Biology and evolution of reduced-genome microbes

An interesting trend in cell interaction is the evolution of intracellular microorganisms, such as parasites and endosymbionts, which frequently present a reduced genome, usually accompanied by decreased metabolic and/or cellular capabilities. Such reduction is noticed in bacteria and in unicellular eukaryotes, such as fungi and protozoa, when they are compared with their most closely related free-living relatives, and points towards significant aspects of genomic and cellular evolution. Living together brings about genome shortening, including genes for DNA repair and transcriptional regulation, loss of mobile elements, lowering of GC content, reduction or loss of codon-usage bias and diminution of intergenic region length that often leads to gene overlapping. Other interesting aspects of intracellular life are related to gene loss from the endosymbiont or parasite, or transfer of endosymbiont genes to the host nucleus. A diminished metabolic capability for the intracellular organism usually results in these instances, although biosynthetic pathways required by interaction with the host(s) are typically maintained. A notorious aspect of intracellular life is organelle origin, such as the mitochondrion and the chloroplast, originated from mutualistic relationships between prokaryotes and eukaryotes. Furthermore, symbiosis as a whole, which comprehends associations ranging from beneficial to harmful, has been investigated in environmental impact studies as well as in health programs to control several types of diseases.

The aim of this Research Topic is to highlight the latest research on all aspects of the biology, evolution, and biochemistry of host-parasite and host-endosymbiont interactions. Works on unicellular eukaryotes and/or bacterial lineages will be emphasized.