Genetic basis of pigmentation loss and eye evolution in Asellus aquaticus

People in this lab are probably well aware of the interesting freshwater isopod Asellus aquaticus, who has contributed to two PhD-theses in our group by Fabrice Eroukhmanoff and Kristina Karlsson. Fabrice has published two thesis-papers on the quantitative genetic basis of ecotype divergence in body size and pigmentation, in Molecular Ecology and Journal of Evolutionary Biology (forthcoming).

What we demonstrated in these papers was that both morphological traits (size-traits) and pigmentation had significant additive genetic basis, both between and within populations. What we did not show, however, was the specific molecular-genetic basis of these heritable traits, i. e. which loci, how many and their genomic locations. That was not our goal, however, and Asellus aquaticus has not been a major model organism in molecular genetics.

But things seems to change. A forthcoming paper in PNAS by a US-based group presents a genome-wide linkage map involving 117 markers for Asellus aquaticus, and discuss the genetic mechanisms behind loss-of-pigmentation phenotypes and loss-of-vision phenotypes among blind, pale and cave-dwelling populations of this enigmatic species. The abstract is found below. I predict that the findings in that study will also have implications for ecotype divergence within lakes, such as between the ecotypes we have studied in southern Sweden.

Genetic basis of eye and pigment loss in the cave crustacean, Asellus aquaticus

Protas ME, Trontelj P, Patel NH.

Department of Molecular and Cell Biology, Center of Integrative Genomics, and Department of Integrative Biology, University of California, Berkeley, CA 94720-3200.

Abstract

Understanding the process of evolution is one of the great challenges in biology. Cave animals are one group with immense potential to address the mechanisms of evolutionary change. Amazingly, similar morphological alterations, such as enhancement of sensory systems and the loss of eyes and pigmentation, have evolved multiple times in a diverse assemblage of cave animals. Our goal is to develop an invertebrate model to study cave evolution so that, in combination with a previously established vertebrate cave system, we can address genetic questions concerning evolutionary parallelism and convergence. We chose the isopod crustacean, Asellus aquaticus, and generated a genome-wide linkage map for this species. Our map, composed of 117 markers, of which the majority are associated with genes known to be involved in pigmentation, eye, and appendage development, was used to identify loci of large effect responsible for several pigmentation traits and eye loss. Our study provides support for the prediction that significant morphological change can be mediated through one or a few genes. Surprisingly, we found that within population variability in eye size occurs through multiple mechanisms; eye loss has a different genetic basis than reduced eye size. Similarly, again within a population, the phenotype of albinism can be achieved by two different genetic pathways-either by a recessive genotype at one locus or doubly recessive genotypes at two other loci. Our work shows the potential of Asellus for studying the extremes of parallel and convergent evolution-spanning comparisons within populations to comparisons between vertebrate and arthropod systems.