Mutualisms may evolve increasing interdependence.

Close interactions between species that have evolved over long periods of time are called symbioses (singular, symbiosis). One of the best-studied symbiotic mutualisms involves aphids (and their insect relatives) and closely associated bacteria. The insects suck plant sap for food, and the bacteria live in tissues that develop in the insects adjacent to their digestive system. The insects provide a home for the bacteria at some physiological cost but gain an important benefit: Plant sap contains relatively few nutrients, and the bacteria provide their hosts with essential amino acids. The bacteria benefit from a stable and favorable environment at the cost of some loss of nutrients supplied to the aphids. The aphids pass the bacteria from mother to daughter in egg cells, guaranteeing that offspring will have bacteria as effective as those associated with earlier aphid generations. When the interaction between species drives reciprocal adaptations in both participants, there is a possibility for long-term coevolution because the descendants of each side will also be associated with each other, just as were past generations. Any mutations that arise can affect both sides of the partnership because neither aphids nor bacteria are found without the other.

DNA evidence suggests that aphids and their bacteria have been associated for 100 million years or more, and that the bacterial genomes have been gradually losing genes they would need for life outside the insects (Fig. 47.8). Some bacterial genes have in fact been acquired by the insects. We saw a similar process of coevolution in Chapter 26 in the example of mitochondria and chloroplasts, which are probably nature’s most dramatic examples of coevolved mutualism.

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HOW DO WE KNOW?

FIG. 47.8

Have aphids and their symbiotic bacteria coevolved?

BACKGROUND Aphids are small insects that are common pests of garden plants. Specialized cells in the aphids harbor populations of the bacterium Buchnera that provide their host with amino acids essential for growth. Mother aphids pass the bacteria through their eggs to their daughters.

HYPOTHESIS Aphids and Buchnera bacteria have coevolved for millions of years.

EXPERIMENT Evolutionary ecologist Nancy Moran sequenced the DNA of bacteria and the aphids they come from to establish the phylogenetic relationships of aphid species and their symbionts. She reconstructed the phylogeny of both groups and compared their relationships. If the two phylogenies matched each other, the hypothesis of coevolution would be supported.

RESULTS The phylogenetic trees of aphids and their associated bacteria matched perfectly, just as if the bacteria were a gene of the aphids instead of a separate group. This matching of phylogenetic trees was seen both among aphid species in different genera and for aphid populations within a single species. A DNA-based molecular clock further showed that aphids, and the insects related to them, have all coevolved with these bacteria over nearly 200 million years.

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FIG. 47.8

CONCLUSION A single group of Buchnera has been passed down through the generations and the diversification of its aphid hosts for nearly 200 million years. Host–symbiont systems can be stable over long time intervals and show long-term coevolution.

FOLLOW-UP WORK Researchers are studying the changes in the genomes of bacteria living inside aphids to see how they lose genes necessary for living independently of their aphid hosts.

SOURCE Moran, N. A., et al. 1993. “A Molecular Clock in Endosymbiotic Bacteria Is Calibrated Using the Insect Hosts.” Proceedings of the Royal Society. London, Series B, 253:167–171.

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While some mutualisms involve close interactions between specific species, others are less particular. For example, sweet-tasting fruits evolved in flowering plants, attracting mammals and birds that disperse their seeds, but not in response to any particular bird or mammal species. Similarly, flowers evolved in response to insect pollinators such as bees and flies, whose own adaptations for visiting flowers evolved in response to flower availability.