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.