Power and efficiency: trade-offs in resource use and links to dormancy in a facultative parthenogen, Daphnia pulicaria

Background/Question/Methods

Trade-offs are often invoked to explain the maintenance of genetic variation in natural populations, yet how ecological and evolutionary processes interact to maintain this diversity is often poorly understood.  Daphnia pulicaria inhabiting lakes in Michigan display both among- and within-population variation in two seemingly unrelated traits: sensitivity to resource fluctuation and allocation to sexual reproduction, which is also allocation to dormancy.  In terms of feeding, some genotypes are “powerful”, meaning they are superior competitors when resource levels are high (spring) but suffer reductions in growth rate when resources decline in summer and fall.  Other genotypes are “efficient”, meaning they are less sensitive to fluctuations in resources, but are inferior competitors during the spring when resources are plentiful.  During allocation to sexual reproduction in spring, some genotypes invest more in males and haploid eggs than do others. We collected D. pulicaria genotypes from 7 lakes in spring and summer raising all genotypes in the laboratory on spring and summer-like resource conditions.  We tested the hypothesis that variation in both resource acquisition traits and dormancy strategies are maintained via a series of tradeoffs.  We predict that trade-offs exist in genotypes between resource sensitivity, minimum resource requirements, and investment in dormant offspring.

Results/Conclusions

Previous work has demonstrated predictable temporal variability in resources from spring to summer, with the highest resources observed in spring.  Our previous studies indicate far more among-genotype variation in clonal growth rate under high resource conditions than under lower resources.  Ongoing feeding assays demonstrate both within- and among lake variation in sensitivity to resources.  Results of our laboratory assays indicate that genotypes collected from all 7 lakes in the spring and 3 lakes with persistent populations in the summer contain both powerful and efficient clones.  In some lakes, Daphnia pulicaria does not persist into the summer (therefore investing in dormancy as a means of temporal escape).  We have observed persistent among-lake variation in allocation to sexual reproduction in Daphnia pulicaria populations including a significant negative correlation between the average population-level investment in male production and the proportion of genotypes that did not produce males.  Variations in life history strategies of resource exploitation and reproduction offer insight into the maintenance of genetic diversity of ecologically relevant traits in natural populations.