Initial organization of the Drosophila dorsoventral axis depends on an RNA-binding protein encoded by the squid gene.

Females mutant for the newly identified squid (sqd) gene are sterile and lay eggs that display only dorsal structures. The resulting embryos are also dorsalized even if fertilized by wild-type sperm. The gene acts midway through oogenesis at about the time dorsoventral (D/V) axis is established within the growing egg chamber. The sqd gene encodes at least three distinct proteins generated by alternative RNA processing that are members of a well-characterized family of RNA-binding proteins. At least one Sqd isoform is essential in somatic tissues. The ventralizing mutations gurken (grk), torpedo (tor), and cornichon are all epistatic to sqd. Strong alleles of grk and top can act as dominant suppressors of sqd dorsalization. A model of D/V axis formation is presented postulating that squid is needed to organize a concentration gradient of a morphogen originating in the germinal vesicle.