The components of the Daphnia pulex immune system as revealed by complete genome sequencing

We found that some immune pathways, in particular the TOLL pathway, are fairly well conserved between insects and Daphnia, while other elements, in particular antimicrobial peptides, could not be recovered from the genome sequence. We also found considerable variation in gene family copy number when comparing Daphnia to insects and present phylogenetic analyses to shed light on the evolution of a range of conserved immune gene families.All metazoans appear to have an innate immune system based on a distinct set of gene products that play recognition, regulatory and response roles [1]. However, within this set of genes, there is striking diversity, as the enormous spectra of host habitats and specialist biological enemies drive the evolution of immune systems [2-6]. To date, knowledge of immune system function in arthropods has been based on a very few model organisms, and the majority of studies have used two members of a single insect order, specifically the Dipterans Drosophila melanogaster and Anopheles gambiae. While the genes associated with immunity are now being characterised in other insect genomes [7,8], these additional exemplars do not yet span the diversity of Arthropoda. To gain fuller understanding of both the evolutionary origins of the insect immune system, and of any underlying patterns in arthropod immune system function and diversification, it is necessary to examine species in other arthropod subphyla, such as the Crustacea.Daphnia (waterfleas, Family Daphniidae, Order Branchiopoda) species are employed as model organisms for a wide range of evolutionary and ecological topics, including the evolution of immunity, coevolution and virulence [4,9-14]. Daphnia and many of their parasites can be manipulated experimentally, and laboratory experiments have revealed a wealth of genetic diversity for responses to infection [9]. Especially useful is the fact that Daphnia are cyclical parthenogens, and thus can be maintained in the lab as clonal lineages, e