Wnts are secreted cysteine-rich glycoproteins with diverse roles in animal development (reviewed in Cadigan and Nusse, 1997). Frizzled proteins are cell surface Wnt receptors characterized by an extracellular cysteine-rich domain (CRD)

Wnts are secreted cysteine-rich glycoproteins with diverse roles in animal development (reviewed in Cadigan and Nusse, 1997). Frizzled proteins are cell surface Wnt receptors characterized by an extracellular cysteine-rich domain (CRD) followed by seven transmembrane segments (Vinson et al., 1989; Wang et al., 1996). Members of both protein families have been conserved in species as evolutionarily distant as nematodes and humans (Wang et al., 1996). The prototypic member of the Frizzled family is the Drosophila tissue-polarity gene frizzled (fz), which was identified because of its phenotype in the adult cuticle (Adler et al., 1987; Gubb and Garcia, 1982). Null alleles of fz are viable as adults but their bristles, hairs and ommatidia lose their characteristic polarities (Adler et al., 1987; Gubb and Garcia, 1982; Vinson and Adler, 1987), a condition that is referred to as a tissue or planar polarity phenotype. Historically, tissue polarity signaling has been studied without reference to Wnt signaling, although both signaling pathways are known to be affected by mutations in the dishevelled gene (Theisen et al., 1994; Krasnow et al., 1995). A second member of the Frizzled family in Drosophila, Drosophila frizzled 2 (Dfz2), was identified by sequence homology (Bhanot et al., 1996). Both fz and Dfz2 are expressed during embryonic and larval life (Adler et al., 1990; Bhanot et al., 1996), with fz expression in the embryo also being maternally derived (Park et al., 1994). Mutations affecting only the Dfz2 gene have not been described. The most intensively studied member of the Wnt family is the Drosophila segment polarity gene wingless (wg). wg has multiple essential functions during Drosophila development (reviewed in Klingensmith and Nusse, 1994). During embryogenesis, it is required for epidermal patterning 4175 Development 126, 4175-4186 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV7744