Our study suggests that, during infection in humans, H7N9 viruses have undergone adaptive evolution to adapt to their new host environment and that the sites where parallel evolution occurred might play roles in crossing species barriers and respond to the new selection pressures arising from their new host environments.

Our understanding of the ecological and evolutionary context of novel infections is largely based on viral diseases, even though bacterial pathogens may display key differences in the processes underlying their emergence. For instance, host-shift speciation, in which the jump of a pathogen into a novel host species is followed by the specialization on that host and the loss of infectivity of previous host(s), is commonly observed in viruses, but less often in bacteria. Here, we suggest that the extent to which pathogens evolve host generalism or specialism following a jump into a novel host will depend on their level of adaptation to dealing with different environments, their rates of molecular evolution and their ability to recombine. We then explore these hypotheses using a formal model and show that the high levels of phenotypic plasticity, low rates of evolution and the ability to recombine typical of bacterial pathogens should reduce their propensity to specialize on novel hosts. Novel bacterial infections may therefore be more likely to result in transient spillovers or increased host ranges than in host shifts. Finally, consistent with our predictions, we show that, in two unusual cases of contemporary bacterial host shifts, the bacterial pathogens both have small genomes and rapid rates of substitution. Further tests are required across a greater number of emerging pathogens to assess the validity of our hypotheses. This article is part of the theme issue ‘Dynamic and integrative approaches to understanding pathogen spillover’.

Many pathogens infect multiple hosts, and spillover from domestic to wild species poses a significant risk of spread of diseases that threaten wildlife and humans. Documentation of cross‐species transmission, and unraveling the mechanisms that drive it, remains a challenge. Focusing on co‐occurring domestic and wild felids, we evaluate possible transmission mechanisms and evidence of spillover of “Candidatus Mycoplasma haemominutum” (CMhm), an erythrocytic bacterial parasite of cats. We examine transmission and possibility of spillover by analyzing CMhm prevalence, modeling possible transmission pathways, deducing genotypes of CMhm pathogens infecting felid hosts based on sequences of the bacterial 16S rRNA gene, and conducting phylogenetic analyses with ancestral state reconstruction to identify likely cross‐species transmission events. Model selection analyses suggest both indirect (i.e., spread via vectors) and direct (i.e., via interspecific predation) pathways may play a role in CMhm transmission. Phylogenetic analyses indicate that transmission of CMhm appears to predominate within host species, with occasional spillover, at unknown frequency, between species. These analyses are consistent with transmission by predation of smaller cats by larger species, with subsequent within‐species persistence after spillover. Our results implicate domestic cats as a source of global dispersal and spillover to wild felids via predation. We contribute to the emerging documentation of predation as a common means of pathogen spillover from domestic to wild cats, including pathogens of global conservation significance. These findings suggest risks for top predators as bioaccumulators of pathogens from subordinate species. Focusing on co‐occurring domestic and wild felids, we evaluate evidence of spillover and mechanisms of transmission for an erythrocytic bacterial parasite of cats. We undertake analyses of disease prevalence, transmission modeling, pathogen genotyping based on sequences of the bacterial 16S rRNA gene, and phylogenetic analysis with ancestral state reconstruction. Our results implicate domestic cats as a source of global dispersal and spillover of pathogens to wild felids via predation.

White-spotted flower chafer adult (Protaetia brevitarsis, Coleoptera: Scarabaeidae), a serious omnivorous pest in regions with multiple fruits and crops, was studied to gain a deeper understanding of its damage patterns. DNA molecular tracking technology was used to identify host plant residues in adult P. brevitarsis midgut, and plant species with the most availability were determined during their growing season. Combining the 2019 and 2021 results, it was found that adults in the multi-cropped area fed on 32 plant species from 23 families, with grape (Vitis vinifera, 40%), peach (Prunus perisica, 23%) and mulberry (Morus alba, 14%) making up the majority of their diet. Some adults fed on multiple plant hosts, with four species detected in one adult and two to three species detected in one-third of adults. Adults shifted among host species during the season, moving from mulberry or grape to peach and then back to grape. These results provide a scientific basis for in-depth research to develop green integrated control technologies against P. brevitarsis adults.

