Long-lost relative claims orphan gene: oskar in a wasp.

Abstract

“To us as embyrologists and men the formation of an embryo has appeared to be everything, the history of the germ cells a secondary item of no particular moment. Nature, on the other hand, reverses the relative importance of the two, setting the germ-cells in the place of honour, as linking the remote past with the distant future.” In 1902, the vertebrate embryologist James Beard wrote these words in his monograph on germ cells in the skate Raja batis [1]. Over 110 years later, germ line specification and development are indeed major areas of investigation in the fields of developmental biology and evolution. August Weismann's description of the germ line as containing “unalterable accessory idioplasm” [2] may sound suspiciously mythical to modern readers. Nevertheless, we now know that in some animals, a special cytoplasm containing conserved gene products is indeed transmitted from oocyte to embryonic germ cells, and again to oocytes in the next generation. This “germ plasm” is necessary and sufficient for germ cell formation, and its molecular basis is best understood in the fruit fly Drosophila melanogaster. Germ plasm in some form has been described in oocytes and embryos of most “higher insects” (Holometabola: e.g., flies, wasps, and butterflies) as well as in other animals such as frogs and fish. However, in “lower insects” (Hemimetabola: e.g., grasshoppers, mayflies, and cockroaches) and in most other animals, nothing like germ plasm or inherited germ line determinants have been reported. In mice, the best studied example of such cases, inductive signals from specific somatic cells cause neighbouring cells to adopt germ cell fate. Comparative analyses suggest that most animals may use inductive signaling rather than germ plasm to specify germ cells, including animals branching close to the base of the animal tree (e.g., sponges and cnidarians). This has led to the hypothesis that the ancestral mechanism for animal germ cell specification may have been based on inductive signaling, meaning that germ plasm-driven mechanisms would have evolved independently several times in animal radiation [3]. How such a novel mechanism could have evolved remains unclear. In a recent paper [4], Jeremy Lynch and colleagues provide evidence that a critical component of germ plasm in insects is more ancient than previously thought, and that the driving force for this novel mechanism was the evolution of a novel gene.