BSPP Presidential Meeting 1999

Biotic Interactions in Plant-pathogen Associations

Session I - Within-taxon population interactions

Population consequences of vegetative (in)compatibility in fungi.
Rolf F. Hoekstra, Fons Debets and Maarten Nauta

Wageningen University, The Netherlands E-Mail:

Work from our laboratory on three topics related to vegetative incompatibility in fungi will be discussed:

(1) Causes of the high levels of polymorphism for vegetative incompatibility found in many species. Theoretical population genetic models assuming homogeneous populations indicate that selection against transmission of a harmful cytoplasmic element is a more plausible explanation than is selection against a nuclear gene to explain the extent of incompatibility observed. These models predict only moderate levels of polymorphism. However, even genetic drift alone may produce large numbers of incompatibility types under certain conditions.

(2) Transmission rates of cytoplasmic genetic material in incompatible confrontations. Both in Podospora anserina and in Neurospora crassa low frequency transfer of mitochondrial plasmids between vegetatively incompatible partners can be demonstrated. The potential consequences of low frequency horizontal transfer of plasmids associated with the fungal senescence phenotype will be discussed.

(3) Interactions between vegetative incompatibility and the sexual cycle. Several observations point to an involvement of vegetative incompatibility in the sexual cycle. First, in species like Neurospora crassa vegetative incompatibility is associated with the mating type idiomorphs. We have shown that in the absence of mating type vegetative incompatibility transmission of plasmids from the conidial paternal parent was ten times higher than in normal crosses. We have evidence that conidia that are vegetatively compatible with the maternal parent may enter the maternal tissue by somatic fusion, thus obtaining access to the maternal resources and initiating new fruiting bodies.

Why study the population genetics of plant associated bacteria?
Paul Rainey

Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.

Population genetics is the study of evolutionary change. It is concerned with natural genetic diversity, its causes, its distribution and its biological significance. More specifically, population genetics is concerned with the basic forces of evolution, that is, mutation, recombination, migration, natural selection and genetic drift, and the contribution that these forces make to the nature and rate of evolutionary change. Knowledge of the effects of these forces on populations can provide information on likely future directions of evolutionary change and, conversely, patterns of genetic variability can field information about the processes that generated. The link between the effects of specific evolutionary forces and patterns of variation is provided by the theory of population genetics, which gives evolutionary biology its most significant and far-reaching body of theory.

The study of prokaryote populations represents a significant intellectual goal in itself, but in addition, it has real practical significance. The rapid generation times and large population sizes of bacteria suggest that evolutionary change will be swift. This is clearly evident in the rapid spread of antibiotic resistance genes in infectious bacteria and in the sudden outbreak of disease in plants. An understanding of the genetic structure of microbial populations provides a framework within which epidemic outbreaks of pathogens can be monitored and traced. It also enables the spread of drug and pesticide resistance to be managed, the efficacy and safety of genetically engineered micro-organisms intended for environmental applications to be assessed and biological control strategies to be rationally designed. In addition, population genetics has a positive contribution to make to the debate on the "species concept" in prokaryotes and provides a guide to the practical problems of bacterial classification.

The value of population genetics as a means of understanding changes in patterns of virulence and drug resistance is widely recognised by those concerned with infectious human pathogens. It is also recognised by a small number of plant pathologists, most notably those concerned with pathogenic fungi. However, with the exception of a small number of studies, the role and value of population genetics in the study of natural bacterial populations in the environment (both saprophytes and pathogens) is often neglected. In this talk I will discuss basic concepts in bacterial population genetics using examples from our own studies on the genetic and ecotypic structure of Pseudomonas populations.