Clonal erosion in overwintering populations of Daphnia pulex
Final Report Abstract
Daphnia are a key stone species in lake ecosystems and therefore a model system in ecology. Their ecological success is greatly due to their dual reproductive mode: females reproduce parthenogenetically when environmental conditions are favourable but switch to sexual reproduction and resting egg production in unfavourable conditions. Winter conditions in a lake are stressful because of food deterioration. Overwintering as ephippial egg would therefore seem to be a safe solution. Females surviving as parthenogens, however, have a large advantage during the first algal bloom in spring because they can start reproduction immediately. In lakes both overwintering strategies occur. To investigate how these alternative life-history strategies can co-exist we investigated the clonal composition of a Daphnia pulex population in a small mesotrophic lake in Norway. We hypothesized that clonal diversity should be low during the winter months due to high mortality rates but would be restored in the spring most likely from the resting egg bank. Daphnia were sampled every month from September 2007 to September 2010 and approximately 60 individuals per month were genotyped using 11 microsatellite markers. Marker resolution was high and in the 936 individuals genotypes 582 different genotypes could be detected. As expected the seasonal pattern of genotypic diversity was much higher during the summer months and lower during winter. Turnover rate of genotypes from month to month was high and only very few genotypes could be found for longer time intervals (more than one month). Newly emerging genotypes very likely came from the resting egg bank rather than from immigrants as genotypic variation was constituted by different combination of alleles already present in the population rather than by new alleles. A major drawback of our study was the illegal introduction of fish into the lake probably in the spring of 2007. Before that no pelagic predators had been present in the lake – one of the reasons why we picked this unique ecosystem for our studies. Unfortunately, the presence of fish predators in lake could only be verified in August 2008 when we had already sampled Daphnia for almost a year. The low individual number of D. pulex found in several investigated months and the unexpectedly high genotypic diversity in lake Myravann might therefore be influenced by predation pressure. Our data show that only a few D. pulex individuals survive the winter as parthenogenetic adults. Still within the cooperation of the University of Bergen we found no adaptations towards one of the over-wintering strategies. Females most likely use a mixed strategy and randomly invest in ephippia in the fall but still try to survive the winter as parthenogenetics.
Publications
- (2010) Coexisting overwintering strategies in Daphnia pulex: A test of genetic differences and growth responses. Limnology and Oceanography 55(5): 1893–1900
W. Lampert, K. P. Lampert & P. Larsson
