The Scaling of Host Density with Richness Affects the Direction,Shape, and Detectability of Diversity-Disease Relationships

Pathogen transmission responds differently to host richness and abundance, two unique components of host diversity. However, the heated debate around whether biodiversity generally increases or decreases disease has not considered the relationships between host richness and abundance that may exist in natural systems. Here we use a multi-species model to study how the scaling of total host community abundance with species richness mediates diversity-disease relationships. For pathogens with density-dependent transmission, non-monotonic trends emerge between pathogen transmission and host richness when host community abundance saturates with richness. Further, host species identity drives high variability in pathogen transmission in depauperate communities, but this effect diminishes as host richness accumulates. Using simulation we show that high variability in low richness communities and the non-monotonic relationship observed with host community saturation may reduce the detectability of trends in empirical data. Our study emphasizes that understanding the patterns and predictability of host community composition and pathogen transmission mode will be crucial for predicting where and when specific diversity-disease relationships should occur in natural systems.     

Ecology of rodent-associated hantaviruses in the Southern Cone of South America: Argentina, Chile, Paraguay, and Uruguay

Thirteen hantavirus genotypes, associated with at least 12 sigmodontine reservoir rodents, have been recognized in the four countries that represent the Southern Cone of South America. Host-virus relationships are not as well defined as in North America; several Southern Cone hantaviruses appear to share a common host and some viruses do not occur throughout the range of their host. Although hantavirus-host relationships in the Southern Cone are less strictly concordant with the single-host-single-virus pattern reported elsewhere, recent studies suggest that much of the ambiguity may result from an incomplete understanding of host and hantavirus systematics. Although some Southern Cone host species are habitat generalists, some sympatric species are habitat specialists, helping to explain how some strict host-virus pairings may be maintained. In some cases, host population densities were higher in peridomestic habitats and prevalence of hantavirus infection was higher in host populations in peridomestic habitats. Seasonal and multiyear patterns in climate and human disturbance affect host population densities, prevalence of infection, and disease risk to humans. Unusually high hantavirus antibody prevalence in indigenous human populations may be associated with frequent and close contact with host rodents. Ongoing studies are improving our understanding of hantavirus-host ecology and providing tools that may predict human risk.     

The effects of host race, gender, and host plant distribution on alighting behavior, mating, and oviposition in Eurosta solidaginis

Eurosta solidaginis Fitch (Diptera: Tephritidae) has formed host races on Solidago altissima L. and Solidago gigantea Ait. (Asteraceae), and reproductive isolation between these host races is brought about in part by host‐associated assortative mating. Any non‐assortative mating creates the potential for gene flow between the populations, and we investigated the conditions that favored non‐assortative mating. We hypothesized that the frequency of non‐assortative mating would be influenced by differences in the behaviors of the host races and sexes and by the presence and pattern of distribution of the two host species. To test these hypotheses, we caged flies on four combinations of 32 potted host plants: all S. altissima, all S. gigantea, and cages with both host species arranged in either two pure species blocks or randomly dispersed. We recorded the number of flies of each host race that alighted on each host species and the frequency of mating within and between the host races. Males of both host races were observed on plants more frequently than females. Flies of the host race from S. gigantea (gig flies) were observed on plants in greater absolute numbers, and they mated more frequently than flies of the host race from S. altissima (alt flies). In all treatments, gig flies of both sexes were found on non‐natal host plants significantly more frequently than alt flies, and gig females showed a weaker preference for their host species than did gig males or alt flies of either gender for their respective natal hosts. Assortative mating predominated in all treatments, and flies from each host race mated more frequently in cages containing their own host plant. The frequency of non‐assortative mating varied among treatments, with the matings between alt ♀ × gig ♂ being more common in the pure S. altissima treatment and the gig ♀ × alt ♂ being more frequent in the pure S. gigantea and random treatments. Matings between gig ♂ × alt ♀ were more common overall than the reciprocal mating, because gig males were more active in pursuing matings and in alighting on the non‐natal host plant than alt flies. Non‐assortative matings were more frequent in the random than in the block treatments, but this difference was not significant. Because of strong selection against oviposition into the alternate host, we hypothesized that host plant distribution would not affect oviposition preference. We tested this hypothesis by examining the oviposition behavior of naïve, mated females in two treatments in which both host species were present: either arranged in blocks or randomly dispersed. Females oviposited only into their natal host, regardless of host plant distribution.     

