Classification of prokaryotic genetic replicators: between selfishness and altruism

Abstract

Prokaryotes harbor a variety of genetic replicators, including plasmids, viruses, and chromosomes, each having different effects on the phenotype of the hosting cell. Here, we propose a classification for replicators of bacteria and archaea on the basis of their horizontal-transfer potential and the type of relationships (mutualistic, symbiotic, commensal, or parasitic) that they have with the host cell vehicle. Horizontal movement of replicators can be either active or passive, reflecting whether or not the replicator encodes the means to mediate its own transfer from one cell to another. Some replicators also have an infectious extracellular state, thus separating viruses from other mobile elements. From the perspective of the cell vehicle, the different types of replicators form a continuum from genuinely mutualistic to completely parasitic replicators. This classification provides a general framework for dissecting prokaryotic systems into evolutionarily meaningful components.

Keywords: archaea; bacteria; cell vehicles; classification; prokaryotes; replicators.

Figure 1

Classes of replicators differ by their intercellular mobility and by the type of relationship with the cell vehicle. The scale is arbitrary.

Figure 2

Transitions between different classes of replicators. Solid arrows denote interconversion of replicators from different classes, and broken arrows denote integration of replicators from different classes such that the recipient class mediates the transmission of the donor class.