Required for visual system development. May transcriptionally regulate genes necessary for optic lobe invagination and Bolwig's nerve formation.

(UniProt, Q27350)

Ocelli always absent; eyes usually reduced to small groups of ommatidia, and occasionally missing; eye field sometimes in form of an eye stalk protruding from head with an irregular arrangement of ommatidia; heavy ommatidial disruption with many receptor cells missing. Optic lobes reduced in size, and many flies have no lamina. The reduced volume of adult optic lobes is due to accentuated degeneration of precursor neurons that occurs to a certain degree in normal pupal development (Fischbach, 1983); the increased severity in the mutant includes degeneration of axons in second optic chiasma (Fischbach and Technau, 1984); sol enhances this kind of degeneration, but acts on a separate set of precursors for columnar visual system neurons--as confirmed by anatomical analysis of sol; so double mutant, which ends up with tiny, rudimentary optic lobes (Fischbach and Technau, 1984); sol; so also leads to a central brain that is smaller than normal due to missing afferents from visual system (Fischbach and Technau, 1984); more specifically, there is a reduction in number of axons in anterior optic track in so, and combining so with sol causes a further reduction, but again, these two genes act independently on separate subsets of such axons (Fischbach and Lyly-Hunerberg, 1983). Histological studies reveal that the eye-antenna disc in third-instar larvae appears normal until differentiation begins, at which time cell death is observed (Hofbauer and Campos-Ortega). More extreme at elevated temperatures; lethal at 30; temperature-sensitive period for eye defect in third instar. Survival sensitive to elevated temperature at all developmental stages (Ransom, 1980, DIS 55: 126). Mosaic studies demonstrate that so acts in developing eye tissue and that the resulting reduction in retinal innervation leads to death of cells in the lamina and breakdown of medulla and lobula-complex neuropil (Fischback and Technau). Nonphototactic (Benzer, 1967, Proc. Nat. Acad. Sci. USA 58: 1112-19) and visual orientation almost absent (Bulthoff, 1982, DIS 58: 31). Studies of circadian rhythms in so show eclosion to be normally periodic (Engelmann and Honneger, 1966); adult activity rhythms are robust, in that so, even when thoroughly eyeless, responds to light:dark cues such that it entrains to these conditions (is periodically active vs. inactive, and anticipates the environmental transitions) and subsequently free-runs with obvious circadian periodicities in constant darkness (Helfrich and Engelmann, 1983; Dushay, Rosbash, and Hall, 1989); however, these behavioral rhythms are frequently aberrant, e.g., with "split" active components appearing after several days of free-run and with dual periodicities extractable from the locomotor data (Helfrich, 1986); nearly all adults are dual-period when so combined with sol (Helfrich, 1986).

Heterozygote has very small glazed eyes; more extreme than Drmio/+;soD/+/+ has a very clear phenotype, but weaker than that of Drmio/+ (i.e., about the size of Gla/+, but not glazed). Heterozygote has excellent viability; homozygous lethal.