Evolutionary change in metabolic rate of Daphnia pulicaria in response to the invasive predator Bythotrephes longimanus

Metabolic rate is a trait that can be hypothesized to evolve in response to a change in predation. In the current study, we address this question by utilising an invasive event by the predatory zooplankton Bythotrephes longimanus in Lake Mendota, Wisconsin, US. This invasion dramatically impacted the prey Daphnia pulicaria, causing a ~60% decline in their biomass. Using a resurrection ecology approach, we compared the metabolic rate of D. pulicaria clones originating from prior to the Bythotrephes invasion with that of clones having evolved in the presence of Bythotrephes. We observed a 7.4% reduction in metabolic rate among post-invasive clones compared to pre-invasive clones. This change is in the opposite direction to what might be expected to evolve in response to increased predation. The evolution of a lower metabolic rate may instead be due to a habitat shift in the prey species into deeper and less productive waters and associated changes in the optimal metabolic rate.

Draft Genomes for One Microcystis-Resistant and One Microcystis-Sensitive Strain of the Water Flea, Daphnia pulicaria

Daphnia species are well-suited for studying local adaptation and evolutionary responses to stress(ors) including those caused by algal blooms. Algal blooms, characterized by an overgrowth (bloom) of cyanobacteria, are detrimental to the health of aquatic and terrestrial members of freshwater ecosystems. Some strains of Daphnia pulicaria have demonstrated resistance to toxic algae and the ability to mitigate toxic algal blooms. Understanding the genetic mechanism associated with this toxin resistance requires adequate genomic resources. Using whole genome sequence data mapped to the Daphnia pulex reference genome (PA42), we present reference-guided draft assemblies from one tolerant and one sensitive strain of D. pulicaria, Wintergreen-6 (WI-6) and Bassett-411 (BA-411), respectively. Assessment of the draft assemblies reveal low contamination levels, and high levels (95%) of genic content. Reference scaffolds had coverage breadths of 98.9% - 99.4%, and average depths of 33X and 29X for BA-411 and WI-6, respectively. Within, we discuss caveats and suggestions for improving these draft assemblies. These genomic resources are presented with a goal of contributing to the resources necessary to understanding the genetic mechanisms and associations of toxic prey resistance observed in this species.

Transgenerational plasticity in the eye size of Daphnia

It is well established that environmental signals can induce phenotypic responses that persist for multiple generations. The induction of such ‘transgenerational plasticity’ (TGP) depends upon the ability of organisms to accurately receive and process information from environmental signals. Thus, sensory systems are likely intertwined with TGP. Here we tested the link between an environmental stressor and transgenerational responses in a component of the sensory system (eye size) that is linked to enhanced vision and ecologically relevant behaviours. We reared 45 clones of Daphnia pulicaria in the presence and absence of a low-quality resource (cyanobacteria) and evaluated shifts in relative eye size in offspring. Our results revealed divergent shifts in relative eye size within- and across-generations. Parental Daphnia that were fed cyanobacteria produced a smaller eye than Daphnia fed high-quality algae. Such differences were then reversed in the offspring generation; Daphnia whose mothers were fed cyanobacteria produced larger eyes than Daphnia that were continually fed green algae. We discuss the extent to which this maternal effect on eye size is an adaptive response linked to improved foraging.