Bedbugs Evolved before Their Bat Hosts and Did Not Co-speciate with Ancient Humans

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The evolutionary and molecular features of the broad-host-range plant pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a pathogenic fungus that infects hundreds of plant species, including many of the world's most important crops. Key features of S. sclerotiorum include its extraordinary host range, preference for dicotyledonous plants, relatively slow evolution, and production of protein effectors that are active in multiple host species. Plant resistance to this pathogen is highly complex, typically involving numerous polymorphisms with infinitesimally small effects, which makes resistance breeding a major challenge. Due to its economic significance, S. sclerotiorum has been subjected to a large amount of molecular and evolutionary research. In this updated pathogen profile, we review the evolutionary and molecular features of S. sclerotiorum and discuss avenues for future research into this important species.     

Morphological shifts in island-dwelling birds: the roles of generalist foraging and niche expansion

Abstract Passerine birds living on islands are usually larger than their mainland counterparts, in terms of both body size and bill size. One explanation for this island rule is that shifts in morphology are an adaptation to facilitate ecological niche expansion. In insular passerines, for instance, increased bill size may facilitate generalist foraging because it allows access to a broader range of feeding niches. Here we use morphologically and ecologically divergent races of white-eyes (Zosteropidae) to test three predictions of this explanation: (1) island populations show a wider feeding niche than mainland populations; (2) island-dwelling populations are made up of individual generalists; and (3) within insular populations there is a positive association between size and degree of foraging generalism. Our results provide only partial support for the traditional explanation. In agreement with the core prediction, island populations of white-eye do consistently display a wider feeding niche than comparative mainland populations. However, observations of individually marked birds reveal that island-dwelling individuals are actually more specialized than expected by chance. Additionally, neither large body size nor large bill size are associated with generalist foraging behavior per se. These latter results remained consistent whether we base our tests on natural foraging behavior or on observations at an experimental tree, and whether we use data from single or multiple cohorts. Taken together, our results suggest that generalist foraging and niche expansion are not the full explanation for morphological shifts in island-dwelling white-eyes. Hence, we review briefly five alternative explanations for morphological divergence in insular populations: environmental determination of morphology, reduced predation pressure, physiological optimization, limited dispersal, and intraspecific dominance.     

Sexual selection and ecological generalism are correlated in antbirds

Sexual selection is thought to counteract natural selection on the grounds that secondary sexual traits are inherently costly and evolve at the expense of naturally selected traits. It is therefore commonly predicted that increased sexual selection is associated with decreased physiological tolerance or ecological plasticity. Using phylogenetic comparative methods, we test this prediction by exploring relationships between traits assumed to be sexually selected (plumage dichromatism and song structure) and traits assumed to be naturally selected (altitudinal range and habitat range) in a diverse family of tropical birds. Contrary to expectations, we find that taxa with higher levels of dichromatism, and lower song pitch, occupy a wider variety of habitats and elevations. In other words, indices of sexual selection are positively related to two standard measures of ecological generalism. One interpretation of this pattern is that sexual selection combines synergistically with natural selection, thereby increasing physiological tolerance or the propensity to adapt to novel environments. An alternative possibility is that ecological generalism increases population density, which in turn promotes sexual selection in the form of greater competition for mates. Overall, our results suggest that a synergism between natural selection and sexual selection may be widespread, but the processes underlying this pattern remain to be investigated.     

The ‘generalism’ debate: misinterpreting the term in the empirical literature focusing on dietary breadth in insects

We here attempt to show, using three broad insect groups – Lepidoptera (mainly leaf‐chewing larval herbivores and nectar‐sucking adults), parasitic Hymenoptera (mainly endoparasitoids, especially of other insects) and aphids (sap‐sucking plant parasites) – how the terms ‘generalist’, usually equated with levels of phagy (oligo‐ and polyphagy), and ‘specialist’ (monophagy), still in widespread parlance, have often been misrepresented. Probably, the reality of generalism, be that herbivorous, predatory and parasitic, can only be demonstrated by detailed empirical field observations plus the use of high‐resolution molecular (DNA) markers, including sequencing, and thereby determining whether the organism in question is really a homogeneous species population over a wide geographical range, or rather comprises a series of morphologically similar/identical cryptic, host‐adapted specialist populations. In the case of insects, the largest group of terrestrial animals on the planet, even if it can be shown that certain species are indeed polyphagous and feed on a variety of plant hosts (herbivores) or prey species (predators and parasitoids), nevertheless, the range of these food items may be highly selective and restricted, depending on morphological–genetical (biochemical/chemical)–behavioural constraints. In the end, while some animals appear to be generalist, this situation may well be illusory. Our present recognition of the term is at best inappropriate, and at worse, inaccurate, as we demonstrate in the examples given, mostly insects. In the meantime, we suggest that the terms used should be re‐defined as four broad classes of specialism–generalism, although the apparent ‘generalism’ is itself conditional on proof following further empirical analyses.     

