| Summary: | <p>Abstract</p> <p>Background</p> <p>Cytochrome P450s (CYPs) in animals fall into two categories: those that synthesize or metabolize endogenous molecules and those that interact with exogenous chemicals from the diet or the environment. The latter form a critical component of detoxification systems.</p> <p>Results</p> <p>Data mining and manual curation of the <it>Daphnia pulex </it>genome identified 75 functional CYP genes, and three CYP pseudogenes. These CYPs belong to 4 clans, 13 families, and 19 subfamilies. The CYP 2, 3, 4, and mitochondrial clans are the same four clans found in other sequenced protostome genomes. Comparison of the CYPs from <it>D. pulex </it>to the CYPs from insects, vertebrates and sea anemone (<it>Nematostella vectensis</it>) show that the CYP2 clan, and to a lesser degree, the CYP4 clan has expanded in <it>Daphnia pulex</it>, whereas the CYP3 clan has expanded in insects. However, the expansion of the <it>Daphnia </it>CYP2 clan is not as great as the expansion observed in deuterostomes and the nematode <it>C. elegans</it>. Mapping of CYP tandem repeat regions demonstrated the unusual expansion of the CYP370 family of the CYP2 clan. The CYP370s are similar to the CYP15s and CYP303s that occur as solo genes in insects, but the CYP370s constitute ~20% of all the CYP genes in <it>Daphnia pulex</it>. Lastly, our phylogenetic comparisons provide new insights into the potential origins of otherwise mysterious CYPs such as CYP46 and CYP19 (aromatase).</p> <p>Conclusion</p> <p>Overall, the cladoceran, <it>D. pulex </it>has a wide range of CYPs with the same clans as insects and nematodes, but with distinct changes in the size and composition of each clan.</p>
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