Insights into the evolutionary process of genome degradation

Studies of noncoding and pseudogene sequence diversity, particularly in Ric kettsia, have begun to reveal the basic principles of genome degradation in microorganisms. Increasingly, studies of genes and genomes suggest that th ere has been an extensive amount of horizontal gene transfer among microorg anisms. As this inflow of genetic material does not seem generally to have resulted in genome size expansions, however, degenerative processes must be at the very least as widespread as horizontal gene transfer. The basic pri nciples of gene degradation and elimination that are being explored in Rick ettsia are likely to be of major importance for our understanding of how mi crobial genomes evolve.