Response to Increase in Salinity in a Tropical Euplotes Species

The fact that ciliates of the Euplotes genus are widely distributed in different environmental conditions including extreme temperatures suggests that they have evolved specific adaptation strategies. We collected a strain of Euplotes rariseta from a tropical sandy beach of Misali Island annexed to Pemba Island in the Zanzibar Archipelago, Tanzania. The strain was found to be able to form cysts in response to increased sea water salinity. This behavior can be interpreted as an adaptation strategy evolved to respond to the frequent increase in salinity during tide changes. This same behavior has been previously reported for strains of E. rariseta from an equatorial area (Dallai et al., Monit Zool Ital,1987.). In the Pemba strain, cysts started to form when the salinity reached 40%o and their number increased up to a salinity of 70%o for about 50% of the cell culture. The cells that did not encyst stopped their division and then gradually died. Cysts were found to resist for at least a month in high salinity and normal cells rapidly excysted when salinity went back to 35%o. To investigate whether a specific sea water ion was responsible for triggering the encystment process, we tested the increase in concentration of NaCl, MgCl2 and CaCl2 in artificial sea water. While CaCl2 only impaired the ciliary beating and MgCl2 had a similar but weaker effect, the increase in NaCl, even at a total salinity of 40%o, rapidly promoted encystment in about 50% of the cell culture. In order to obtain an expression profile of E. rariseta cells at different stages of the encystment process, we extracted RNA from cells at increasing salinity, from 30%o to 55%o, and we are studying RNA expression levels by RNAseq with Illumina sequencing, followed by a differential gene expression analysis. Changes in expression levels of expected stress genes were also measured by quantitative PCR.