DIFFERENT LEVELS OF RAS ACTIVITY CAN SPECIFY DISTINCT TRANSCRIPTIONALAND MORPHOLOGICAL CONSEQUENCES IN EARLY DROSOPHILA EMBRYOS

The terminal portions of the Drosophila body pattern are specified by the localized activity of the receptor tyrosine kinase Torso (Tor) at each pole of the early embryo. Tor activity elicits the transcription of two 'gap' genes, tailless (tll) and huckebein (hkb), in overlapping but distinct domains by stimulating the Ras signal transduction pathw ay, Here, we show that quantitative variations in the level of Ras act ivity can specify qualitatively distinct transcriptional and morpholog ical responses. Low levels of Ras activity at the posterior pole direc t tll but not hkb transcription; higher levels drive transcription of both genes, Correspondingly, low levels of Ras activity specify a limi ted subset of posterior terminal structures, whereas higher levels spe cify a larger subset, However, we also show that the response to Ras a ctivity is not uniform along the body, Instead, levels of Ras activity which suffice to drive tll and hkb transcription at the posterior pol e fail to drive their expression in more central portions of the body, apparently due to repression by other gap gene products, We conclude that tll and hkb transcription, as well as the terminal structures, ar e specified by two inputs: a gradient of Ras activity which emanates f rom the pole, and the opposing influence of more centrally deployed ga p genes which repress the response to Ras.