The evolution of parasite virulence, superinfection, and host resistance

We analyze the evolutionary consequences of host resistance (the ability to decrease the probability of being infected by parasites) for the evolution of parasite virulence (the deleterious effect of a parasite on its host). When only single infections occur, host resistance does not affect the evolution of parasite virulence. However, when superinfections occur, resistance tends to decrease the evolutionarily stable (ES) level of parasite virulence. We first study a simple model in which the host does not coevolve with the parasite (i.e., the frequency of resistant hosts is independent of the parasite). We show that a higher proportion of resistant host decreases the ES level of parasite virulence. Higher levels of the efficiency of host resistance, however, do not always decrease the ES parasite virulence. The implications of these results for virulence management (evolutionary consequences of public health policies) are discussed. Second, we analyze the case where host resistance is allowed to coevolve with parasite virulence using the classical gene-for-gene (GFG) model of host-parasite interaction. It is shown that GFG coevolution leads to lower parasite virulence (in comparison with a fully susceptible host population). The model clarifies and relates the different components of the cost of parasitism: infectivity (ability to infect the host) and virulence (deleterious effect) in an evolutionary perspective.     

Relative abundance, rate of increase, and fungal infections of freshwater phytoplankton

Fungal infections of seven species of phytoplankton were examinedin relation to the host species' exponential rate of net increase,and to their proportional contribution to the total phytoplanktonbiovolume. Infections were observed to increase at biovolumeproportions of the host species of as low as 1%. In most algalspecies, infected cells were always found at higher proportions,with the exception of Stephanodiscus rotula and Fragilaria crotonensis.In these two species, high proportions of biovolume were reachedwithout any evidence of infected cells. The increase in infectedcells was usually associated with a growing host population,whereas peak and decreasing densities of infected cells wereusually observed when host populations were declining. The resultsshow that the fungal parasites can exist on their host populationeven if it comprises only a small fraction of the total phytoplanktonbiovolume, and that the parasites become evident while the hostpopulation is still increasing.     

Immunity,antigenic heterogeneity,and aggregation of helminth parasites

Empirical studies of helminth parasites reveal that the distribution of parasite burdens in their host populations is highly aggregated. This aggregation is fundamental to the ecology and epidemiology of helminth parasites. Results from a stochastic model predict that aggregation of helminth parasites is inversely related to the intensity of host immunity. Aggregation also decreases with antigenic heterogeneity and increases with heterogeneity in transmissibility among parasite strains. It is also found that the degree of aggregation is greater when immunity affects parasite fecundity than when immunity acts on host susceptibility. Potential relevance of this result for assessing the influence of vaccines that target either host susceptibility or parasite fecundity on the level of aggregation and consequent effects on drug resistance and disease prevalence are discussed.     

Rates of parasitism,but not allocation of egg resources,vary among and within hosts of a generalist avian brood parasite

Brown-headed cowbirds (Molothrus ater) deposit their eggs into the nests of other birds, which then raise the cowbird chick. Female cowbirds thus have limited options for impacting their offspring’s development via maternal effects compared to most other passerines. Cowbirds can impact their offspring’s phenotype by choosing among potential host nests, and by adjusting egg resources based on host characteristics. To examine whether cowbirds exhibit either or both of these strategies, we investigated rates of cowbird parasitism and egg investment (egg size, yolk-to-albumen ratio, and yolk testosterone and androstenedione) among and within host species in a shrubland bird community. We found that the probability of being parasitized by cowbirds, controlling for host status as a cowbird egg accepter or rejecter and ordinal date, varied significantly among host species, indicating an apparent preference for some hosts. Parasitism rates did not differ with host size, however, and despite variation in cowbird egg size among host species, this variation was not related to host size or cowbird preference. Among host species with eggs that are larger than those of the cowbird, cowbirds were significantly more likely to parasitize nests with relatively smaller eggs, whereas parasitism rates did not vary with relative egg size in host species with smaller eggs. There was no evidence for variation in cowbird egg components among or within host species. Our data indicate that cowbirds discriminate among host nests, but do not appear to adjust the composition of their eggs based on inter- or intraspecific host variation.     

