Future directions
The strong association of habitat of origin with the physiological ability to recover from rapid desiccation in the vegetative stage provides a context to formulate hypotheses about the evolution of vegetative desiccation tolerance in Tetradesmus , ranging from a single origin in the terrestrial species (in which case the aquaticT. sp. “raciborskii ” returned to the aquatic habitats secondarily), to the evolution of vegetative desiccation tolerance independently in each of these desert taxa.
Recovery of one aquatic strain (T. obliquus , UTEX 393) at 80% RH reveals that aquatic algae tolerate desiccation stress to some extent. This ability may prove to be important for discovering evolutionary and molecular mechanisms of the emergence of desiccation tolerance inTetradesmus. Aquatic Tetradesmus are common in fresh waters from around the world (e.g., Ismagulova et al. 2018), including regions where they experience seasonal or intermittent water restrictions, which should require an ability of cells to survive periodic environmental stresses, including desiccation. Many aquatic algae form tolerant dormant stages either as vegetative cells or as reproductive stages, such as zygotes (e.g., Starr 1955; Evans 1958; Rengefors et al. 1998). The genes imparting tolerance to these dormant stages may have been co-opted for vegetative desiccation tolerance by terrestrial Tetradesmus in a similar way that genes providing desiccation tolerance to seeds were co-opted in the vegetative tissues in desiccation-tolerant angiosperms (e.g., Illing et al. 2005; Costa et al. 2017; VanBuren et al. 2017). Future comparative omics studies of the closely related species of Tetradesmus from different habitats and having distinct desiccation tolerant phenotypes will enable investigations of the origins of the desiccation machinery in this group, and may reveal the evolutionary transitions that enable these algae to occupy such diverse environments.
Acknowledgements The authors thank Dr. X. Sun and Dr. M. Abril from the Bioscience Electron Microscopy Laboratory at University of Connecticut for their help in sample preparation and assistance with TEM, and C. O’Connell from the Advanced Light Microscopy Facility at UConn for assistance with fluorescence microscopy. We also thank Drs. B. Goffinet, N, Patel, J. Seemann, Y. Yuan, and J. Wegrzyn for helpful discussions.