oskar Predates the Evolution of Germ Plasm in Insects

Summaryoskar is the only gene in the animal kingdom necessary and sufficient for specifying functional germ cells [ 1 and 2]. However, oskar has only been identified in holometabolous (“higher”) insects that specify their germline using specialized cytoplasm called germ plasm [ 3]. Here we show that oskar evolved before the divergence of higher insects and provide evidence that its germline role is a recent evolutionary innovation. We identify an oskar ortholog in a basally branching insect, the cricket Gryllus bimaculatus. In contrast to Drosophila oskar, Gb-oskar is not required for germ cell formation or axial patterning. Instead, Gb-oskar is expressed in neuroblasts of the brain and CNS and is required for neural development. Taken together with reports of a neural role for Drosophila oskar [ 4], our data demonstrate that oskar arose nearly 50 million years earlier in insect evolution than previously thought, where it may have played an ancestral neural role, and was co-opted to its well-known essential germline role in holometabolous insects.

► First discovery of an oskar ortholog in a basally branching insect without germ plasm
► oskar is not required for germ cell specification or axial patterning in crickets
► Cricket oskar is required for embryonic neuronal specification and axon patterning
► oskar was co-opted into germ cell specification pathway in “higher” insects