Category: Molecular Biology
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The paths of evolutionary change at the molecular level frequently result in convergent solutions. Examples of molecular convergence range from gene regulation and genome structure to critical enzymes and other proteins important in cellular to whole organ structure and function.
Both patterns of gene loss and gene transfer from one organism to another frequently recur. For example, members of the two major animal groups, the deuterostomes and protostomes, have independently lost genes used redundantly in amino acid synthesis, and several parasitic bacteria (e.g. spirochaetes and Mycoplasma genitalium, with its tiny 580kb genome) have lost genes for vital cellular functions that are instead carried out by their host. Massive 'horizontal' gene transfer to host organisms has occurred in many pathogenic, endosymbiotic bacteria, in some parasitic protistans and, interestingly, in groups as different as choanoflagellates and rotifers. In a remarkable example of molecular convergence, the mitochondrial oxidase gene Cox2 of green algae (Chlorophyta) and apicomplexan protists was transferred to the nucleus and subsequently divided into two genes at the same position. In terms of whole genetic regulatory networks, it is clear that at certain levels convergence occurs; one case of this is the reduction in pelvis size of sticklebacks and manatee, caused by identical but independent changes in a shared vertebrate developmental pathway.
Organisms have evolved a number of defence mechanisms to cope with pathogens, including anti-microbial compounds and immune systems. The innate immune systems of animals and plants are convergent, sharing trans-membrane and intracellular defence proteins comparable in structure and function, while the adaptive immune systems of vertebrates finds strongly independent parallels in jawless fish, and even to some extent the limited 'memory' of insect immune systems. Tetrodotoxin (or tetrodoxin) is a lethal defence compound synthesised by symbiotic bacteria and recruited by an unusual range of animals including puffer fish, blue-ringed octopus, two unrelated frogs and the newt Taricha. Saxitoxin is produced by the venus clam Saxidomus (a bivalve), one macroscopic alga and a few octopus, crustacean, fish, cyanobacteria and dinoflagellate species. For both tetrodoxin and saxitoxin, animals protect their nervous systems from toxic effects by specific and convergent amino acid changes in sodium voltage-gated channels (these channels themselves being convergent between animals, a protistan heliozoan known as Actinoryne contractilis and an alkaline-tolerant bacterium). A number of bacteria have converged on molecular adaptations to saline environments, including archeal halobacteria and the eubacterium Salinibacter ruber. In a particularly striking case, eubacteria and certain archael bacteria have also independently evolved flagellar motors, constructing them from similar proteins.
Enzymes are proteins that accelerate biochemical reactions, and they provide many examples of convergence. Identical active site structure may evolve, such as the serine-histidine-aspartate 'triad' in trypsin and subtilisin, or different routes may taken by structurally distinct proteins to catalyse an identical substrate (e.g. β-lactamases can function via serine protease or zinc 'metalloprotease' groups). The enzyme carbonic anhydrase (CA) converts CO2 + H20 ↔ HCO3- (bicarbonate) + H+, which is a reversible reaction critical for processes as diverse as photosynthesis, respiration, biomineralisation and kidney function. Not surprisingly, therefore, CA enzymes are ubiquitous, and yet they appear to have evolved at least five (or six) times independently, and various CA families are known in animals, plants, bacteria and certain algae (e.g. diatoms, haptophytes). Peroxidases are typical anti-microbial enzymes, creating toxic oxidising conditions via generation of highly reactive hydrogen peroxide (H2O2) or organic hydroperoxidases. An array of peroxidases is known to have evolved in animals, plants, fungi and bacteria, variously depending on heme (iron-based) cofactors, active cysteine or selenocysteine to function.
Aside from enzymes a wealth of other proteins critical to life show clear convergences, from respiratory proteins to silks. Astonishingly, the copper-based respiratory protein haemocyanin evolved independently in arthropods and molluscs, while iron-based β-haemoglobins show convergent gene duplication patterns in birds and mammals (as well as between monotreme and therian mammals). Eye lenses are transparent due to the properties of crystallin proteins, independently recruited from microbes many times in animal evolution and co-opted from roles in stress resistance (e.g. heat shock proteins). Interestingly, genes for crystallins made of very different proteins are driven by almost identical promoter regions in the genomes of scallops and vertebrates! Proteins with elastic properties show convergences as various levels, from elastins in the vertebrate and cephalopod aorta to the shared characteristics of resilin, abductin, elastin and even gluten from plant seeds. Bivalves such as the well-known mussels have attachment structures with both elastic and more rigid silk fibroin-like regions, and the 'pen shell' Pinna has a byssus of 'sea-silk' threads, reminiscent of arthropod silk. Silk evolved for various functions at least three times in arachnids (spiders, spider-mites and some pseudoscorpions) and many more times in the insects, especially at larval stages. Among many convergent functions: spiders, spider-mites and caddis fly larvae build webs; silk nests are built by weaver ants, leafhoppers, 'webspinners' and pseudoscorpions; the silk-worm Bombyx makes silk cocoons; a few (brave) spiders and hilarinid flies offer silken 'nuptial gifts', and spiders, spider mites and moth larvae use silk lines for 'ballooning' into the air.
