[Coral-List] stress tolerance alteration

[Bill Allison]

 Ye, S., Bhattacharjee, M. & Siemann, E. Stress tolerance alteration in the
freshwater cnidarian green hydra (*Hydra viridissima*) via symbiotic algae
mutagenesis. *Symbiosis* *82, *189–199 (2020).
https://doi.org/10.1007/s13199-020-00712-w
Abstract

Symbiotic organisms such as corals are threatened by changing climate
because they are sensitive to stress, and may be unable to adapt quickly
due to the long host generation time. Instead of selecting for stress
tolerant hosts, manipulating their symbiotic microbes has been proposed
because microbes affect holobiont phenotypes and they have rapid life
cycles. Because it is time-consuming to isolate stress tolerant symbionts
from the wild even when appropriate types exist, mutation and selection of
symbionts is a promising alternative approach. Using green hydra (*Hydra
viridissima*) and symbiotic algae (*Chlorella variabilis* NC64A), we found
symbiont mutagenesis in vitro altered their UV-B resistance as well as that
of holobionts receiving mutated algae. In addition, hydra UV-B tolerance
was positively correlated to that of the algae they were hosting, as the
hydra associated with UV-B tolerant algal strains exhibited higher UV-B
resistance. However, chronic low-level UV-B selection decreased algal
resistance to acute high-level UV-B, which was unexpected, and did not
affect UV-B resistance of holobionts. The variations in algal UV-B
tolerance and hydra UV-B tolerance were largely due to mutagenesis rather
than selection. Our results suggest symbiont mutagenesis and trait-based
identification may be more effective than assisted evolution in holobiont
phenotype alteration, and it highlights the need to characterize symbiont
traits in vitro that are correlated to stress tolerance they can confer to
hosts, which may have application in conservation, agriculture and forestry.