EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION II. A TEST OF THE DENSITY-DEPENDENT SELECTION HYPOTHESIS

This study tests the hypothesis that one evolutionary advantage of sexual reproduction is that it produces genetically variable progeny with a density-dependent advantage mediated by resource partitioning or pest pressure. Our experimental approach involved planting separate plots of sexually-derived and asexually-derived tillers of the grass Anthoxanthum odoratum in density gradients at the two natural sites from which the source material was taken. The sexual progeny displayed a significant fitness advantage compared to the asexual progeny. But, in contrast to the expectations of the density-dependent selection hypothesis, the advantage of the sexually produced progeny is most marked at lower densities. Thus, the results of this experiment and our previous report (Antonovics and Ellstrand, 1984) seem to best support the frequency-dependent selection hypothesis for the advantage of sexual reproduction.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. III. MATERNAL AND PATERNAL EFFECTS DURING SEEDLING ESTABLISHMENT

To determine whether genetic differences in fitness components exist among seeds and seedlings in a natural population, weighed propagules of six parents of Anthoxanthum odoratum from a reciprocal diallel cross were planted into the parental source population, a mown field. Seed families of maternal genotypes differed in germination success, while paternal families showed no detectable differences. Differential germination success could not be attributed to propagule weight. Seed families ranked differently in germination percentage in different blocks. No survivorship differences among parental seed families could be detected. There were significant cross × block × germination and cross × block × survivorship interactions; different crosses performed better or worse in different blocks. In some cases, crosses sired by different fathers within a maternal seed family differed in germination or survivorship, suggesting that natural selection may be capable of discriminating among juvenile genotypes within a maternal family despite the presence of large overall maternal effects. These results indicate that seedling establishment may differ according to genotype and that microsite heterogeneity may maintain genetic variation in juvenile traits in natural plant populations.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. IV. EFFECT OF NEIGHBOR RELATEDNESS AND APHID INFESTATION ON SEEDLING PERFORMANCE

To investigate the effect of neighbor relatedness in seedling populations, propagules of six Anthoxanthum odoratum parents, produced from a reciprocal diallel cross, were planted into the parental source population, a mown field. The propagules were either surrounded by four unrelated neighbors in a 1 cm square, four sibling neighbors, or no neighbors. About 45% of the emerging seedlings were attacked by aphids (Schizaphis graminum). Aphid infestation significantly reduced seedling survivorship and did not differ with parental genotype or neighbor relatedness; plants without neighbors had a slightly higher infestation risk. Plants without neighbors had lower survivorship than those surrounded by unrelated neighbors although this difference was only significant for plants infested by aphids. When infested by aphids, plants surrounded by siblings had lower survivorship than plants surrounded by non-relatives, suggesting the operation of minority advantage. These results are consistent with the frequency-dependent selection hypothesis for the evolution and maintenance of sexual reproduction.

What is a vector?

Many important and rapidly emerging pathogens of humans, livestock and wildlife are 'vector-borne'. However, the term 'vector' has been applied to diverse agents in a broad range of epidemiological systems. In this perspective, we briefly review some common definitions, identify the strengths and weaknesses of each and consider the functional differences between vectors and other hosts from a range of ecological, evolutionary and public health perspectives. We then consider how the use of designations can afford insights into our understanding of epidemiological and evolutionary processes that are not otherwise apparent. We conclude that from a medical and veterinary perspective, a combination of the 'haematophagous arthropod' and 'mobility' definitions is most useful because it offers important insights into contact structure and control and emphasizes the opportunities for pathogen shifts among taxonomically similar species with similar feeding modes and internal environments. From a population dynamics and evolutionary perspective, we suggest that a combination of the 'micropredator' and 'sequential' definition is most appropriate because it captures the key aspects of transmission biology and fitness consequences for the pathogen and vector itself. However, we explicitly recognize that the value of a definition always depends on the research question under study.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Co-occurrence and hybridization of anther-smut pathogens specialized on Dianthus hosts

Host specialization has important consequences for the diversification and ecological interactions of obligate pathogens. The anther-smut disease of natural plant populations, caused by Microbotryum fungi, has been characterized by specialized host-pathogen interactions, which contribute in part to the isolation among these numerous fungal species. This study investigated the molecular variation of Microbotryum pathogens within the geographic and host-specific distributions on wild Dianthus species in southern European Alps. In contrast to prior studies on this pathogen genus, a range of overlapping host specificities was observed for four delineated Microbotryum lineages on Dianthus hosts, and their frequent co-occurrence within single-host populations was quantified at local and regional scales. In addition to potential consequences for direct pathogen competition, the sympatry of Microbotryum lineages led to hybridization between them in many populations, and these admixed genotypes suffered significant meiotic sterility. Therefore, this investigation of the anther-smut fungi reveals how variation in the degrees of host specificity can have major implications for ecological interactions and genetic integrity of differentiated pathogen lineages.

A translation of the Linnaean dissertation The Invisible World

This study presents the first translation from Latin to English of the Linnaean dissertation Mundus invisibilis or The Invisible World, submitted by Johannes Roos in 1769. The dissertation highlights Linnaeus's conviction that infectious diseases could be transmitted by living organisms, too small to be seen. Biographies of Linnaeus often fail to mention that Linnaeus was correct in ascribing the cause of diseases such as measles, smallpox and syphilis to living organisms. The dissertation itself reviews the work of many microscopists, especially on zoophytes and insects, marvelling at the many unexpected discoveries. It then discusses and quotes at length the observations of Münchhausen suggesting that spores from fungi causing plant diseases germinate to produce animalcules, an observation that Linnaeus claimed to have confirmed. The dissertation then draws parallels between these findings and the contagiousness of many human diseases, and urges further studies of this 'invisible world' since, as Roos avers, microscopic organisms may cause more destruction than occurs in all wars.

