Involved in regulation of planar cell polarity in the compound eye where it is required for correct specification of the R3 and R4 photoreceptor cells by regulating Fz activity in the R3/R4 precursor cells (PubMed:11893338). This is likely to occur through creation of an ft gradient so that the equatorial R3/R4 precursor cell has a higher level of ft function than its polar neighbor (PubMed:15548581). Also required for planar cell polarity of wing hairs (PubMed:12540853, PubMed:15240556). Mediates heterophilic cell adhesion in vitro and is required to stabilize ds on the cell surface (PubMed:15240556). Involved in regulation of eye imaginal disk size (PubMed:23667559). Upstream component of the Hippo pathway where it is likely to act as a cell surface receptor involved in regulation of tissue size and is required for the localization and stability of ex (PubMed:16996265). Probably acts as a cell surface receptor for ds (PubMed:20434335).

(UniProt, P33450)

Viable alleles characterized using ft1. Abdomen short and fat. Thorax broad. Wings short and broad with crossveins much closer together than normal. Scutellum shortened; scutellar bristles far apart. Viability good. Second- and third-instar larvae, particularly when there is little yeast in the food, show vacuoles in cytoplasm of salivary gland cells. Two waves of vacuole formation; vacuoles may be membrane-bound lipoprotein bodies (Chandhuri, 1969, DIS 44: 118). Tip of X disfigured, possibly as a result of several small puffs intermingled with hard, non-puffed bands. In about 1% of larvae, salivary glands distally expanded and crooked [Slizynski, 1964, Cytologia (Tokyo), 29: 330-36 (fig.)]. Lethal alleles characterized in study of ft8 (formerly fd = floppy disc) by Bryant et al.. ft8 classified as an amorphic mutation based on the similarity in lethal phenotype of ft8/ft8 and ft8/Df(2L)M25A-11. Larvae characterized by imaginal-disc hyperplasia such that mutant discs are much larger and more convoluted than wild type; the disc remains a single epithelial layer but in the highly convoluted proximal regions, of the wing disc at least, the columnar cells give way to cuboidal epithelial cells, which are deficient in cytoskeletal elements. Pupariation is delayed 3.2 days in mutants and the discs contain 122,000 cells at the end of nine days compared to 50,000 cells in wild type discs, which attain full growth at 5 days. Disc phenotype autonomous in transplants into adult hosts. Occasional mutant pupae reach the pharate-adult stage so that adult structures can be studied. Abdomens normal; eyes often swollen and may be split into two parts; extra head bristles; distal parts of antennae and legs may be missing; wings often fail to evaginate. Legs most severely affected; joints short and thick; missing tarsal joints and claws and tarsal fusions; increased bristle densities with deviant chaetal polarities; frequent outgrowths and ingrowths of cuticle; the latter giving rise to cuticle bound vesicles within the legs.

Wings large, held out from sides at 45-90 angle, curved downward, and somewhat pointed. Vein L1 thickened; crossveins closer together, sometimes broken. Thoracic and vertical bristles duplicated in majority of flies. G/ft has exaggerated ft phenotype. Partially inhibited by ds/+ and much inhibited by ds/ds. Homozygous lethal. RK2.

Does not show G phenotype. Allelic to ft but does not exaggerate ft. Lethal in combination with G. RK2.

Lethal shortly after pupation; imaginal discs two to four times normal size. Usually display abnormal shapes and folding patterns; occasionally appear normal. Metamorphosed fragments in normal larval hosts exhibit hair patterns; occasional single bristles seen; often only naked cuticle forms.