Sensory Adaptation Mechanisms in Swarm Development

Abstract

INTRODUCTION
Microorganisms that dramatically alter the expression of their genomes may be used as models to study genetic regulatory mechanisms (several examples are discussed in this volume). One aspect of development is the program of genetic switching events – cells differentiating in accord with their changing patterns of gene expression. To understand this process it is necessary to understand how transcription and translation of genes are turned on and off. A second aspect of development is the dynamic interaction between the cell and its environment. Two cells with the same internal levels of expressed genes can react differently as they experience different external conditions. In development, therefore, a specific progression of changing extracellular conditions can cause a series of intracellular modifications in metabolite levels, in activities of proteins, and in the expression of genes. In this paper we are concerned with the responses of a simple cell to external environmental changes and the way in which optimal responses provide high selection pressure for genetic change.

A bacterium seeded in nutritive medium grows, multiplies, consumes nutrients, and produces wastes. These activities generate gradients at the boundary of the developing colony, cells move in response to these gradients, and the colony spreads outward. The leading edge travels at a fixed rate, and as long as an individual remains at the front its surroundings are rich and constant. Cells that fall back must adapt to a medium preconditioned by those who have gone before; the unused nutrients and by-products of individuals at the front...