INTRODUCTION

The extravagant diversity of microbes has only been fully appreciated with the development of comparative genomics. Comparisons of gene repertoires among prokaryotes have revealed striking differences among species and even among strains of the same species. For example, the genomes of three Escherichia coli strains have been shown to share only 40% of their genes, with most of the remaining genes being strain-specific (Welch et al., 2002). More generally, although most prokaryotic genomes contain thousands of genes, only a handful can be identified as truly ubiquitous in modern organisms. This so-called ‘core’ of universal genes has received much interest from evolutionary biologists because it probably represents a relic of the last universal common ancestor (LUCA) and provides valuable information for reconstructing the tree of life. It has also been viewed as the sine qua non condition of life, since no living organism seems able to survive without it. However, perhaps more interesting is the paucity of these ubiquitous genes, as it shows the formidable evolutionary plasticity of biological systems and points to the mechanisms necessary for acquiring and generating new genes.

WHAT'S IN A GENOME?

An inventory

All cellular organisms have in common the use of DNA as the support of genetic information, RNA as an intermediate of protein expression and the same genetic code (with only a few exceptions) as well as catabolism and metabolism based on a limited number of amino acids and sugars.