Genetics, experience, and host-plant preference in Eurosta solidaginis: implications for host shifts and speciation

Host-associated mating is crucial in maintaining the partial reproductive isolation between the host races of Eurosta solidaginis (Diptera: Tephritidae), a fly that forms galls on Solidago altissima and S. gigantea. (We refer to flies reared from S. gigantea as gigantea flies and those reared from S. altissima as altissima flies.) We measured the host preference of males and females of both host races, F1 hybrids between the host races, F2, and backcrosses to both host races. Male and female altissima flies and female gigantea flies had high host fidelity, whereas male gigantea flies had low host fidelity. This result suggests that there may be gene flow between the host races due to nonassortative mating that occurs when male gigantea mate with altissima females on S. altissima. This indicates assortative-mating mechanisms in addition to host-associated mating are required to produce the partial reproductive isolation between the host races that has been observed. Nongenetic factors had no influence on host preference. Larval conditioning did not influence host preference: reciprocal F1 hybrids reared in S. altissima and S. gigantea both preferred S. gigantea. Adult experience had no impact on host preference: females preferred their natal host plant regardless of which host they encountered first as an adult. The hypothesis that maternal effects influence preferences was rejected because male and female flies did not show a consistent preference for the host plant of their mother. We also found no evidence that preference was a sex-linked trait because F1 and backcrosses to the host races with different combinations of X chromosomes from the two host races preferred S. gigantea. Our results indicate that host preference is not determined by a large number of genes because preference of hybrids did not correspond to the proportion of the genome derived from each host race. The strength of the ovipuncture preference for S. gigantea by gigantea females, the females of both reciprocal F1 hybrids, the backcross to gigantea, and F2s indicates that preference is inherited nonadditively at a limited number of loci. The F1 female hybrids, however, had a weaker host preference for S. gigantea than the pure gigantea host race, indicating that there may be incomplete dominance or modifier loci. Males had different host preference patterns than females, with individual male gigantea and male F1 hybrids usually exhibiting preference exclusively for S. gigantea or S. altissima. One hypothesis explaining the difference in host preference between males and females is that the same gene influences both female and male host preference, but it is a sex-influenced gene. Thus, males carrying the gene for S. gigantea preference have an intermediate host preference, whereas females have a strong host preference to S. gigantea. In summary, we found that the host preference that produces host-associated mating is inherited nonadditively at a relatively small number of loci on autosomal genes. This mode of inheritance meets the assumptions of models of sympatric speciation, indicating that the host races could have evolved in sympatry.     

New species, phylogeny, host-associations and geographic distribution of genus Cryptosporella (Gnomoniaceae, Diaporthales)

The phylogeny of Cryptosporella is revised to include recently discovered species. Eight species new to science are described and two new combinations are proposed, raising the total number of species accepted in Cryptosporella to 19. The species delimitation and phylogeny for Cryptosporella are determined based on analyses of DNA sequences from three genes (β-tubulin, ITS and tef1-α), comparative morphology of sexual structures on their host substrate, and host associations. The inferred phylogeny suggests that Cryptosporella has speciated primarily on Betulaceae with 16 species occurring on hosts in that plant family. The host range of most species seems to be narrow with nine species reported from a single host species or subspecies and seven species occurring on plants within a single host genus. A key to species is provided. The known distribution of Cryptosporella is expanded to mountain cloud forests of the provinces of Chiriquí in Panama and Tucumán in Argentina.     

Systematics, distribution, and host specificity of grass endophytes.

Clavicipitaceous endophytes (Ascomycetes) are distributed worldwide in many grasses and sedges forming a perennial and often mutualistic association with their hosts. Most endophytes appear to produce alkaloid toxins in infected plants. The high frequency of infection in many grasses and in certain grassland communities may indicate a selective advantage of infected over non-infected host plants due to their toxic effects on grazing animals and insects. Field observations and artificial inoculations of seedlings have demonstrated a high degree of specificity of most endophytes to their host plant, particularly in asexual, seed-borne endophytes. Specific isozyme genotypes found on several host species suggest that host-specific physiological races may occur. Knowledge of host range and host specificity is vital for potential applications of endophytes in pest control.     

