Symbiosis and modern biology

The recognition of symbiotic relationships has had a revolutionary impact on modern biological thought. The idea that mitochondria and chloroplasts are transformed by symbiotic bacteria provides a common thread to the biological world and raises hope of finding other symbiotic wonders among life's diversity. Plants and animals have acquired new metabolic capabilities through symbioses with bacteria and fungi. Mammalian herbivores and termites digest cellulose with the help of microbial symbionts. The luminescent bacteria contained in the specialized light organs of some fishes and squids produce marine bioluminescence. Diverse animal life around deep-sea vents is based on symbiosis with bacteria that oxidize hydrogen sulfide and chemosynthetically fix carbon dioxide into carbohydrates. Associations between fungi and algae have resulted in unique morphological structures called lichens. Early land plants formed associations with mycorrhizal fungi, which greatly facilitated their phosphorous uptake and thus played a significant role in the plants' ability to colonize terrestrial habitats. Evolutionary changes in organisms and their gene pools are not restricted to nuclear events and sexual mechanisms. Horizontal gene transfer between species has been documented in all forms of life. Bacterial cells possess plasmids and viruses that transfer new genetic properties from one cell to another. Many virulence factors in pathogenic bacteria are expressed through plasmid-borne genes. Similarly, bacteria become resistant to antibiotics when they incorporate plasmids with genes for antibiotic resistance. Horizontal gene transfer has been suggested in the evolution of flowers, fruits, and storage structures from gall-forming insects and viruses. The role of viruses as genetic engineers is gaining importance in evolutionary biology. The Rhizobium-legume symbiotic relationship is an excellent example of how host cells and bacterial symbionts within root nodules undergo transformation, which allows the bacterial cells to fix nitrogen-converting atmospheric nitrogen into a chemical form that can be taken up by plants. Within the host cells, Rhizobia acting as bacteroids behave as temporary cell organelles that fix nitrogen. Intragenomic conflict is an evolutionary force. The evolution of sex was a form of genomic conflict management. Uniparental inheritance of cytoplasmic genes, mating types, and many features of sexual behavior may have evolved as a result of evolutionary conflict. The two-sex model that is widespread throughout the diversity of life may have been the result of ancient intracellular symbiosis. The Red Queen hypothesis suggests that harmful parasites and virulent pathogens exert selection pressure on their hosts so that sexual selection is maintained. Parasites, pathogens, and their hosts are involved in a microevolutionary "arms race" and in time, the symbionts' offense and the host defenses produce cycles of coadaptations.

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