Horizontal gene transfer among genomes: The complexity hypothesis

Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes. Extensive horizontal transfer has occurred for operational genes (those involved in housekeepin g), whereas informational genes (those involved in transcription, translati on, and related processes) are seldomly horizontally transferred. Through p hylogenetic analysis of six complete prokaryotic genomes and the identifica tion of 312 sets of orthologous genes present in all six genomes, we tested two theories describing the temporal flow of horizontal transfer, We show that operational genes have been horizontally transferred continuously sinc e the divergence of the prokaryotes, rather than having been exchanged in o ne, or a few, massive events that occurred early in the evolution of prokar yotes. In agreement with earlier studies, we found that differences in rate s of evolution between operational and informational genes are minimal, sug gesting that factors other than rate of evolution are responsible for the o bserved differences in horizontal transfer. We propose that a major factor in the more frequent horizontal transfer of operational genes is that infor mational genes are typically members of large, complex systems, whereas ope rational genes are not, thereby making horizontal transfer of informational gene products less probable (the complexity hypothesis).