Monday, January 2, 2012

Evolutionary consequences of alewife divergence

As described in several posts on this blog, recent research has shown that evolutionary diversification (in species such as alewives and sticklebacks) can differentially impact ecological processes. Such work is important because it sets the stage for there to be an ongoing feedback between ecological and evolutionary forces within these systems. However, natural systems are inherently complex and genetic divergence in one organism, and associated ecological impacts of these changes, may alter the selective landscape and promote a series of evolutionary changes that propagate throughout the food web. The potential significance of such a ‘cascade of evolutionary change’ has been the focus of our recent research on interactions between populations of a fish predator, the alewife (Alosa pseudoharengus), and their zooplankton prey.

In lakes in Connecticut, the presence or absence of passages to the coastal ocean lead to lakes with anadromous alewives that migrate seasonally between marine and freshwater environments or lakes with permanent populations of landlocked alewives. Adult anadromous alewives migrate into lakes to spawn during each spring (March-May), and young-of-the-year (YOY) alewives spend ~6 months in freshwater before migrating back to the ocean each fall. In these lakes, large zooplankton, such as Daphnia, are abundant each spring, but are eliminated by alewife predation in early summer. Conversely, landlocked alewives are present in lakes year-round and, therefore the abundance of large zooplankton is consistently low. Previous work has shown that small-bodied zooplankton (i.e., Bosmina) dominate the zooplankton community in lakes with landlocked alewives and has promoted morphological divergence (gill raker spacing, gape size, preferred prey size) of landlocked alewife from anadromous ancestors.

Our first step was to determine if variation among these populations of alewives (and the ecological impacts of this variation) drive evolutionary divergence in their prey. We collected multiple populations of a single species of zooplankton (Daphnia ambigua) from lakes with anadromous and landlocked alewives (and also lakes without any alewife) and compared them for life history variation after several generations of rearing in a common environment. We found that Daphnia from lakes with anadromous alewife differ very strongly for many life history traits than Daphnia from lakes with landlocked alewife or no alewife. Furthermore, Daphnia from lakes with anadromous alewife typically displayed ‘higher fitness’ for the traits that were evaluated; they grew faster, developed more rapidly, and had larger clutches than Daphnia from lakes with landlocked lakes. The causal mechanism is likely related to variation in the annual migratory cycles of alewife and the known differences in lake seasonality. Daphnia in lakes with anadromous alewife are only present during the spring (when alewife predation intensity is low), and lakes in Connecticut are colder during the spring than summer. As a result, these evolutionary changes are best explained as an adaptation to a very short growing season and colder environment (i.e. countergradient variation) that are an indirect byproduct of intense seasonal predation by anadromous alewives.

We also explored the impact of alewife variation on the evolution of phenotypic plasticity in Daphnia. The presence of predators and the chemicals released by predators (kairomones) induce phenotypic changes in the life history, morphology, and behavior of prey. For instance, Daphnia typically develop faster and produce more offspring in the presence of fish chemical cues. A key requirement for the evolution of such plasticity is the existence of spatial and temporal environmental heterogeneity. Theory indicates that plasticity is favored when the environment is variable but predictable. In our lakes, landlocked alewives are present year-round, while anadromous alewives migrate predictably between marine and freshwater environments. Because these differences in migratory behavior have divergent impacts on Daphnia abundances, intraspecific variation in alewives may influence the evolution of plasticity in Daphnia. To explore this possibility, we reared Daphnia from lakes with and landlocked and anadromous alewives in the presence and absence of alewife chemical cues. This work revealed evidence for differences in plasticity among our focal populations of Daphnia. Daphnia from lakes with anadromous alewife responded much more strongly to the presence of alewife chemical cues for several life history traits (clutch size, size at maturation, growth, etc.) including the extent to which they engage in sexual reproduction (Daphnia alternate between asexual and sexual bouts of reproduction).

Overall, this research has revealed significant evolutionary consequences of intraspecific variation in alewives. The next step in this line of research is to determine if these evolutionary interactions between alewife and zooplankton have an ecological imprint in lakes.

Walsh, M. R., and D. M. Post. 2012. The impact of intraspecific variation in a fish predator on the evolution of phenotypic plasticity and investment in sex in Daphnia. Journal of Evolutionary Biology 25: 80-89

Walsh, M. R., and D. M. Post. 2011. Interpopulation variation in a fish predator drives evolutionary divergence in prey in lakes. Proceedings of the Royal Society B 278: 2628-2637

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