Posterior organizing center defined by:
Posterior organizing center established by localization of nanos mRNA to posterior pole plasm via actin and oskar-dependent anchoring. Translation of Nanos in posterior blastoderm represses hunchback via Pumilio, permitting posterior gap gene expression and abdomen formation. Simultaneously Nanos specifies pole cells germline precursors. Gurken marks dorsal follicle side, Bicoid anterior, Dorsal ventral embryonic nuclei; none define posterior center. Nanos mutation eliminates abdominal segments producing head thorax only. Nanos gradient therefore provides second axis information complementary to anterior Bicoid morphogen gradient during blastoderm patterning and germline specification.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Nanos defines posterior organizing center - repression of hunchback translation.