Figure 8. Model for the regulation of ch organ and oenocyte formation by Ato, Abd-A, and Sens.

Schematics summarize the regulation of primary ch organ (EGF-independent, green), secondary ch organ (EGF dependent, blue), and oenocyte (red) development under wild type and spitz mutant conditions. (A) Wild type thoracic segments (T2/T3) contain three 1° scolopodia (C1-C3), whereas the abdominal segments also have two 2° scolopodia, an lch1 organ (C4), and oenocytes (Oe). (B) In spitz¹ mutant segments, neither 2° scolopodia nor Oes form within the abdominal segments. (C) A typical Ato-expressing thoracic segment (PrdG4;UAS-Ato) has an extra 1° scolopodia (green) and an extra 2° scolopodia (blue), whereas abdominal segments have two extra 1° scolopodia (green), two extra 2° scolopodia (blue), and no Oes. (D) In the absence of spi, the number of induced scolopodia in Ato-expressing segments (spi¹; PrdG4;UAS-Ato) is decreased by half in both the thoracic and abdominal segments as no 2° SOPs are recruited. (E) Ato and Abd-A expressing thoracic and abdominal segments (PrdG4;UAS-Ato,UAS-AbdA) develop similarly with approximately two extra 1° scolopodia (green) and six extra 2° scolopodia (blue) as well as two-three oenocytes. (F) In the absence of spi, Ato and Abd-A expressing segments (spi¹; PrdG4;UAS-Ato,UAS-AbdA) only induce an average of two extra 1° scolopodia. (G) A typical Ato and Sens expressing thoracic segment (PrdG4;UAS-Ato,UAS-Sens) develops an average of two extra 1° scolopodia (green) and three extra 2° scolopodia (blue), whereas abdominal segments have five extra 1° and five extra 2° scolopodia. (H) In the absence of spi, Ato and Sens expressing segments (spi¹; PrdG4;UAS-Ato,UAS-Sens) induce an average of two 1° scolopodia in the thorax and five 1° scolopodia in the abdomen. (I) Model for the regulation of 2° scolopodia and oenocyte formation by the Ato, Abd-A, and Sens transcription factors. Both oenocyte and 2° SOP cell formation are dependent upon Spi-mediated EGF secretion from a sending cell. If a 1° ch organ SOP cell expresses Ato and Abd-A, then Sens-mediated repression of rhomboid (rho) expression is abolished, the Rho protein is expressed, and Spi is secreted. Spi binds the EGF-receptor on the receiving cell and if the receiving cell expresses the Salm transcription factor then oenocytes are induced. In contrast, if the Spi-receiving cell expresses the Ato/Sens transcription factors then oenocyte formation is inhibited and 2° SOP cells develop.