UC Riverside

The survival of coral reefs in the face of environmental stress is dependent on understanding the cellular and molecular mechanisms that govern Cnidaria-Symbiodiniaceae symbiosis. However, the relative genetic intractability of the Symbiodiniaceae symbiont has led to a bottleneck in the availability of Symbiodinaceae mutants that can be applied towards this purpose. This work describes a protocol for the utilization of UV mutagenesis and downstream screening process for the discovery of Symbiodiniaceae mutants with phenotypes of interest and their application. Algal photosynthesis plays an important role in the symbiosis, assisting in the spread of symbionts to host cells throughout the host gastrodermal tissue, as well as providing fixed carbon to the host. This role makes it of interest for investigation. Symbiodiniaceae mutants lacking the ability to perform photosynthesis were generated following the protocol and applied to in hospite conditions across multiple cnidarian species. The results show that photosynthetic activity is not required for the establishment of algal association, suggesting different criteria are required for association within the host cell. Furthermore, the rate of algal division has been shown to differ among strains of Symbiodiniaceae, leading to differences in the symbiosis phenotype. Low algal division rate has been hypothesized to confer temperature stress, warranting further investigation into this phenotype. As such, a mutant with a suppressed division rate in mixotrophic culture conditions was also discovered and applied to answer questions about the symbiosis. Reduced algal division rate was observed in hospite, as well as a reduced final algal density and animal size compared to the original Symbiodiniaceae strain. While these initial investigations into the role of photosynthesis and algal division rate have provided insight into these aspects of the symbiosis, further work can be performed with these mutants, particularly work pertaining to temperature stress resistance. The successful creation and application of these mutants showcase the utility of UV mutagenesis and screening processes as a valid approach for the future discovery of Symbiodiniaceae mutants with other phenotypes of interest.