Molecular genetic mechanisms of axial patterning: Mechanistic insights into generation of axes in the developing eye

All multicellular organisms require axial patterning to transform a single-layer organ primordium to a three-dimensional organ. It involves delineation of anteroposterior (AP), dorsoventral (DV), and proximodistal (PD) axes. Any deviation in this fundamental process results in patterning and growth defects during organogenesis. The Drosophila eye is an excellent model to study axial patterning. In the Drosophila eye, DV lineage is the first axis to be determined, which is followed by generation of the AP axis. The default state of the Drosophila early eye primordium is ventral, and the dorsal fate is established by onset of expression of dorsal eye fate selector pannier (pnr)in a group of cells on the dorsal eye margin. The boundary between dorsal and ventral compartments is the site for activation of Notch (N) signaling and is referred to as the equator. Activation of N signaling is crucial for initiating the cell proliferation and differentiation in the developing Drosophila eye imaginal disc. This chapter will focus on (a) how axial patterning occurs in the developing Drosophila eye, (b) how the developing eye field gets divided into dorsal and ventral cell populations, and (c) how DV patterning genes contribute toward the growth and patterning of the fly retina.