Assessing disturbance-sensitivity and generalism in mammals: Corroborating a hump-shaped relationship using a hemerobiotic approach

Hemeroby is an indicator widely used in plant ecology to evaluate the level of disturbance of the optimal habitat of a species. Hemeroby can be scaled on a range of ten point scores, higher scores of hemeroby meaning higher level of disturbance. In this study we applied two indicators of sensitivity to disturbance based on the concept of hemeroby: HS_i (mean hemeroby score), calculated on the habitat types where each species occurs, and the recently proposed HH_i (mean hemerobiotic entropy), i.e. the level of generalism of a species with respect to the range of levels of disturbance where the species occurs. Both indices are based on the position and range of species along a gradient of disturbance, from pristine to completely human-made habitats. From a recent regional atlas of mammals, it was possible to calculate the ecological preferences for a large number of habitat types for 36 common mammal species for Latium (Central Italy). From the occurrences of mammals in each habitat, we calculated the HS_i (here rescaled: HS_rescaled) and HH_i indices. The relationship between habitat-related disturbance (HS_rescaled) vs. generalism (HH_i) of species showed a hump-shaped relationship peaking at intermediate levels of HS_rescaled, suggesting that generalism is maximum at intermediate levels of disturbance and corroborating analogous results obtained for birds. Alien mammal species exhibited higher averaged values of HS_i when compared to autochthonous species, supporting the evidence regarding the close relationship between alien species and more disturbed ecosystems. The application of the two indices to mammals could open new perspectives in conservation and management of species inhabiting a wide range of differently disturbed habitat types.     

The early life of a leaf‐cutter ant colony constrains symbiont vertical transmission and favors horizontal transmission

Colonial organisms host a large diversity of symbionts (collectively, parasites, mutualists, and commensals) that use vertical transmission (from parent colony to offspring colony) and/or horizontal transmission to disperse between host colonies. The early life of some colonies, characterized by the dispersal and establishment of solitary individuals, may constrain vertical transmission and favor horizontal transmission between large established colonies. We explore this possibility with the miniature cockroach Attaphila fungicola, a symbiont of leaf‐cutter ants and the mutualist fungal gardens they cultivate. The early life of a leaf‐cutter colony is characterized by the dispersal of a female alate (winged “queen”) carrying a fungal pellet, and the subsequent establishment of a foundress (workerless “queen”) raising her incipient fungal garden and colony. Roaches hitchhike on female alates during leaf‐cutter nuptial flights, which strongly suggests that roaches are vertically transmitted to foundresses and their incipient colonies; however, weak compatibility between roaches and incipient gardens may constrain roach vertical transmission. Reciprocally, opportunities for horizontal transmission between large established colonies with abundant fungal gardens may weaken selection against roach‐induced harm (virulence) of incipient gardens. We use a laboratory experiment, behavioral observations, field surveys, and a transmission model to estimate the effect roaches have on the survivorship of incipient gardens and the frequency of roach vertical transmission. Contrary to traditional assumptions, our results indicate that roaches harm incipient gardens and predominantly use horizontal transmission between established leaf‐cutter colonies. Ultimately, “costs of generalism” associated with infecting disparate stages of a host''s lifecycle (e.g., incipient vs. established colonies) may constrain the vertical transmission of roaches and a broad range of symbionts.     

The commonness of rarity in a deep-sea taxon

The generality and drivers of rarity, defined along the axes of geographic range, population size and habitat specificity, have received considerable scientific attention for well over a century. Yet, studies that examine rarity holistically among these three attributes are limited, especially among invertebrate and marine taxa. The perceived paradox of deep-sea species, with often low population size but large geographic ranges, remains poorly resolved and understood. Here I assess seven forms of rarity and their drivers in deep-sea bivalves across the Atlantic Ocean. Rarity appears to be a common trait among deep-sea bivalves, with nearly 85% of the species exhibiting some form of rarity. Bivalves also showed a strong bimodal pattern of very common and very rare species. Geographic range, population size and habitat specificity were all heavily right skewed. Taxonomic superfamilies, body size, energy availability, temperature, depth and latitude, all significantly predicted geographic range, population size and habitat specificity. In a few cases, these patterns were counter to theoretical expectations. The drivers of rarity appear to be predictable from knowledge of the intrinsic biological and extrinsic environmental context of the species. These findings have major implications for deep-sea conversation, especially as anthropogenic threats are increasing.