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Bacteria are single celled micro-organisms of astonishing diversity and abundance that first appeared more than 3.5 billion years ago, in the Archean period. The pattern of bacterial diversification into distinct groups is not certain, but molecular evidence suggests that the common ancestor may have been thermophilic, living in hot, hydrothermal ecosystems. The gut bacterium E. coli is cultured for use in many laboratory experiments, and others such as Shigella and Salmonella are feared as pathogens. Early in the history of life some lineages of nucleated cells (eukaryotes) acquired organelles for respiration and photosynthesis by capturing and integrating, through endosymbiosis, certain forms of bacteria. For example, α-proteobacteria were incorporated to become the respiring mitochondria of eukaryotes, and cyanobacteria were engulfed (apparently on several independent occasions) to form photosynthetic chloroplasts.
Bacteria are a fertile source of information on evolutionary convergence, with a prime example being experiments with E. coli carried out to investigate the consequences of "re-running the tape of life". Ancestral populations can be kept in stasis and compared with a range of descendants after thousands of generations (due to the very fast nature of bacteria replication), an approach which so far has shown that evolutionary trajectories repeatedly take the same course even when run many times independently.
Bacteria from four distantly related groups (e.g. gram-positive actinobacteria and cyanobacteria such as Anabaena and Nostoc) have been independently recruited for nitrogen fixation in the root nodules of leguminous plants and other plants including hornworts and the water fern Azolla (which both recruit different species of Anabaena for N fixation). All of these bacteria use nitrogenase enzymes to reduce atmospheric nitrogen (N2), fixing it as ammonia (NH3) for use in synthesis of essential compounds such as DNA and amino acids.
Among bacteria adapted to extremes of temperature, salinity, acidity or alkalinity (termed 'extremophiles'), notable convergences with certain Archaea (or 'archebacteria', unicellular micro-organisms unlike bacteria or eukaryotes) occur. One such example concerns the similarity between the extremely saline tolerant ('hyper-halophilic') bacterium Salinibacter and the archeal Halobacterium salinarium that also inhabits brines. Heat-tolerance, or 'thermophily' appears to have evolved in several bacterial groups, including Thermotogales (e.g. Thermotoga maritime), Aquificales (e.g. Aquifex), Deinococcus-Thermus (Deinococcus, Thermus) and Chloroflexi (Chloroflexus aurantiacus). Magnetotactic bacteria synthesize crystals of iron minerals (typically magnetite or iron sulphide) to orientate themselves in the ambient magnetic field and find the optimum position in an area of redox boundaries. This remarkable ability evolved at least twice, namely in the α- and δ-proteobacteria.
The propulsive flagellum of bacteria is a complex molecular motor that appears to have evolved convergently (by co-option of pre-existing proteins) within the bacteria and also in the Archea. Certain cyanobacteria have light sensitive pigments (e.g. opsin in Anabaena) and extraordinarily the cyanobacterium Leptolyngbya has an 'eye-spot' for phototaxis. Finally, bacteria appear able to 'communicate' using quorum sensing, as social insects do, to enhance their co-operative activity when forming structures (e.g. biofilms), attacking prey or commencing dispersal.
