Conservation and divergence of chemical defense system in the tunicate Oikopleura dioica revealed by genome wide response to two xenobiotics

Oikopleura genome surveys for key genes of the chemical defensome suggested a reduced repertoire. Not more than 23 cytochrome P450 (CYP) genes could be identified, and neither CYP1 family genes nor their transcriptional activator AhR was detected. These two genes were present in deuterostome ancestors. As in vertebrates, the genotoxic compound BaP induced xenobiotic biotransformation and oxidative stress responsive genes. Notable exceptions were genes of the aryl hydrocarbon receptor (AhR) signaling pathway. Clo also affected the expression of many biotransformation genes and markedly repressed genes involved in energy metabolism and muscle contraction pathways.Oikopleura has the smallest number of CYP genes among sequenced animal genomes and lacks the AhR signaling pathway. However it appears to have basic xenobiotic inducible biotransformation genes such as a conserved genotoxic stress response gene set. Our genome survey and expression study does not support a role of AhR signaling pathway in the chemical defense of metazoans prior to the emergence of vertebrates.Animals protect themselves from xenobiotic or endogenous harmful chemicals by biotransformation and disposition of the compounds. Chemical defense mechanisms are well characterized in vertebrates. Recent comparisons of genomes from distantly related metazoans suggest conservation of the chemical defensome [1-3]. Oikopleura dioica belongs to larvacean tunicates that have considerable importance in the marine ecosystem and for the vertical flux of carbon in the form of discarded food-filtering house [4,5]. Oikopleura is surrounded by a complex mucopolysacharide house used to filter food particles from a large volume of water [6,7], and the therefore it can potentially be affected by marine pollution. O. dioica can be kept in culture for many generations and has a very short generation time (6 days at 15°C) [8]. Although tunicates are the closest living relatives of vertebrates [9], a recent study revealed