Switch,disperse, repeat: host specificity is highly flexible in rodent-associated Eimeria

Interplay between conserved host specificity and occasional host switches is an important process determining the evolution of host-parasite systems. Here, we address the dynamics of host switches at the population level in rodent-associated Eimeria. Focusing mainly on two ecologically similar host groups, Murinae and Arvicolinae, we show that the Eimeria infecting those hosts form a complex system of many genetic lineages with different host specificities. The broad geographic distribution of lineages indicates that they are well-established genetic forms which retained their host specificities while spreading across large geographic areas. We also demonstrate that genetic structure is only partially reflected by morphological traits.     

Costs,benefits, and loss of vertically transmitted symbionts affect host population dynamics

The costs and benefits of symbiotic interactions may vary with host and symbiont ontogeny. Effects of symbionts at different stages of host development or on different host demographic rates do not contribute equally to fitness. Although rarely applied, a population dynamics approach that integrates over the host life cycle is therefore necessary for capturing the net costs or benefits and, thus, the mutualistic or parasitic nature of symbioses. Using the native, disturbance‐specialist grass Agrostis hyemalis, we asked how a symbiotic endophyte affected the population dynamics of its host and how imperfect vertical transmission influenced symbiont frequency in a late successional environment. A size‐structured integral projection model (IPM) parameterized with experimental field data showed that greater rates of individual growth and reproduction for endophyte‐symbiotic (E+) hosts outweighed their lower rates of survival, leading to a net positive effect of symbiosis on equilibrium plant population growth (slower rate of extinction). Given that populations under going successional transitions are unlikely to be at an equilibrium size structure, we also conducted transient analysis that showed an initial short‐term cost to endophyte symbiosis. We used a megamatrix approach to link E? and E+ IPMs via imperfect vertical transmission and found that this parameter strongly influenced the frequency of symbiosis via complex interactions with host demographic rates. Overall, our population dynamics approach improves the ability to characterize the outcome of symbiotic interactions, and results suggest that particular attention should be paid to interactions between the rate of vertical transmission and host demography.     

Conditioning of leaf-shape discrimination by chemical cues in the butterfly,Battus philenor

In a large outdoor screened enclosure, female pipevine swallowtail butterflies (Battus philenor) were trained to search selectively for leaves of a shape similar to that of the Aristolochia host species to which they were exposed as adults. Conditioning was reversed by exposure to an alternative host species with a different leaf shape. Results indicated that contact with a host plant without oviposition was sufficient to induce a change in alighting responses to leaves of different shapes. Enclosure assays employing non-host species possessing leaves of different shapes and treated with host extracts demonstrated that females associate the shape of their alighting substrate with host chemicals present on the surface of that substrate. Results suggested that contact with host extracts without oviposition can induce changes in alighting responses to leaves of different shapes. A hypothesis is presented in which the primary host species for which females in an east Texas population learn to search varies seasonally with changes in host leaf chemistry.     

Host abundance,durability, basidiome form and phylogenetic isolation determine fungivore species richness

Species with close associations to a specific host species, such as parasites and phytophages, make immense contributions to biodiversity. Hence, factors determining the variation in species richness among hosts are a main focus of ecological research. Investigations of determining factors of fungivorous species among host species are still scarce. Based on ecological patterns of parasites and phytophages, we hypothesized that the species richness of tree‐fungus beetles of the family Ciidae (Coleoptera) would increase with increasing basidiome size, niche diversity of the growth form, durability, increasing abundance and decreasing phylogenetic isolation of the host fungus. Our generalized least‐squares model, controlled by host phylogeny, revealed that Ciidae species richness increases with host abundance, but decreases with host phylogenetic isolation. In contrast with our prediction, Ciidae species richness was higher in annual basidiomes than in perennials. Pileate basidiomes revealed higher species richness than resupinate and stipitate basidiomes, which may be interpreted as being a result of their higher host niche diversity. The importance of host abundance, measured on the landscape scale, corroborates that fungivore species richness among macrofungal hosts is determined by factors similar to those that determine parasite and phytophage species richness among their hosts. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 699–708.     

