In mutualistic interactions between species, each species uses the other as a resource. That is, each exploits the other, and the degree of exploitation may determine whether an interaction is mutualistic or parasitic. Mutualisms include interactions both between free-living organisms, such as plants and pollinating animals, and between symbionts, one of which spends most of the life cycle on or in the other. Microbes are partners in many symbiotic mutualisms. Mutualists often have adaptations for encouraging the interaction or even nurturing the associate, such as foliar nectaries in plants, which attract ants that defend the plants against herbivores, or the root nodules of legumes, which house and nourish nitrogen-fixing rhizobial bacteria. In some intimate symbioses, the symbiont functions as an organ or organelle, as in the case of host-specific bacteria that reside within special cells in aphids and supply essential amino acids to their host.
For each mutualist, the interaction has both a benefit and a cost. Legumes, for example, obtain nitrogen from rhizobia, but expend energy and materials on the symbionts. Excessive growth of the rhizobia would reduce the plant's growth to the point of diminishing its fitness. Likewise, excessive proliferation of mitochondria or plastids, which originated as symbiotic bacteria, would reduce the fitness of the eukaryotic cell or organism that carries them. Thus, selection will always favor protective mechanisms to prevent overexploitation by an organism's mutualist. Whether selection on a mutualist favors restraint depends on how much an individual's fitness depends on the fitness of its individual host. When a mutualist can readily move from one host to another, as pollinating insects can from plant to plant, it does not suffer from the reproductive failure of any one host, and selfishness or overexploitation may be favored. For example, many pollinating insects "cheat." The larvae of yucca moths (Tegeticula) feed on developing yucca seeds in flowers that their mothers actively pollinated. However, several species of Tegeticula have independently lost the pollinating behavior, having evolved the habit of ovipositing in flowers that other species have already pollinated. Moreover, the pollinating species lay only a few eggs in each flower, so that the few larvae do not consume all the developing seeds. This reproductive restraint has evolved in response to a defensive tactic of the plant, which aborts developing fruits that contain more than a few eggs. However, the "cheater" species of Tegeticula circumvent the plant's defense by laying eggs after the developmental window for fruit abortion, and they lay so many eggs that the larvae consume most or all of the seeds. Deception and cheating has also evolved in some plants, such as orchids that provide no reward to the naive bees that visit them; other orchids mimic the female sex pheromone of an insect species, the males of which effect pollination by "copulating" with the flower.
Vertical transmission of a symbiont favors restraint and reciprocal benefit, just as it favors lower virulence in parasites, because the fitness of the individual symbiont is then proportional to its host's reproductive success. This principle can explain why internal symbionts such as aphids' bacteria or corals' zooxanthellae (or eukaryotes' mitochondria) divide at rates commensurate with their host's growth. It is conceivable that hosts may evolve mechanisms to prevent horizontal transmission (mixing) of symbionts and thus maintain conditions under which "selfishness" would be disadvantageous to the symbiont. By extension, such principles explain the conditions for the evolution of coordination versus conflict among different genes in a single genome, i.e., the evolution and maintenance of integrated organisms.
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