Local adaptation among geographically distant clones of the cosmopolitan freshwater ciliate Meseres corlissi.: I.: Temperature response

We used the oligotrich ciliate Meseres corlissi as a model organism to test for local, ecophysiological adaptation among globally distributed, but rare, freshwater ciliates. We investigated the temperature response of 8 M corlissi clones originating from, 2 temperate localities in Austria, 1 clone from a (sub)tropical habitat in China and 1 clone from a warm-temperate habitat in Australia. The Austrian clones were isolated either in late autumn to early winter or in summer. The results were compared to those of a previous investigation with a M, corlissi isolate from the tropical Dominican Republic. The temperature reaction norm was investigated in laboratory cultures with a small cryptophyte Cryptomonas sp. as food at saturating food levels. Experiments were conducted at temperatures ranging from 7.5 to 30 degrees C. We measured growth rate, cell volume, and cellular production at each temperature. We found statistically significant differences in the thermal sensitivity of several clones that increased with increasing geographical distance and habitat difference. Overall, the temperature reaction norm (population growth rate vs. temperature) of the temperate Austrian isolates was statistically significantly different from that of the warm-temperate Australian clone, and from the tropical and subtropical isolates from the Dominican Republic and China, respectively. The minimum temperature tolerated by the clones was positively related to the minimum temperature encountered in their natural habitats. Our study provides evidence that freshwater ciliates are physiologically adapted to their respective climatic habitat.