Chapter 5 : Multiple Chromosomes

Abstract:

Some species of bacteria have more than one chromosome. Scientific dogma can be challenged at any time by the acquisition of new data or the reinterpretation of old data, but dogma is not easy to change. The word "plasmid" has drifted considerably from its original meaning. Chromosomes and plasmids can be circular or linear and both vary widely in size. With hindsight and the acquisition of new data, initial definitions and hypotheses often appear naive. Plasmids encode nonessential functions and can be considered dispensable. Chromosomes encode essential functions. The functions encoded by a replicon may be the most useful method in defining it as a plasmid or a chromosome. Multiple chromosome-like replicons (MCLRs) are widespread among bacteria. Genome architecture varies widely even between closely related bacterial species and may influence lifestyle. It has been hypothesized that genome architecture may reflect lifestyle; however, there is still no clear correlation between genome architecture and lifestyle. A bacterium can acquire a second chromosome by chromosome disruption, gene insertions into preexisting plasmids, and chromosome capture from another bacterium. To date, evidence suggests that multiple chromosomes have arisen during the course of evolution by two of the three possible mechanisms. Multiple chromosomes may increase sequence diversity by facilitating intrachromosomal recombination and/or horizontal gene transfer. Gene duplication without deletion may occur between multiple chromosomes. Multiple chromosomes may be a way of varying gene dosage during replication.