Lineage-specific duplications and losses of spo -like genes in Daphnia and insects. An evolutionary scenario based on the observed distribution of spo-like (CYP307) genes in arthropods. Since insects are believed to have evolved from crustaceans living in freshwater environments  and the only spo-like gene observed in Daphnia belongs to the CYP307A subfamily, the ancestral arthropod spo gene was likely a CYP307A gene. An early duplication, which probably occurred after insects diverged from crustaceans, gave rise to spo (CYP307A1) and spot (CYP307B1). In Drosophila, the ancestral spo-like gene, referred to as spok (Cyp307a2), underwent a second round of duplication in which an intronless retrogene arose. In Drosophila this gene is referred to as spo. Note that another round of gene duplication, which occurred in the line of Drosophila evolution leading to the subgenus Drosophila , is not shown.