Fundamental Concepts in Symbiotic Interactions: Light and Dark, Day and Night, Squid and Legume.

The legume-Rhizobium symbiosis and that between Euprymna scolopes and Vibrio fischeri show some surprising physiological similarities as well as differences. Both interactions rely on exchange of signal molecules, some of which are derived from bacterial cell surface molecules. Although the legume-Rhizobium symbiosis is nutritionally based as are many animal-microbe symbioses, it is not obligate because the plant initiates nodule formation only when the soil is deficient in nitrogen. In contrast, the squid-Vibrio symbiosis is obligate for the squid but is not nutritionally based. Rather, the bacteria produce light, which enables the animal to evade predators. These similarities and differences are described and discussed in term of the overall question of whether or not these two symbiotic relationships have evolved from commensal or pathogenic/parasitic interactions between prokaryotes and eukaryotes.