Exploring cyanobacterial mutualisms

Cyanobacterial symbioses with eukaryotes are ancient associations that are widely distributed in aquatic and terrestrial environments. Cyanobacteria are a significant driving force in the evolution of their hosts, providing a range of services including photosynthesis, nitrogen fixation, UV protection, and defensive toxins. Although widespread, cyanobacteria occur in a limited range of hosts. Terrestrial symbioses are typically restricted to lichens and early evolved plants, and aquatic symbioses to sessile or slow-moving organisms. This review examines the underlying evolutionary processes that may have lead to these patterns. It also examines the facts that the degree of integration between symbiont and host, and the mode of transmission of the symbiont, do not appear to be an indication of how old the symbiosis is or how important it is to host well-being. Biparental transmission of symbionts may prolong the survival of gametes that persist in the environment, increasing chances of fertilization.