oskar displays a unique evolutionary origin resulting from a Horizontal Domain Transfer from a probably bacterial endosymbiont onto an ancestral insect genome. The OSK domain is of bacterial origin and fused with the LOTUS domain through a linker domain. This event must have happened just prior to the divergence with the Crustacean, the insect's sister group, as oskar can be found as early as the Zygentoma but does not seem to exist in Crustacean.
