Quantitative genetics of phosphorus content in the freshwater herbivore, Daphnia pulicaria.

Phosphorus (P) is essential for growth of all organisms, and P content is correlated with growth in most taxa. Although P content was initially considered to be a trait fixed at the species level, there is growing evidence for considerable intraspecific variation. Selection on such variation can thus alter the rates at which P fluxes through food webs. Nevertheless, prior work describing the sources and extent of intraspecific variation in P content were not genetically explicit, confounded by unknown genetic background and evolutionary history. We constructed an F2 recombinant population of the dominant freshwater grazer, Daphnia pulicaria to mitigate such issues. F2 recombinants exhibited considerable variation in growth rate, P content (0.49%-1.97%), P use efficiency (PUE; 51-208 mg biomass/mg P), and correlated traits such as hatching time of resting eggs, in common garden conditions. These results clearly demonstrate the scope of genetic recombination in generating variation in ecologically relevant traits. The absence of environmental selection is a likely component driving such variation not observed in natural settings. Although phosphoglucose isomerase (PGI) genotype was significantly associated with variation in hatching time of resting eggs, contrary to prior work with less rigorous designs, and allelic variation at the PGI locus did not explain variation in P content and PUE of Daphnia, indicating that such quantitative traits are under polygenic control. Together, these results suggest that although there is considerable genetic scope for variation in key ecologically relevant traits, such as P content and efficiency of P use, these traits are likely under strong stabilizing selection, most likely due to selection on growth rate and size. Importantly, our observations suggest that anthropogenic alterations to P supply due to eutrophication could alter selection on these traits, thereby rapidly altering the role Daphnia plays in the P cycle of lakes.

Phototactic behavior of native Daphnia in the presence of chemical cues from a non-native predator Bythotrephes.

Chemical cues are used by many taxa to communicate within and among species. Behavioral defenses induced by predator cues are a mechanism by which prey species resist or avoid predator attack. This study examined the egg bank of native Daphnia species in a lake that has been invaded by Bythotrephes longimanus, an invertebrate zooplanktivore native to northern-central Europe and Asia (initial invasion 1994, population boom in 2009). Daphnia resting eggs from both pre- and post-B. longimanus invasion lake sediments were hatched and established as isofemale clonal lines. Phototactic behavior (a proxy for vertical migration behavior) was assessed in the presence and absence of B. longimanus cue. This was done to evaluate the hypothesis that the heavy predation imposed by B. longimanus would have been selected for Daphnia clones that are more negatively phototactic in the presence of B. longimanus cue, because B. longimanus is a visual predator. The behavior of the clones derived from pre-B. longimanus era resting eggs was not significantly different from the behavior of the clones from the post-B. longimanus era and exposure to predator cue did not affect the phototactic response of the clones. There was a significant difference in the phototactic behavior of the three Daphnia species tested (Daphnia ambigua, Daphnia mendotae, and Daphnia pulicaria). These results suggest that predation by B. longimanus is not the main factor that is influencing the phototactic behavior of Daphnia in the lake. Other factors such as fish predation may be playing a more significant role in this system.