|Topic title||Teaser text||Availability|
|Foregut fermentation in mammals||Foregut fermentation is best known from the ruminants, such as cattle, deer and giraffes, that regurgitate and rechew their food to aid microbial digestion. However, they are not the only mammals to have evolved this digestive strategy...||Available|
|Foregut fermentation in birds||A foregut-fermenting bird was long considered a paradox. But what about the hoatzin, a curious South American bird known locally as the "stinking pheasant" thanks to its smell of fresh cow manure?||Available|
|Horizontal gene transfer in bdelloid rotifers, bacteria and protists||n/a||Unavailable|
|Pufferfish (and inflation)||Pufferfish are some of the most extraordinary fish to have evolved, especially because of their capacity to swallow water and inflate themselves to something like a football. Not only that but some representatives can be deadly to the unwary diner...||Available|
|Lichens: fungal association with cyanobacteria and green algae||n/a||Unavailable|
|Bacterial cell shapes||A fascinating example of convergence in bacterial cell shape is the independent evolution of multicellularity in magnetotactic bacteria, with striking similarities to the arrangement seen in eukaryotic green algae.||Unavailable|
|Halotolerance in bacteria and protistans||n/a||Unavailable|
|Collagen in animals and bacteria||n/a||Available|
|Membranes and vesicle formation in bacteria||Examples include endoplasmic membranes with a capacity to attach ribosomes in E. coli, and in the hyperthermophile Archaea what are evidently vesicles that are believed to have budded from a cytoplasmic membrane.||Unavailable|
|Magnetotactic bacteria||Magnetotactic bacteria provide some excellent examples of convergent evolution. In particular the ability to synthesize iron compounds has evolved at least twice, respectively employing iron oxide (magnetite) and iron sulphide.||Available|
|Extremophiles: Archaea and Bacteria||Surely, no organism can survive in boiling water or brines nine times the salinity of seawater? Wrong - some archaea and bacteria have independently evolved adaptations to such extreme environments...||Available|
|Sodium voltage-gated ion channels||Sodium voltage-gated ion channels are vital to electric signal transmission, but it is less widely appreciated that they are convergent and have evolved at least twice in groups outside the animals.||Unavailable|
|Light sensitivity and eye-spots in bacteria||Light sensitivity based on opsins is well documented, notably in the cyanobacterium Anabaena where it is involved with photosynthesis and in particular the production of key pigments.||Unavailable|
|Nitrogen-fixing bacteria in legumes||One convergent avenue to obtaining nitrogen is to employ symbiotic bacteria that typically are found in root nodules, perhaps best known in the legumes.||Unavailable|
|Bacterial carboxysomes (and other microcompartments)||It is now clear that the cellular construction of at least the eubacteria is more complex than realized, and includes organelle-like structures known as microcompartments, of which the best known are the carboxysomes.||Available|
|Lysozyme||Lysozymes are common antibacterial enzymes that protect our eyes and nose from infection, but some animals have recruited them for a rather different purpose...||Available|
|Gut fermentation in herbivorous animals||Ever tried eating a newspaper? Don't. Plant cell walls contain cellulose, which is notoriously difficult to digest. Considering that all vertebrates lack the enzymes to attack this polysaccharide, how do so many of them manage to survive on a plant diet?||Available|
|Carbonic anhydrase in vertebrates, plants, algae and bacteria||Carbonic anhydrase is extremely convergent and may have evolved as many as six times. The most familiar variants are α, β and γ carbonic anhydrases.||Available|
|Evolutionary experiments: "Re-running the tape of life" in bacteria||Re-running the tape of life presupposes that history can be run many times independently, and this of course can be readily achieved with bacteria.||Unavailable|
|Bacterial communication and co-operation||Interesting convergences between bacteria and eukaryotes include aspects of communication linked to quorum sensing and enhanced cooperative activity in terms of formation of biofilms, mass attack on prey and dispersal.||Unavailable|
|Bacterial flagellar motors||The bacterial flagellum has proved to be a cause celebre because of its high-jacking by the “intelligent design” movement who argue that it is “irreducibly complex” and therefore could not have evolved by Darwinian processes.||Unavailable|
|Gene regulation and cell cycles in bacteria and eukaryotes||Regulation of gene networks and cell cycles are of particular importance for convergence because genomic organization in bacteria shows significant differences from the eukaryotes.||Unavailable|
|Sap feeding and honey-dew production in insects||Interestingly, it has now been shown that the saliva of the aphids has an analogue to the anti-coagulant properties of blood suckers, subverting the wound repair mechanism of the plant.||Available|
|Chloroplast and mitochondrial plastid origins||Not only are there intriguing parallels in the story of gene loss in chloroplasts and mitochondria, but there is also the re-invention of bacterial pathways, such as oxidation of quinols.||Available|
|Recruitment of endosymbiotic bacteria in insects||Independently the sap-feeding aphids and psyllids have recruited γ-proteobacteria, respectively best known from Buchnera and Carsonella, in an intimate and obligate symbiotic relationship.||Unavailable|
|Genome reduction in bacteria and animals||Interestingly, many parasitic bacteria show dramatic and parallel gene reduction independent of each other, although here various vital functions are now undertaken by the host.||Unavailable|