My PhD research aims to untangle the relationship between Rhopalodian diatoms and their nitrogen-fixing endosymbionts called spheroid bodies (SBs) by studying their evolutionary history and present-day diversity. Only two SB genomes have been sequenced so far along with limited genetic data for the diatom (Nakayama et al. 2014 and 2017), which isn’t enough to answer outstanding questions. So, a key part of my project is expanding available resources for studying Rhopalodiaceae and SBs by building a culture collection of strains from around the world. So far, I have established cultures for 11 strains across at least 5 species. For two of these strains, I have assembled complete genomes for the SB and diatom mitochondria and chloroplast, along with preliminary nuclear genome data. Additionally, we have gotten gene expression data for the SB from one species which tells us about what genes the SB is using. Through expanding these data, I can begin to uncover what SB genes are necessary to function, how the diatom supports and controls SBs, and how this translates to Rhopalodian diatom distribution and diversity in the real world.

Beyond applying this research to how SBs impact Rhopalodiaceae, this system is also a remarkable chance to understand the early stages of endosymbiosis. While there are many well-understood examples of highly integrated endosymbionts, there are very few examples of endosymbionts early in this evolutionary process. The Rhopalodiaceae-SB system is an amazing opportunity to understand how endosymbionts evolve and integrate with their hosts, as well as how endosymbionts of unicellular hosts may differ from multicellular hosts.

Image description: Spheroid bodies in three different species from Rhopalodiaceae. Top: Rhopalodia gibba Bottom left: Epithemia sorex Bottom Right: Rhopalodia gibberula