Horizontal Gene Transfer in Halobacteria
Matthew S. Fullmer, J. Peter Gogarten and R. Thane Papke
from: Halophiles: Genetics and Genomes (Edited by: R. Thane Papke and Aharon Oren). Caister Academic Press, U.K. (2014)
The Halobacteria are a class of Archaea that have been fundamentally shaped by Horizontal Gene Transfer (HGT). The mechanisms for HGT are not well understood, or are unreported. A noteworthy exception exists for the genus Haloferax, where a novel mating system exists that includes the fusion of cytoplasm between two cells. Despite shallow insight into mechanisms evidence from phylogenetics and population genetics studies demonstrate that these organisms have been able to exchange genes since their distant origins and continue to actively do so today. Single gene studies have uncovered transfer of halobacterial rhodopsins into diverse lineages such as the fungi and multiple bacterial taxa, construction of novel biosynthetic pathways, homologous recombination of parts or whole ribosomal proteins and RNAs, as well as divergent tRNA synthetases being exchanged between distant lineages. Furthermore, the very origin of the Halobacteria appears to have resulted from an influx of genes from the bacterial domain, which reshaped the fundamental metabolism from an anaerobic chemoautolithotrophic methanogen into a facultative aerobic heterotroph. Population genetics analysis demonstrated that gene flow with phylogenetically defined populations is so frequent that allele distributions resemble that of sexually reproducing eukaryotes, and acts as both a homogenizing and diversifying evolutionary force. Given all of the evidence for abundant recombination into, out of and between these lineages, how then do new, distinct, lineages such as these stably emerge? The answer appears to lie in a balance between recombination as a cohesive force holding populations together as entities recognizable as taxonomic units, and barriers to that transfer for promoting diversification. A primary candidate appears to be geographic barriers that reduce gene transfer between populations sufficiently to allow regional signatures to emerge read more ...