Roles of carbohydrate reserves for local adaptation to low temperatures in the freeze tolerant oligochaete Enchytraeus albidus
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Geographic variation in cold tolerance and associated physiological adaptations were investigated in the freeze tolerant enchytraeid Enchytraeus albidus (Oligochaeta). Specimens from Svalbard, Greenland (Nuuk), Iceland (Hólar and Mossfellsbær) and continental Europe [Norway (Bergen), Sweden (Kullen) and Germany] were reared in the laboratory in a common-garden experiment. The aim was to test for variations in minimum lethal temperature, freeze duration tolerance, carbohydrate reserves and metabolic rate among the populations. Cold tolerance was related to environmental temperature of the respective location. Populations from the coldest climatic regions were able to tolerate freezing down to at least −15 °C and endured being frozen at −5 °C for 27–48 days, respectively. Populations from milder climates had a lower freeze duration tolerance (about −9 °C) and endured −5 °C for a shorter period (between 9 and 16 days). Glucose accumulation and glycogen reserves varied significantly between populations, but was not related directly to cold tolerance. Metabolic rate varied significantly between populations, but was not significantly related to cold tolerance. The metabolic rates at −2 °C of frozen and unfrozen worms from Germany and Svalbard were tested. The metabolic depression due to freezing of these populations was relatively small (<50 %), suggesting that the large carbohydrate accumulations may also be important as fuel during long-term freezing at moderately low temperatures. Differences in metabolic depression may partly explain the difference in cold tolerance of these two populations, however, the mechanisms behind local adaptation to low winter temperatures in these enchytraeid populations seem more complex than earlier studies have indicated.
KeywordsCryoprotectant Glucose Metabolic rate Cold tolerance Freeze duration
We thank Prof. Bent Christensen, University of Copenhagen, Denmark, for the verification of the taxonomy of the enchytraeids and Vladimir Kostal, University of South Bohemia, Czech Republic, for constructive comments on an earlier version of the manuscript. Further, we thank Hans Malte, Aarhus University, Denmark, for helping with analysing CO2 data. This study was supported by Sapere Aude DFF-Starting grants from The Danish Council for Independent Research (J.O. and J.G.S.) and Danish Research Council (M.H.).
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