Cell lineage analysis of germ cells of Drosophila melanogaster

Abstract

THE initial events of differentiation of genetically identical nuclei within the eggs of higher organisms are generally believed to be triggered by a heterogeneous distribution of substances within the egg^1,2. One of the clearest examples of such differential distribution of determinative substances concerns the posterior polar plasm of some insects. Morphological observations suggest that all cleavage nuclei in such insects have identical developmental potential, and that only the few nuclei that chance to migrate to the posterior polar region and thus to include a sufficient quantity of the morphologically distinguishable polar plasm, acquire the capacity to give rise to the germ line³. Various experimental approaches^4–8 have provided very strong, although not compelling ⁹ evidence for such a causal relationship between the polar plasm and germ line differentiation. Genetic fate mapping experiments described here strongly suggest that the germ cells of Drosophila melanogaster originate from the posterior-most region of the blastoderm, and they unequivocally rule out the possibility that the germ cells originate from a mid-dorsal position¹⁰. The posterior-most location of the germ cells supports, or at least is consistent with, the notion that the polar plasm determines whether or not a given cleavage nucleus will acquire the capacity to differentiate into a germ cell.