| Of particular importance to studies of symbiosis is an understanding of what factors are critical for shaping the complex interactions between host and symbiont. Environmentally transmitted symbioses present a unique association where both host and symbiont must persist in the environment until the symbiosis is established, resulting in the evolution of life histories potentially very different than those observed in vertically or horizontally transmitted symbioses. The symbiosis between certain members of the Vibrionaceae, including Vibrio fischeri and Vibrio logei, and sepiolid squids (Mollusca: Cephalopoda) is a unique system to study the evolution of symbiosis within an environmentally transmitted organism. This thesis explores the maintenance of this symbiosis at a number of biological and ecological "levels" in order to understand the ongoing forces shaping the evolution of this association. At the population level, fluorescent in situ hybridization was used to determine the distribution of environmental vibrios in Hawaii, Australia, and Southern France. Results demonstrate that to some extent, host presence is important to symbiont distribution, but many other factors influence these bacterial communities. Studies on the genetic distribution of host-associated vibrios using population genetic techniques provided evidence that symbionts in Southern Australia are closely related, but those in Northeastern Australia are more closely related to Hawaiian strains. In addition, Hawaiian populations contain native strains as well as strains closely related to those from Thailand, suggesting a secondary colonization event. At the individual level, the nature of the symbiosis was defined as a mutualism by comparing the highly correlated intensities of host-perceived and Vibrio-produced light. The results from these experiments suggest that Eurpymna scolopes benefits from the symbiosis through counterillumination to avoid predators during nocturnal activities. At the molecular level, transcripts expressed exclusively in the host light organ and seawater by V. fischeri strains from E. scolopes and E. tasmanica were identified, providing unique insight into genes under selection for host specificity and environmental persistence. Results of this thesis have provided unique insights to how biological interactions at a number of ecological levels shape the evolutionary history of partners in this environmentally transmitted mutualism. |
