Fig. 2

CATSIR, a method to differentiate if carbon incorporated into a growing tumor, is derived from ingested food or from host tissues. a The protocol starts with genetic crosses of flies that will generate offspring with tumors in the eye-antennal disc. Larvae develop on C3-type food from egg laying until day 6, when the larvae are fully developed. At day 6, the exponential growth of the tumor starts and the larvae is then moved to C4-type food, obtaining a situation where the host tissues have C3-labeled carbon and ingested food nutrients have C4-labeled carbon. At day 8, the carbon composition as well as total carbon mass of the tumor is measured, allowing a determination of where the growing tumor is obtaining carbon to expand its biomass. b δ13C measurements of indicated tissue from Ras^V12,scrib^−/− larvae. In addition to the standard C4 food, larvae were moved to other food variants – one with only sugars (C4 sug), one with only yeast (C4 yea) and one without any nutrients (starved). See also Additional file 1: Figure S1 for an illustration using simulated data of how the δ13C measurements can be used to determine where a growing tumor is obtaining carbon