title: Oskar predates the evolution of germ plasm in insects.
abstract: oskar is the only gene in the animal kingdom necessary and sufficient for specifying functional germ cells. However, oskar has only been identified in holometabolous ("higher") insects that specify their germline using specialized cytoplasm called germ plasm. 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, 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.
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.
authors.forename: Ben
authors.initials: B
authors.lastname: Ewen-Campen
authors.forename: John R
authors.initials: JR
authors.lastname: Srouji
authors.forename: Evelyn E
authors.initials: EE
authors.lastname: Schwager
authors.forename: Cassandra G
authors.initials: CG
authors.lastname: Extavour
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology; D005075:Biological Evolution; D019521:Body Patterning; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics*; D005854:Germ Cells / Q000502:physiology; D006135:Gryllidae / Q000196:embryology / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D009420:Nervous System / Q000196:embryology
chemical_list: D029721:Drosophila Proteins; C091675:osk protein, Drosophila