Arthropod CYPomes illustrate the tempo and mode in P450 evolution.

The great diversity of P450 genes in a variety of organisms is well documented but not well explained. The number of CYP genes in each species is highly variable and this is shown here for arthropod, mainly insect CYPomes. Pairs of recognizable orthologs are but a small portion of the CYPome, but species- or lineage-specific expansions of CYP subfamilies are consistently observed. These "blooms" of CYP genes have their origin in multiple gene duplications, although some subfamilies expand and others do not. Stochastic birth and death models of CYP gene proliferation are sufficient to explain blooms, and speciation events may play important roles in CYPome diversity between lineages. Mitochondrial clan P450s are a monophyletic group of genes that has seen several blooms in insects, but apparently not in vertebrates.