Autoparasitism, interference, and parasitoid-pest population dynamics

Autoparasitoids ("heteronomous hyperparasitoids") are parasitoids that lay female eggs on homopteran hosts and male eggs on juvenile parasitoids of either the same species or another species. Males develop as hyperparasitoids and eventually kill the juvenile parasitoid. We present a series of stage-structured models that investigate the effects of autoparasitism on population dynamics. Autoparasitism causes density-dependent mortality on juvenile parasitoids and therefore has a stabilizing effect. This also leads to an increase in host population abundance. In most cases an autoparasitoid leads to higher host equilibrium densities than a comparable primary parasitoid (except when the primary parasitoid is arrhenotokous (sexual) and the autoparasitoid has a low preference for attacking parasitized hosts or can attack the parasitized host for only a small portion of its development). When male autoparasitoids are followed explicitly in the models, mate limitation reduces the stabilizing effect of autoparasitism and leads to a further increase in host abundance. Coexistence of an autoparasitoid with a nonprimary parasitoid or second autoparasitoid is possible when the level of conspecific autoparasitism is greater than the level of heterospecific autoparasitism. When an autoparasitoid coexists with a primary parasitoid, the resulting host density is always greater than that with only the primary parasitoid. Therefore, autoparasitoids have the potential to disrupt control achieved by primary parasitoids. When two autoparasitoids coexist, the resulting host density is always lower than that attained by either autoparasitoid alone. The effects of autoparasitism are compared with those of other forms of interference competition.     

Community structure of helminth parasites of the tuna, Euthynnus affinis, from the Visakhapatnam coast, Bay of Bengal

An analysis is made of the community structure of the helminth parasites of the tuna Euthynnus affinis collected off the coast of Visakhapatnam, Bay of Bengal. The helminth fauna comprised 23 species, consisting of two monogeneans, 18 digeneans, one larval cestode, one nematode and one acanthocephalan. Didymozoids, represented by 14 species, were the dominant members of the parasite spectrum. The parasite assemblage is characterized by species richness as well as a high diversity. Most of the parasites found, the didymozoids in particular, were host specialists, exhibiting a high degree of host specificity. The fauna comprised three core species, four satellite species and many secondary species, with no potential for interaction among them but the degree of predictability is high in that each infracommunity comprised two or three core species superimposed by a few secondary species. Many host factors such as the varied diet, high vagility, long life span and endothermy appear to have contributed to the development of the species rich and diverse parasite communities in the tuna. There was evidence for a decrease in parasite density and an increase in diversity with increase in host size, indicating that host size has a profound influence on the component community structure.     

Oomycetes, effectors, and all that jazz

Plant pathogenic oomycetes secrete a diverse repertoire of effector proteins that modulate host innate immunity and enable parasitic infection. Understanding how effectors evolve, translocate and traffic inside host cells, and perturb host processes are major themes in the study of oomycete-plant interactions. The last year has seen important progress in the study of oomycete effectors with, notably, the elucidation of the 3D structures of five RXLR effectors, and novel insights into how cytoplasmic effectors subvert host cells. In this review, we discuss these and other recent advances and highlight the most important open questions in oomycete effector biology.     

Timing of Spawning and Host-nest Choice for Brood Parasitism by the Japanese Minnow,Pungtungia herzi,on the Japanese Aucha Perch,Siniperca kawamebari

A field study of the breeding ecology of the Japanese aucha perch, Siniperca kawamebari, and brood parasitism by the Japanese minnow, Pungtungia herzi, on nests of the perch was carried out from 1989 to 1991. Observations of perch nests under natural conditions in 1990 showed that brood parasitism by the minnow was concentrated on host nests in which nest owners had just begun their nesting cycle. When spawned in a perch nest with recently spawned perch eggs, parasite eggs always hatched earlier than host eggs. An experiment with imitation perch eggs in 1991 confirmed that changing colour of host eggs was the cue for the parasites to distinguish between different developmental stages of host eggs. Parasite eggs rapidly disappeared without guarding by a host male (Baba et al. 1990). This loss was caused by predation by fishes. Parasite fry left the nest immediately after hatching, so parasite eggs spawned in a host nest in an early stage should be well guarded until they hatch. In the field, minnows deposited their eggs in perch nests which had larger numbers of newly spawned perch eggs. Since the perch males always deserted their nests when their own eggs disappeared, the parasite's choice of host nests with larger numbers of host eggs may ensure survival of the parasite eggs. The timing of egg deposition and choice of host nest by the minnow appear to be adaptive in terms of brood parasitism on nests of the perch.