Daphnia and arthropod phylogeny
The D. pulex genome also has the potential to contribute to resolving long-standing debates on arthropod phylogeny. Current views of arthropod phylogenetic relationships are based mainly on two types of datasets - molecular genetic data and morphological characters - and this has led to partly contradictory concepts of arthropod phylogeny. There is now almost universal agreement-that arthropods derive from a common ancestor, and that crustaceans and insects are sister groups [13]. However, some issues of arthropod relationships remain controversial, for example the question of whether insects, crustaceans and myriapods form a monophyletic-group. Crustaceans show the greatest diversity of body organization and development among arthropods [14] and therefore the phylogenetic relationships within the crustaceans are far from being resolved. Several morphological and molecular studies have questioned the monophyly of crustaceans, and either Branchiopoda (such as Daphnia) or Malacostraca (lobster, shrimps) has been hypothesized to be the sister group to insects [15]. Some recent molecular analyses suggest a sister group relationship of myriapods (millipedes) and chelicerates (spiders) [16]. Interestingly, this suggestion is supported by recent morphological and molecular studies on the development of the nervous system that reveal a surprising degree of similarity between myriapods and chelicerates [17,18]. The morphological support for an insect-crustacean sister-group relationship is mainly based on the comparative analysis of neural characters in higher crustaceans (malacostracans) and insects. For example, in both insects and malacostracans, stem-cell-like neuroblasts have been detected that divide asymmetrically to generate the cells that contribute to the nervous system [14]. But are these neural characters representative of all crustacean groups? Are homologous genes required for the development and the function of the nervous system? With the availability of a branchiopod genome and the development of genetic tools for Daphnia these questions can now be addressed.
Furthermore, using genome sequences of a wide range of organisms, the origin and evolution of neural signaling pathways can be traced, which will broaden our understanding of the evolution of nervous systems. The neurotrophin signaling pathway plays a role in neural development, regeneration and neural plasticity in mammals. Analyzing the Daphnia genome, Wilson [11] shows for the first time that a neurotrophin and both a tyrosine receptor kinase (Trk) and a p75-type neurotrophin-receptor (p75NTR) are present in a protostome, indicating that this pathway existed in the last common ancestor of protostomes and deuterostomes.
To conclude, the initial exploration of the D. pulex genome outlined above proves that with the availability of the genome sequence Daphnia research has entered a new era. New and long-standing questions in ecology and evolution can be addressed and it may finally be possible to link evolutionary and environmental adaptations to the underlying genetic processes.