From microbiology to cell biology: when an intracellular bacterium becomes part of its host cell

Abstract

Mitochondria and chloroplasts are now called organelles, but they used to be bacteria. As they transitioned from endosymbionts to organelles, they became more and more integrated into the biochemistry and cell biology of their hosts. Work over the last 15 years has shown that other symbioses show striking similarities to mitochondria and chloroplasts. In particular, many sap-feeding insects house intracellular bacteria that have genomes that overlap mitochondria and chloroplasts in terms of size and coding capacity. The massive levels of gene loss in some of these bacteria suggest that they, too, are becoming highly integrated with their host cells. Understanding these bacteria will require inspiration from eukaryotic cell biology, because a traditional microbiological framework is insufficient for understanding how they work.

Cicada endosymbionts contain tRNAs that are processed despite having genomes that lack tRNA processing activities

“…Despite the ancient nature and massive genetic integration of organelle with host (85,86), most mitochondria and plastids retain genomes, and thus some level of genetic autonomy. Gene retention patterns in the genomes of highly reduced bacterial symbionts also suggest a hurdle to giving up independence of some processes to the host, especially transcription, translation, and replication (11,12). Although the Hodgkinia genome is lacking many genes involved in these processes, our results here indicate that it is likely complemented by its host to perform them.…”

“…The most gene-poor endosymbiont genomes have lost even seemingly essential genes, like those involved in DNA replication and translation (11). In terms of genome size and coding capacity, these tiny genomes span the gap between their less degenerate endosymbiotic cousins, which retain seemingly minimal sets of genes, and the mitochondria and plastids, which have lost most genes involved in replication, transcription, and translation (11,12). These extremely gene-poor insect endosymbiont genomes thus provide an opportunity to learn more about key adaptations enabling co-dependent and integrated endosymbioses, but in associations that are younger and more labile than the classic cellular organelles.…”

No genome is an island: toward a 21st century agenda for evolution

“…Prominent examples of endosymbiosis with clear adaptive benefits include root nodule formation by nitrogen‐fixing bacteria Sinorhizobium spp. on leguminous plants, Symbiodinium dinoflagellate endosymbionts that confer photosynthetic capacity to reef‐building corals, and numerous bacterial endosymbionts that provide essential nutrient biosynthesis and digestive capabilities to insects and other invertebrates that feed on plant tissues , . Intracellular bacteria can provide such important adaptive benefits to the hosts that they frequently become “obligate” endosymbionts essential to host survival .…”

“…Although not as phylogenetically deep in evolutionary history as the symbiogenetic origins of mitochondria and plastids, endosymbiotic cell mergers have continued throughout the course of eukaryotic evolution. s, [36][37][38] We know this to be the case because independent endosymbiotic associations were established following evolutionary events that separated eukaryotic hosts into distinct taxonomic groups. The only exception to ongoing endosymbiosis is that adaptively beneficial intracellular endosymbionts seem to be missing in vertebrates.…”

“…on leguminous plants, Symbiodinium dinoflagellate endosymbionts that confer photosynthetic capacity to reef-building corals, and numerous bacterial endosymbionts that provide essential nutrient biosynthesis and digestive capabilities to insects and s http://shapiro.bsd.uchicago.edu/No_Genome_is_an_ Island_Extra_References_6.html other invertebrates that feed on plant tissues. t, 37 Intracellular bacteria can provide such important adaptive benefits to the hosts that they frequently become "obligate" endosymbionts essential to host survival. u In addition to the energetic, nutritional, biosynthetic, and digestive capabilities listed above, the presence of bacterial endosymbionts frequently confers resistance to parasites and viruses.…”