Existing and emerging pathogens pose unusual challenges for conservation because of their potential to drive rapid changes in the numerical abundance and genetic composition of wild host populations. An increasing number of studies indicate that host genetic diversity plays an important role in buffering populations against widespread epidemics, and that parasites represent powerful selective agents in natural populations. The observation that infectious diseases might be both mitigated by and rapidly change the genetic composition of host populations gives new significance to the role of host genetic diversity in species conservation. Less clear is the role that pathogen evolutionary change plays in the emergence and spread of new diseases, but recent examples indicate that humans might be selecting unknowingly for rapid changes in pathogen biology through habitat fragmentation, climate shifts and environmental pollution. Although the risks they pose to endangered species are apparent, pathogens and other natural enemies can be a driving force behind species and genetic diversity in natural populations, and preserving interacting networks of coevolving populations should enable hosts to respond better to future disease threats.

Rapid evolutionary dynamics and disease threats to biodiversity

Here we address the controversy between plant pathologists and invertebrate zoologists as to the genetic basis of infection. We show that the alternative models proposed by these groups represent two ends of a continuum. Specifically, the gene-for-gene (GFG) model of plant pathologists represents one end of a continuum, where a very broad host range is expected to occur in one pathogen genotype. The matching alleles (MA) model, favoured by invertebrate zoologists, represents the opposite end of the same continuum, where an exact genetic match is required for infection. Since it is known that matching-alleles models give highly dynamical behaviour and selection for recombination if parasites exert strongly negative effects on host fitness, we were especially interested in determining the nature of any such behaviour in the interior regions of the MA–GFG continuum. We found that the highly dynamical aspects of matching-alleles models were observed across most of the continuum, and that recombination would increase geometric mean fitness over about half of the continuum.

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Infection genetics: gene-for-gene versus matching-alleles models and all points in between

A major assumption of models of the evolution of plant resistance to disease is that plant resistance involves fitness costs. To test this assumption, a field experiment was performed so that a quantitative-genetic analysis could be used to detect fitness costs to Ipomoea purpurea of resistance to different fungal isolates of Colletotrichum dematium, a pathogenic fungus causing the disease anthracnose. This experiment yielded no evidence that resistance to anthracnose involves direct fitness costs. Nevertheless, trade-offs in plant fitness that were unrelated to resistance were detected between different disease environments. Tolerance, defined as the ability to compensate in part for fitness decrements caused by disease, was found to involve fitness costs. Halfsib families that were more tolerant of disease had lower fitness in the absence of disease. The possibility that the cost of tolerance could obscure fitness costs of resistance is explored.

Costs and benefits of plant responses to disease: resistance and tolerance

Sexually transmitted diseases (STDs) have been generally thought of as a small subset of infectious diseases, rather than as an important group of diseases that occur in numerous species. In this paper, we have (1) briefly reviewed theoretical studies on the dynamics of STDs; (2) documented the distribution of STDs in the animal kingdom; and (3) investigated whether STDs have characteristics which distinguish them from other infectious diseases. The dynamics of STDs should differ from those of ordinary infectious diseases because their transmission depends on the frequency rather than density of infectives. With this type of transmission, there is no threshold density for disease spread, and the conditions for host-pathogen coexistence are more restrictive. Nevertheless, a wide variety of disease characteristics may allow a sexually transmitted pathogen to coexist with its host. We found over 200 diseases for which there was evidence of sexual transmission. They occurred in groups as diverse as mammals, reptiles, arachnids, insects, molluscs and nematodes. Sexually transmitted pathogens included protozoans, fungi, nematodes, helminths, and cancerous cell lines, as well as bacteria and viruses. Detailed comparison of the characteristics of sexually transmitted mammalian diseases with those that are transmitted by non-sexual means, showed that STDs cause less mortality, are longer-lived in their hosts, are less likely to invoke strong immune responses, have narrower host-ranges, and show less fluctuation in prevalence over time. These shared features are related to mode of transmission rather than either host or pathogen taxonomic affiliation. This suggests an evolutionary explanation based on shared ecologies rather than one based on phylogenetic history.

Sexually transmitted diseases in animals: ecological and evolutionary implications.

Plant diversity sustains all animal life, and the genetic diversity within plants underpins global food security. This text provides a practical and theoretical introduction to the strategies and actions to adopt for conserving plant genetic variation, as well as explaining how humans can exploit this diversity for sustainable development. Notably readable, it initially offers current knowledge on the characterization and evaluation of plant genetic resources. The authors then discuss strategies from in situ and ex situ conservation to crop breeding, exploring how these can be used to improve food security in the face of increasing agrobiodiversity loss, human population growth and climate change. Each chapter draws on examples from the literature or the authors' research and includes further reading references. Containing other useful features such as a glossary, it is invaluable for professionals and undergraduate and graduate students in plant sciences, ecology, conservation, genetics and natural resource management.

M. A. Parker

Papers

Disease and plant population genetic structure.