Figure 18-14 The bcd “anteriorless” mutant phenotype can be rescued by wild-type cytoplasm or purifed bcd 1 mRNA. (a) In embryos derived from bcd mothers, the anterior (head and thoracic) segments do not form, producing the anteriorless phenotype. (b) If anterior cytoplasm from an early wild-type donor embryo is injected into the anterior of a recipient embryo derived from a bcd mutant mother, a fully normal embryo and larva are produced. Cytoplasm from any other part of the donor embryo does not rescue. (c) Injection of bcd 1 mRNA into the anterior of an embryo derived from a bcd mutant mother also rescues the wild-type segmentation pattern. (Ab 5 abdomen; He 5 head; Th 5 thorax.) [After C. H. Nüsslein-Volhard, G. Frohnhöfer, and R. Lehmann, Science 238, 1987, 1678.] bcd + mRNA + Ab He Th Ab Th (a) (b) (c) 588 Chapter 18 • The Genetic Basis of Development Cell-to-cell signaling and the Drosophila dorsal–ventral axis In the examples considered thus far, products inside the cell determined the position of future body parts: these products were mRNAs or larger macromolecular assem-
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