Cospeciation between Hosts and Symbionts:

I. Introduction Mutualistic associations between eukaryotic hosts and their microbial partners have long been an interest of ecologists and evolutionary biologists due to the importance of understanding phylogenetic congruence, coevolution, host specificity, infectivity, and maintaining the precarious balance between pathogenicity and innocuousness. Each partner has an important function in the life history of the symbiosis; the " super organism " functions as a result of combining both lifestyles of host and symbiont, enabling the entire association to take advantage of a broader ecological niche than either one could accomplish independently. These types of functional interactions between host and symbiont can only be established when an avenue for the exchange of information has evolved specifically within the symbiosis. Once this occurs, the discourse between the partners (physiological, genetic) allows the association to flourish and become a competitive and dynamic system in an environment where it may not normally exist. The presence of such interactions between highly divergent taxa provides an excellent opportunity to study the interactive roles of coevolving species in processes such as horizontal gene transfer, interspecies gene regulation, allopatric and sympatric routes of speciation, and the development of population structure and dynamics. A number of examples exist that indicate that the formation of a symbiotic relationship allows the host or its symbiont to radiate into newly formed niches: chemoautotrophic bacterial residing within tissues of hydrothermal vent or sewage outfall invertebrates and luminous bacteria within the light organs of monocentrid and anomolopid fish and sepiolid and loliginid squids (Flaygood and Distel, 1993; McFall-Ngai and Ruby, 1991). Although these mutualisms have evolved as part of a new " evolutionary innovation " (Margulis, 1989), there resides the question of how the association is directly affected by either specific host-symbiont interactions, or whether environmental or ecological factors play an important role in the formation of the partnership. Various hypotheses have alluded to the fact that transmission of the symbiosis, whether vertical or horizontal, would also have a major influence in determining whether host-symbiont pairs display parallel modes of speciation or promiscuous patterns of affiliation. Does specificity drive the cospeciation of host-symbiont pairings, or does the fidelity of vertical transmission have a more dramatic effect on determining the coevolution of the mutualism? Does the evolution of specific recognition factors help determine whether a symbiont will maintain fidelity or be promiscuous? Do environmental or ecological factors influence the onset of the symbiosis as well as …