Comparison of population growth rates with anhydrobiotic survival rates across multiple temporal and spatial scales in a Habrotrocha rosa metapopulation
Anhydrobiosis, the phenomenon in which organisms undergo complete desiccation then rehydration, has been thoroughly studied in tardigrades and to a lesser extent some rotifer species. We examined the bdelloid rotifer Habrotrocha rosa which thrives within the rainwater filled pitcher-shaped leaves of Sarracenia purpurea. This carnivorous plant ranges widely throughout North America and, in some areas, experiences midsummer drought-like conditions. During these periods, pitchers often remain empty for a week or more after opening. We were interested in examining recovery from desiccation in two rotifer life history types (fast vs. slow population growth rates) within a bog in southeastern Wisconsin. In addition, we examined the metabolic enzyme Phosphoglucose Isomerase (PGI). We hypothesized that variation in both H. rosa life history and anhydrobiotic survival is correlated with PGI isozyme. Our sampling scheme was designed to provide samples across time as well as multiple spatial scales throughout the bog. Three rotifers were randomly selected from each pitcher sampled, and each became the foundress of a clone representing one H. rosa genotype that was present in the pitcher on the date of collection. We examined PGI isozymes by cellulose acetate gel electrophoresis and evaluated anhydrobiotic survival by rehydrating clones four days after complete desiccation.
We found a statistically significant difference between life history types (fast and slow growing) in terms of anhydrobiotic survival (t = 19.617, p <0.0001). Slow growing clones had fewer survivors after desiccation and rehydration (mean = 8.51 + 0.460 survivors), while fast growing clones had more survivors (mean = 19.49 + 0.481 survivors). We initially believed that this could be due to variations in PGI, which plays a key role in glucose metabolism and the resupply of ATP and could thereby influence reproductive rates. Furthermore, because mitochondrial energy production has been cited as essential for anhydrobiotic survival, variation in PGI could influence the ability to survive in waterless pitcher habitats during mid-summer dry spells. However, all of the H. rosa clones exhibited identical heterozygous genotypes for PGI regardless of growth rate. There was slight variation in the intensity of the protein bands which we believe to be the result of differences in sample density. We suggest that faster growing clones have higher metabolic rates and produce more of the bioprotectants that offset damages associated with complete desiccation. These clones may also be more successful at undergoing the anatomical changes associated with formation of the dehydrated tun.