Decreased detoxification genes and genome size make the human body louse an efficient model to study xenobiotic metabolism

The human body louse, Pediculus humanus humanus, has one of the smallest insect genomes, containing similar to 10 775 annotated genes. Annotation of detoxification [cytochrome P450 monooxygenase (P450), glutathione-S-transferase (GST), esterase (Est) and ATP-binding cassette transporter (ABC transporter)] genes revealed that they are dramatically reduced in P. h. humanus compared to other insects except for Apis mellifera. There are 37 P450, 13 GST and 17 Est genes present in P. h. humanus, approximately half the number found in Drosophila melanogaster and Anopheles gambiae. The number of putatively functional ABC transporter genes in P. h. humanus and Ap. mellifera are the same (36) but both have fewer than An. gambiae (44) or Dr. melanogaster (65). The reduction of detoxification genes in P. h. humanus may be a result of this louse's simple life history, in which it does not encounter a wide variety of xenobiotics. Neuronal component genes are highly conserved across different insect species as expected because of their critical function. Although reduced in number, P. h. humanus still retains at least a minimum repertoire of genes known to confer metabolic or toxicokinetic resistance to xenobiotics (eg Cyp3 clade P450s, Delta GSTs, B clade Ests and B/C subfamily ABC transporters), suggestive of its high potential for resistance development.