Fig. 8. Redundant functions of Ey and Toy in eye-antennal development. Normal head (eye-antennal) development can occur in the absence of Toy if Ey levels are sufficiently high, such as is mostly the case in toy^hdl mutants at 18°C, or in the presence of very low Ey levels if Toy levels are normal as in ey² mutants. Conversely, headless phenotypes are observed in the absence of Toy and at moderate Ey levels, as in most toy^hdl mutants at 28°C, or in the absence of functional Ey as in ey^D mutants. Broken lines indicate where requirement for eye-antennal development is not absolute because it can be compensated by sufficiently high levels of Ey. It is unclear if the pathway of Toy that is parallel to the ey pathway leads through optix and/or eyg, as the diagram suggests. Moreover, additional Toy and Ey functions that are not mediated through so and optix/eyg, respectively, exist to support proper eye-antennal development. Such Toy and Ey functions, like the inhibition of apoptosis, as shown here, or the activation of the cell cycle (Jiao et al., 2001), are required in eye-antennal primordia of stage 12-16 embryos, long before these genes are activated in eye-antennal discs (Kumar and Moses, 2001).

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