"Evolution of the Germ Line: Conservation and Divergence of Molecular Mechanisms"

Germ cells in many animals, including Drosophila, are specified by maternal deposition of localized “germ plasm” that contains molecular determinants of germ cell fate. However, basally branching members of many animal clades are classically described as lacking germ plasm. We use molecular markers and functional genetics in non-traditional model systems to approach these problems. First, we explore the possible evolutionary origins of germ plasm and the role of the oskar gene in germ cell formation. oskar is both necessary and sufficient for specifying germ cells in Drosophila, and was previously thought to have evolved in a last common Holometabolous ancestor. Using a basally branching insect study systems, we show that oskar evolved before the divergence of higher insects, may have played an ancestral role in nervous system development, and that its role in germ cell specification is a recent evolutionary innovation, likely due to co-option of oskar from a somatic function into a germ line specification pathway. Furthermore, we use our basally branching arthropod models to test the hypothesis that a zygotic, inductive signaling mechanism may have been used ancestrally for germ line specification within insects and arthropods. We provide evidence that, in contrast to the Drosophila paradigm, gene products of multiple “germ plasm genes” are not asymmetrically localized during oogenesis or early cleavage stages, and maternal contributions of vasa and piwi are not required for germ cell formation. Moreover, we find deep conservation of inductive signaling mechanisms used to specify germ cells in mice and basal arthropods.