Speciation with gene flow and the genetics of habitat transitions.

Whether speciation can advance to completion in the face of initially high levels of gene flow is a very controversial topic in evolutionary biology. Extensive gene exchange is generally considered to homogenize populations and counteract divergence. Moreover, the role of introgressive hybridization in evolution remains largely unexplored in animals, particularly in freshwater zooplankton in which allopatric speciation is considered to be the norm. Our work investigates the genetic structure of two young ecological species: the pond species, Daphnia pulex and the lake species, Daphnia pulicaria. Phylogenetic and population genetics analyses were conducted on mitochondrial NADH dehydrogenase 5 (ND5) gene, the nuclear Lactate dehydrogenase (Ldh) gene and 21 nuclear microsatellite markers in 416 individuals from habitats with various degrees of permanence. The strong and consistent phylogenetic discordance between nuclear and mitochondrial markers suggests a complex evolutionary history of multiple independent habitat transition events that involved hybridization and introgression between lake and pond Daphnia. On the other hand, the low level of contemporary gene flow between adjacent populations indicates the presence of effective habitat isolating barriers. The Daphnia system provides strong evidence for a divergence-with-gene flow speciation model that involves multiple habitat transition events.

An integrated multi-disciplinary approach for studying multiple stressors in freshwater ecosystems: Daphnia as a model organism.

The increased overexploitation of freshwater ecosystems and their extended watersheds often generates a cascade of anthropogenic stressors (e.g., acidification, eutrophication, metal contamination, Ca decline, changes in the physical environment, introduction of invasive species, over-harvesting of resources). The combined effect of these stressors is particularly difficult to study, requiring a coordinated multi-disciplinary effort and insights from various sub-disciplines of biology, including ecology, evolution, toxicology, and genetics. It also would benefit from a well-developed and broadly accepted model systems. The freshwater crustacean Daphnia is an excellent model organism for studying multiple stressors because it has been a chosen focus of study in all four of these fields. Daphnia is a widespread keystone species in most freshwater ecosystems, where it is routinely exposed to a multitude of anthropogenic and natural stressors. It has a fully sequenced genome, a well-understood life history and ecology, and a huge library of responses to toxicity. To make the case for its value as a model species, we consider the joint and separate effects of natural and three anthropogenic stressors-climatic change, calcium decline, and metal contaminants on daphniids. We propose that integrative approaches marrying various subfields of biology can advance our understanding of the combined effects of stressors. Such approaches can involve the measuring of multiple responses at several levels of biological organization from molecules to natural populations. For example, novel interdisciplinary approaches such as transcriptome profiling and mutation accumulation experiments can offer insights into how multiple stressors influence gene transcription and mutation rates across genomes, and, thus, help determine the causal mechanism between environmental stressors and population/community effects as well as long-term evolutionary patterns.