Introduction

Abstract

Abstract Symbiosis is an association between two or more different species of organisms. The association may be permanent, the organisms never being separated, or it may be long lasting. This definition excludes populations, which are associations between individuals of the same species. Organisms that are involved in a symbiosis may benefit from, be harmed by, or not be affected by the association. Symbiotic associations are common in nature, from bacteria and fungi that form close alliances with the roots of terrestrial plants to those between giant tube worms and sulfur-oxidizing bacteria that live together in the deepest depths of the oceans. No organism is an island-each one has a relationship to other organisms, directly or indirectly. Even humans bear a reminder of an ancient symbiosis-their cells contain mitochondria, organelles which once were symbiotic bacteria. In addition, each of us harbors several types of viruses and bacteria in our skin and intestinal tract. Similarly, chloroplasts in plant cells are organelles which have evolved from ancient symbiotic photosynthetic bacteria. Bactmia which form symbioses with higher forms of life are themselves hosts to symbiotic viruses. Satellite viruses depend on other viruses for their expression. It is difficult to imagine life and its evolutionary history without symbioses (Khakhina, 1992; Sapp, 1994a,b).