Divergent selection often contributes to the origin and maintenance of ecologically relevant trait variation within and among populations. However, phenotypic plasticity can lead to the same phenotypic differences among populations, making it challenging to isolate the causal environmental differences that underlie divergent selection from those that cause plastic responses. Moreover, phenotypic divergence among natural populations often involves multiple environmental gradients that can interact. I am using the freshwater isopod Asellus aquaticus as a model system to investigate how phenotypic variation is maintained through the interplay of natural selection and phenotypic plasticity in multiple ecological contexts.

Asellus aquaticus (male specimen, approx. 9 mm)

Variation in pigmentation in Asellus from Kastanienbaum, Switzerland
Variation in microhabitat: Chara macrophytes and sand patches 

Macrophytes and fish

Rapid evolution of cuticular pigmentation has been documented for Asellus aquaticus in several shallow lakes in southern Sweden (Hargeby et al. 2004/2005, Eroukhmanoff and Svensson 2009). Phenotypic divergence among populations is thought to be adaptive in response to fish predation pressure that is acting along a microhabitat gradient: submerged vegetation, providing structure and affecting background colour, may define which Asellus morphs - dark or light - have higher survival in the presence of visual predators. Using local Asellus populations from Lake Lucerne in Switzerland I am investigating how interactions between putative selective agents, macrophytes and fish, affect survival and phenotypic distributions.
In a large scale mesocosm experiment we established 50 isopod populations, exposed them for 6 months to factorial combinations of macrophyte presence and absence, and different fish densities. After phenotyping of over 4000 isopods, using computer vision techniques, we found strong effects of fish presence on isopod survival, but negligible effects on phenotypes. Instead, pigmentation was consistently stronger in the presence of macrophytes and independent of predator presence.

  • Eroukhmanoff, F., and E. I. Svensson. 2009. Contemporary parallel diversification, antipredator adaptations and phenotypic integration in an aquatic isopod. PloS one 4:e6173.
  • Hargeby, A., J. Johansson, and J. Ahnesjö. 2004. Habitat-specific pigmentation in a freshwater isopod: adaptive evolution over a small spatiotemporal scale. Evolution 58:81–94.
  • Hargeby, A., J. Stoltz, and J. Johansson. 2005. Locally differentiated cryptic pigmentation in the freshwater isopod Asellus aquaticus. Journal of Evolutionary Biology 18:713–721.

 

 Experimental mesocosms used to expose 50 isopod populations to different ecological contexts

Developmental plasticity in isopod pigmentation

Differences between environments can also lead to phenotypically plastic differences between populations. However, the role of plasticity in phenotypic differentiation of populations is hard to assess due to an often broad range of response types in nature. Common gaps of knowledge in the context of plasticity are the role of ontogeny and compensatory dynamics (Metcalfe and Monaghan, 2001), genetic variation in plasticity (Robinson and Beckerman, 2013), and trade-offs (Murren et al., 2015). Using the Asellus system I am investigating whether plasticity may explain some of the patterns of phenotypic variation that are found in natural populations. In highly controlled laboratory experiments I am investigating:

  1. Effects of diet (nutrients, aminoacids, and other compounds) on developmental processes and life history (growth, pigmentation, survival, fecundity).
  2. Genetic (family level) variation in plasticity of growth and pigmentation .
  3. Trade-offs between growth-rates and pigmentation under different dietary scenarios.
  • Metcalfe, N. B., and P. Monaghan. 2001. Compensation for a bad start: grow now, pay later? Trends in ecology & evolution 16:254–260.rnal of Evolutionary Biology 18:713–721.
  • Murren, C. J., J. R. Auld, H. Callahan, C. K. Ghalambor, C. A. Handelsman, M. A. Heskel, J. G. Kingsolver, H. J. Maclean, J. Masel, H. Maughan, D. W. Pfennig, R. A. Relyea, S. Seiter, E. Snell-Rood, U. K. Steiner, and C. D. Schlichting. 2015. Constraints on the evolution of phenotypic plasticity: limits and costs of phenotype and plasticity. Heredity 115:293–301.
  • Robinson, M. R., and A. P. Beckerman. 2013. Quantifying multivariate plasticity: genetic variation in resource acquisition drives plasticity in resource allocation to components of life history. Ecology letters 16:281–290.

 

a) Throughout their developmental phases isopods tend to increase their pigmentation, but that process can depend on environmental conditions: b) isopod reared on high nutrient diet. c) isopod reared on low nutrient diet (both are siblings)