Abstract Ambrosia gall midges (AGMs) represent an intriguing group within the Cecidomyiidae, one of the most diversified dipteran families. AGMs form galls on plants, where they cultivate and consume fungal symbionts (phytomycetophagy). This mutualistic relationship may play a critical role in larval nutrition, gall morphogenesis, and protection against natural enemies. Although most other fungus-farming taxa have been intensively studied, AGMs have largely been neglected. This review synthesizes current knowledge on the diversity, biology, and ecological interactions of AGM, highlighting the intricate relationships with their fungal symbionts. The implications for adaptive radiation and speciation are critically considered, including how fungal associations may have facilitated ecological flexibility and diversification. We also tackle the processes of coevolution, not only between AGM and their fungal symbionts but also involving plants and parasitoids. We identify the most pressing issues and discrepancies in the current understanding the AGM–fungi interactions. Key areas of future research should include elucidating fungal acquisition and transmission mechanisms, determining the specificity and diversity of AGM-associated fungal communities, understanding the evolutionary pathways leading to phytomycetophagy, and addressing taxonomic challenges within the AGM group, where species identification has been complicated by reliance on gall morphology and host specificity. The intricate relationships between ambrosia gall midges and their fungal symbionts provide a compelling model for studying insect–fungus coevolution and mutualism, with significant implication for understanding insect speciation and host plant specialization.

The role of plant–pollinator interactions in the rapid radiation of the angiosperms have long fascinated evolutionary biologists. Studies have brought evidence for pollinator-driven diversification of various plant lineages, particularly plants with specialized flowers and concealed rewards. By contrast, little is known about how this crucial interaction has shaped macroevolutionary patterns of floral visitors. In particular, there is currently no empirical evidence that floral host association has increased diversification in bees, the most prominent group of floral visitors that essentially rely on angiosperm pollen. In this study, we examine how floral host preference influenced diversification in eucerine bees (Apidae, Eucerini), which exhibit large variations in their floral associations. We combine quantitative pollen analyses with a recently proposed phylogenetic hypothesis, and use a state speciation and extinction probabilistic approach. Using this framework, we provide the first evidence that multiple evolutionary transitions from host plants with accessible pollen to restricted pollen from ‘bee-flowers’ have significantly increased the diversification of a bee clade. We suggest that exploiting host plants with restricted pollen has allowed the exploitation of a new ecological niche for eucerine bees and contributed both to their colonization of vast regions of the world and their rapid diversification.

Maize was introduced into opposite sides of Eurasia 500 years ago, in Western Europe and in Asia. This caused two host-shifts in the phytophagous genus Ostrinia, O. nubilalis (the European corn borer; ECB) and O. furnacalis (the Asian corn borer; ACB) are now major pests of maize worldwide. They originated independently from Dicot-feeding ancestors, similar to O. scapulalis (the Adzuki bean borer; ABB). Unlike other host-plants, maize is yearly harvested, and harvesting practices impose severe mortality on larvae found above the cut-off line. Positive geotaxis in the ECB has been proposed as a behavioural adaptation to harvesting practices, allowing larvae to move below the cut-off line and thus escape harvest-mortality. Here we test whether the same behavioural adaptation evolved independently in Europe and in Asia. We sampled eight genetically differentiated ECB, ACB and ABB populations in France and China and monitored geotaxis through the entire larval development in artificial stacks mimicking maize stems. We find that all ECB and ACB populations show a similar tendency to move down during the latest larval stages, a behaviour not observed in any European or Asian ABB population. The behaviour is robustly expressed regardless of larval density, development mode or environmental conditions. Our results indicate that maize introduction triggered parallel behavioural adaptations in Europe and Asia, harvest selection presumably being the main driver.

Virus host range, i.e., the number and diversity of host species of viruses, is an important determinant of disease emergence and of the efficiency of disease control strategies. However, for plant viruses, little is known about the genetic or ecological factors involved in the evolution of host range. Using available genome sequences and host range data, we performed a phylogenetic analysis of host range evolution in the genus Potyvirus, a large group of plant RNA viruses that has undergone a radiative evolution circa 7000 years ago, contemporaneously with agriculture intensification in mid Holocene. Maximum likelihood inference based on a set of 59 potyviruses and 38 plant species showed frequent host range changes during potyvirus evolution, with 4.6 changes per plant species on average, including 3.1 host gains and 1.5 host loss. These changes were quite recent, 74% of them being inferred on the terminal branches of the potyvirus tree. The most striking result was the high frequency of correlated host gains occurring repeatedly in different branches of the potyvirus tree, which raises the question of the dependence of the molecular and/or ecological mechanisms involved in adaptation to different plant species.