Ecological understanding of root-infecting fungi using trait-based approaches

Classification schemes have been popular to tame the diversity of root-infecting fungi. However, the usefulness of these schemes is limited to descriptive purposes. We propose that a shift to a multidimensional trait-based approach to disentangle the saprotrophic-symbiotic continuum will provide a better framework to understand fungal evolutionary ecology. Trait information reflecting the separation of root-infecting fungi from free-living soil relatives will help to understand the evolutionary process of symbiosis, the role that species interactions play in maintaining their large diversity in soil and in planta, and their contributions at the ecosystem level. Methodological advances in several areas such as microscopy, plant immunology, and metatranscriptomics represent emerging opportunities to populate trait databases.

Anthelmintic treatment alters the parasite community in a wild mouse host

Individuals are often co-infected with several parasite species, yet the consequences of drug treatment on the dynamics of parasite communities in wild populations have rarely been measured. Here, we experimentally reduced nematode infection in a wild mouse population and measured the effects on other non-target parasites. A single oral dose of the anthelmintic, ivermectin, significantly reduced nematode infection, but resulted in a reciprocal increase in other gastrointestinal parasites, specifically coccidial protozoans and cestodes. These results highlight the possibility that drug therapy may have unintended consequences for non-target parasites and that host–parasite dynamics cannot always be fully understood in the framework of single host–parasite interactions.

Variation in resistance to multiple pathogen species: anther smuts of Silene uniflora

The occurrence of multiple pathogen species on a shared host species is unexpected when they exploit the same micro-niche within the host individual. One explanation for such observations is the presence of pathogen-specific resistances segregating within the host population into sites that are differentially occupied by the competing pathogens. This study used experimental inoculations to test whether specific resistances may contribute to the maintenance of two species of anther-smut fungi, Microbotryum silenes-inflatae and Microbotryum lagerheimii, in natural populations of Silene uniflora in England and Wales. Overall, resistance to the two pathogens was strongly positively correlated among host populations and to a lesser degree among host families within populations. A few instances of specific resistance were also observed and confirmed by replicated inoculations. The results suggest that selection for resistance to one pathogen may protect the host from the emergence via host shifts of related pathogen species, and conversely that co-occurrence of two species of pathogens may be dependent on the presence of host genotypes susceptible to both.

Two-step infection processes can lead to coevolution between functionally independent infection and resistance pathways

There is growing evidence that successful infection of hosts by pathogens requires a series of independent steps. However, how multistep infection processes affect host-pathogen coevolution is unclear. We present a coevolutionary model, inspired by empirical observations from a range of host-pathogen systems, where the infection process consists of the following two steps: the first is for the pathogen to recognize and locate a suitable host, and the second is to exploit the host while evading immunity. Importantly, these two steps conform to different models of infection genetics: inverse-gene-for-gene (IGFG) and gene-for-gene (GFG), respectively. We show that coevolution under this scenario can lead to coupled gene frequency changes across these two systems. In particular, selection often favors pathogens that are infective at the first, IGFG, step and hosts that are resistant at the second, GFG, step. Hence, there may be signals of positive selection between functionally independent systems whenever there are multistep processes determining resistance and infectivity. Such multistep infection processes are a fundamental, but overlooked feature of many host-pathogen interactions, and have important consequences for our understanding of host-pathogen coevolution.

Evolutionary determinants of genetic variation in susceptibility to infectious diseases in humans

Although genetic variation among humans in their susceptibility to infectious diseases has long been appreciated, little focus has been devoted to identifying patterns in levels of variation in susceptibility to different diseases. Levels of genetic variation in susceptibility associated with 40 human infectious diseases were assessed by a survey of studies on both pedigree-based quantitative variation, as well as studies on different classes of marker alleles. These estimates were correlated with pathogen traits, epidemiological characteristics, and effectiveness of the human immune response. The strongest predictors of levels of genetic variation in susceptibility were disease characteristics negatively associated with immune effectiveness. High levels of genetic variation were associated with diseases with long infectious periods and for which vaccine development attempts have been unsuccessful. These findings are consistent with predictions based on theoretical models incorporating fitness costs associated with the different types of resistance mechanisms. An appreciation of these observed patterns will be a valuable tool in directing future research given that genetic variation in disease susceptibility has large implications for vaccine development and epidemiology.

Biology and evolution of sexual transmission

Sexual reproduction brings together and recombines different genomes. Associated with these contacts is transmission of microorganisms and selfish genetic elements, many of which can be harmful to the host. In organisms with internal fertilization, sexually transmitted infections are caused by pathogens transmitted between the parents participating in mating. Sexual transmission has different epidemiological dynamics from nonsexual transmission in that it is less likely to be dependent on host density, there may be no population density threshold for disease increase, and it is more likely to lead to host extinction. Analysis of the evolutionary pathways that have led to the sexual mode of transmission in pathogens indicates that sexual transmission appears more often to be derived from nonsexual transmission, although the pathways are highly variable, and several groups of pathogens are exceptions to this rule. Sexual transmission has evolved from a wide variety of alternative transmission modes, although rarely from aerially transmitted diseases. More data are needed on the phylogeny and transmission mode of the relatives of sexually transmitted pathogens in order to guide development of animal models and comparative studies.