Symbiosis

Reader's Guide

Symbiosis is a foundational concept in biology, describing the diverse and often intimate relationships between organisms of different species. It can be classified in multiple ways: obligate or facultative, depending on whether the partners depend on each other for survival; ectosymbiosis, where one lives on the surface of another; or endosymbiosis, where one lives inside the other's tissues. The interactions range from competition and amensalism, which reduce fitness, to commensalism, parasitism, and mutualism, where both parties benefit. Symbiosis has driven the evolution of species, enabling colonization of new environments. Symbiogenesis is thought to have helped create eukaryotes as bacteria were incorporated as mitochondria and chloroplasts within cells. Major co-evolutionary relationships include mycorrhiza, pollination by insects, ant protection of acacia trees, seed dispersal by animals, nitrogen fixation by bacteria in legume root nodules, and the mutualistic partnership of algae and fungi in lichens.

Did You Know?

• Endosymbionts often have a much reduced genome, losing protein-coding genes for metabolism and DNA repair.
• An example of amensalism is the black walnut tree secreting juglone, which destroys many herbaceous plants within its root zone.
• Mutualistic relationships may be obligate for both species, obligate for one but facultative for the other, or facultative for both.