Coexisting migration strategies in Daphnia:
Do strong environmental gradients allow for multiple migration behaviours?
From grade school demonstrations to cutting edge research, Daphnia species have been a workhorse of the eco-evolutionary world for decades! Since much is known about their life-history and ecology, Daphnia are an ideal species for testing higher order processes such as the ecological and evolutionary factors most important to the maintenance of diversity. The Round Lake system is particularly useful because it has seasonally co-occurring and visually distinct phenotypes of Daphnia that undergo different diel vertical migration behaviours. These populations have overlapping distributions and share much of the same predators and food resources, which begs the question, “Why don’t all of the Daphnia use the same migration strategy?” This system provides one of the first directly observed cases of distinct and spatially overlapping phenotypes in a natural Daphnia population, which makes the opportunities for testing theories of coexistence and maintenance of diversity with this system exciting! Using field mesocosm experiments and newly developed techniques for controlling Daphnia migration in the field, we hope to identify the mechanisms maintaining Daphnia diversity in Round Lake.
They are small, but they are many! The tea tortrix Adoxophyes honmai is one of the most damaging pests to Japanese tea plantations. The larvae damage the tea by both consuming the leaves for food, and using silk to roll leaves into protective homes. The problem for tea plantations—and the scientific interest for ecologists—is that the populations undergoes multiple outbreak cycles each year. Our recent work found that the outbreaks are large amplitude cycles (‘limit cycles’ in math-speak) caused by the increased life-history rates that occur during warmer parts of the season. But why should evolution favour a life history that gives rise to outbreak cycles? Along with collaborators in the US and Japan, we are using a combination of experiments, time-series analysis and mathematical modelling to study the ultimate cause of these outbreak cycles.
When the charismatic microfauna Daphnia magna is infected with the bacterium Pasteuria ramosa, it undergoes a dramatic alteration of its life history: it stops reproducing and increases in size to up to 200% of its normal biomass. Led by Dr. Clay Cressler, we have been investigating how castration and gigantism result from the parasite redirecting within-host energy to increase its access to resources. Using a novel starvation experiment, we have shown that parasites tap into the energy allocated to growth. The parasite uses castration to increase the energy flow to growth, and gigantism is a passive by-product of this reallocation. We are currently developing qPCR primers that will allow us to track the within-host dynamics of the parasite, as it is unclear which of the parasite’s developmental stages is actually proliferating within the host. Comparison of the dynamics of parasite population growth with host reproduction and growth will give us a much deeper insight into how variation among host genotypes affects infection outcome.
Food opportunities in Round Lake:
How do they influence phenotype and behaviour in Daphnia pulicaria?
Round Lake is oligotrophic and to seek refuge from predators (i.e. fish) the adults need to occupy depths below the warm epilimnion and above the cool, anoxic bottom waters. In summer, when phytoplankton are moderately abundant, pale phenotypes show a strong diel vertical migration (Ariel Gittens, MSc thesis). As summer progresses into fall the population shifts in the number of pales to a mix of hemoglobin-rich phenotypes and pales. This is either in response to a waning phytoplankton supply, to an increase in food in the low oxygen zone, or they have reached a ‘life stage’ where overwintering is in the cards. Certainly “overwintering longevity” will favour the surviving genotypes over the genotypes hatching from a resting egg bank. By looking at the time progression of genotypes through each season in conjunction with available food (carbon and chlorophyll A) we hope to link their behaviours to the most influential changes in their environment.
Daphnia reproduce asexually, so when genetically identical individuals are placed in the same environment, why do we see positive correlations among life history traits? Work from Stefan and Adriana, has investigated the correlated response of life history traits such as growth, reproduction, and survival to a gradient of food quantity and quality. Life history theory predicts that expensive bodily functions like growth and reproduction will be negatively correlated, reflecting trade-offs in the resource budget. Where positive correlations are found, they have been ascribed to variation among individuals in resource acquisition: the greater the amount of resources coming in, the more can be spent. Our experiments have prevented that resource variation and genetic variation, but still show positive correlations, which we believe may be the result of differences in the step in between acquisition and allocation, which we call utilization.
Traditionally, research on evolutionary consequences of community interactions have focused on direct community interactions those interactions imposed on one species by another species, with very little focus on indirect community interactions; those interactions imposed on one species by an intermediating third species. Unfortunately, by focusing only on direct community interactions, it assumes that ecological community interactions have little influence on evolution. Food webs exist within communities and individual species within food webs are not only faced with the direct ecological interactions and environmental change, but also the shifting pressures from indirect interactions with other members of the community which are themselves responding to direct ecological interactions and environmental change. The evolutionary responses to these shifts in pressure may change with indirect effects of predators and competitors. For example, the presence of a competitor may indirectly alleviate predation pressure enabling coexistence between two competitors and in the absence of such an indirect interaction, coexistence would not occur. Additionally, a predator feeding on its prey may be indirectly affected by the quality of food the prey feeds on and as a result shifts its prey choice to a higher quality prey. Given, population densities and resource availability are known selection agents and the two classes of indirect ecological interactions, density and trait mediated effects, it is unrealistic to exclude indirect community interactions from evolutionary studies involving food web communities.