Fundamental concepts in symbiotic interactions: Light and dark, day and night, squid and legume

The legume-Rhizobium symbiosis and that between Euprymna scolopes and Vibri o fischeri show some surprising physiological similarities as well as diffe rences. Both interactions rely on exchange of signal molecules, some of whi ch are derived from bacterial cell surface molecules. Although the legume-R hizobium symbiosis is nutritionally based as are many animal-microbe symbio ses, it is not obligate because the plant initiates nodule formation only w hen the soil is deficient in nitrogen. In contrast, the squid-Vibrio symbio sis is obligate for the squid but is not nutritionally based. Rather, the b acteria produce light, which enables the animal to evade predators. These s imilarities and differences are described and discussed in term of the over all question of whether or not these two symbiotic relationships have evolv ed from commensal or pathogenic/parasitic interactions between prokaryotes and eukaryotes.