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Even viruses can provide insights into convergence - in fact, the extent of viral convergence is probably underestimated. A virus (this Latin word basically means poison) is an infectious particle that is much smaller than a bacterium and usually only visible using an electron microscope. Viruses possess genes and can hence evolve, but neither do they have a cellular structure nor their own metabolism, which is why they depend on their hosts for reproduction. They have been aptly referred to as "organisms at the edge of life" (Rybicki 1990, South African Journal of Science, vol. 86, p. 182). There are a vast number of different viruses that can infect virtually any other organism (even other viruses). The origins of viruses are still not fully understood, but several hypotheses have been proposed.
Interestingly, the coat of a virus shows striking structural similarities with carboxysomes, protein microcompartments of cyanobacteria and chemoautotrophic bacteria that serve to concentrate CO2. Both these polyhedral structures are formed from a tightly fitting array of protein plates, although those of the carboxysome are generally thinner (probably because they are housed in the safety of the cell). They evidently evolved independently and illustrate how in biology the same architectural principles must apply.
Experimental work on the evolution of viruses in the laboratory has furthermore provided convincing evidence for "a limited number of pathways taken during evolution in these viruses" (Wichman, Scott, Yarber & Bull 2000, Philosophical Transactions of the Royal Society of London B, vol. 355, p. 1677). One such example comes from two presumably unrelated viruses with relatively large genomes, viruses of the T4 family that infect bacteria and those of the NCLDV group that target eukaryotes, where the mechanisms of genome evolution were shown to be convergent.
Immune responses against viruses can also be convergent, as exemplified by two species of monkey. Old World macaques and New World owl monkeys have independently come up with the same fusion protein that confers immunity to retroviruses causing immunodeficiency, such as HIV.