Linking allocation to sexual reproduction with resource use efficiency 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.  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 tested the hypothesis that allocation to sexual reproduction is correlated with resource use efficiency by conducting laboratory assays on clones collected from multiple lakes in spring and/or fall of multiple years (2006, 2011, and 2012).  We predicted that “powerful” genotypes will (1) be more prevalent in the spring compared to the fall collections and (2) will show increased allocation to sexual reproduction relative to “efficient” genotypes.

Results/Conclusions

We have observed persistent among-lake variation in allocation to sexual reproduction in Daphnia pulicaria populations in Michigan lakes.  Across seven populations, we found a significant negative correlation between the average population-level investment in male production (1999-2001) and the proportion of genotypes that did not produce males (collected in spring of 2006).  In laboratory assay of genotypes from 2006, 0-25% of each population did not allocate to the production of males.  In recent laboratory assays (of genotypes collected in the fall of 2011) between 50-73% of genotypes produced only female offspring and a maximum of only 36% per lake invested in haploid eggs.  Our previous studies also indicated far more among-genotype variation in clonal growth rate under high resource conditions than under lower resources.  Feeding assays for the clones collected in 2011 and 2012 are ongoing, as is the analysis linking allocation of sexual reproduction to feeding. As natural populations often maintain a surprising amount of genetic diversity within ecologically relevant traits, these variations in life history strategies offer insight into how links between foraging-related traits and life history traits may act to maintain genetic variation in a population.