Short-term activation of the Jun N-terminal kinase pathway in apoptosis-deficient cells of Drosophila induces tumorigenesis

In Drosophila, the JNK pathway eliminates by apoptosis aberrant cells that appear in development. It also performs other functions associated with cell proliferation, but analysis of the latter is hindered by the pro-apoptotic activity. We report the response of apoptosis-deficient cells to transient activation of JNK and show that it causes persistent JNK function during the rest of the development. As a consequence, there is continuous activity of the downstream pathways JAK/STAT, Wg and Dpp, which results in tumour overgrowths. We also show that the oncogenic potential of the Ras-MAPK pathway resides largely on its ability to suppress apoptosis. It has been proposed that a hallmark of tumour cells is that they can evade apoptosis. In reverse, we propose that, in Drosophila, apoptosis-deficient cells become tumorigenic due to their property of acquiring persistent JNK activity after stress events that are inconsequential in tissues in which cells are open to apoptosis. Jun N-terminal kinase (JNK) is necessary for development in tumours, indicating it may play tumour-promoting roles; however, the experimental analysis of the role of JNK in proliferation is hindered by its pro-apoptotic activity. Here the authors carry out experiments in Drosophila with genetic backgrounds that make cells refractory to apoptosis to definitely prove the JNK pathway contribution to tumorigenesis.

A poptosis, i.e. programmed cell death, is a physiological process by which cells trigger their own destruction. Although there is developmentally programmed apoptosis, i.e. the removal of the tail of tadpoles or the articulations of Drosophila legs 1,2 , a major function of apoptosis is the elimination of aberrant cells that may compromise the viability or the fitness of the organism [3][4][5] . Apoptosis also functions to remove, through a process known as cell competition 3,6,7 , oncogenic cells that may arise during development 4,8 .
The study of apoptosis in Drosophila is facilitated by the simplicity of the genetic program, the availability of mutants affecting apoptosis and of methods to manipulate gene activity. Within Drosophila, the wing imaginal disc (the precursor cells of the adult wing and thorax) is a convenient experimental system, for while it normally does not contain cells in apoptosis, it exhibits a strong apoptotic response to ionising radiation or other forms of stress 9 . A simplified version of the apoptosis program is shown in Fig. 1a.
A prime event in the apoptotic response in vertebrates and Drosophila 10-12 is the activation of the Jun N-terminal kinase (JNK) signalling pathway, a member of the MAPKs family involved in various cellular processes 13 . In the Drosophila wing disc, JNK is not normally active-except in a small proximal region (Fig. 1b, c)-but after irradiation, there is overall induction of JNK and subsequent apoptosis 10,11 (Fig. 1d, e), aimed to remove cells damaged by the irradiation. The apoptotic role of JNK also mediates the elimination of oncogenic cells by cell competition, thus revealing its anti-tumour role 4,14 .
In addition to its apoptosis-inducing function, JNK is known to play non-apoptotic roles connected with cell proliferation [15][16][17] . For example, sustained expression of JNK is associated with cancer in vertebrates 18,19 , and in Drosophila we have shown that forcing JNK activity causes overgrowths in apoptosis-defective wing discs 17 . Thus, both in flies and in vertebrates, JNK may function as anti-tumorigenic or pro-tumorigenic, depending on the context.
Another factor associated with tumorigenesis in vertebrates is the inappropriate expression of the Ras-MAP kinase pathway 20,21 . In Drosophila, Ras overexpression can also cause massive overgrowths through interactions with mutations at tumour suppressing genes 22,23 . Significantly, the development of those overgrowths requires JNK activity 3,23 . One noteworthy feature of the overexpression of Ras-MAPK in Drosophila is that it makes cells refractory to apoptosis 24,25 , and a similar observation has been made in mammalian cells 26 .
Our results indicate that cells refractory to apoptosis, they may lack the apoptosis machinery or overexpress the Ras pathway, become tumorigenic due to their property of acquiring sustained JNK activity after its transient activation due to stress or other events.

Results
JNK sustained activity in apoptosis-defective cells. In our experiments, we generated entire discs or disc domains that cannot initiate apoptosis. They may be mutant for the caspase Dronc, or may lack activity of pro-apoptotic genes (see Fig. 1 and Supplementary Fig. 1 for details), or may contain constitutive expression of the Ras-MAPK pathway 24,25 . Then, we analysed the response to events leading to JNK activation such as X-rays, a pulse of p53, or a pulse of JNK itself.
We first analysed JNK response to ionising radiation (3-4000R) using the targets puckered (puc) and metalloprotease 1 (mmp1) to monitor JNK function. Except for the resident JNK expression in a small proximal region (Fig. 1b, c), any additional activity has to be caused by the experimental intervention. We compared the response of discs in which apoptosis is not prevented (controls) with that of discs in which cells cannot undergo apoptosis.
In irradiated discs open to apoptosis, we observe high puc levels 24 h after the irradiation (Fig. 1d), but afterwards those levels decay and eventually disappear by 96 h (Fig. 1f). Cells expressing puc also exhibit Dcp1 effector caspase activity indicating that they are in apoptosis. These results confirm previous work and illustrate the JNK role of removing cells damaged by irradiation 10,11 .
The JNK response to irradiation of apoptosis-deficient discs is very different.
Although the apoptotic response is suppressed, there is a strong activation of the JNK marker puc, especially in the pouch 24 h after X-rays (Fig. 1e). However, unlike the response of dronc + , mutant dronc − discs maintain high puc levels for the rest of the larval development, including pre-pupal stages (Fig. 1g).
We find similar results in discs in which apoptosis is suppressed only in the posterior compartment: 96 h after irradiation, there are high levels of JNK in the posterior but not in the anterior compartment ( Supplementary Fig. 1a, b). Further evidence comes from clones homozygous for the H99 deletion (lacking major pro-apoptotic genes), many of which express JNK markers 96 h after the irradiation, whereas surrounding cells do not ( Supplementary Fig. 1c).
We also performed experiments to follow the lineage of the cells that acquire JNK activity after irradiation, both in cells open to apoptosis (dronc + ) and in apoptosis-defective (dronc − ) cells.
Irradiation of dronc + discs of genotype puc-Gal4 UAS-Flp UAS-GFPDbox act<stop>lacZ activates puc-Gal4 in many of their cells. These cells become labelled with GFPDbox, an unstable form of GFP 27 , to avoid an effect of perdurance, and also produce high levels of the recombinase Flippase. In the majority of those cells, Flippase induces recombination in the act<stop>lacZ cassette, what labels them and their progeny indelibly with LacZ. The puc-expressing cells that do not die generate clones marked with LacZ expression (see Supplementary Fig. 1 for details). Twenty-four hours after X-rays, we find a large number of cells expressing GFPDbox and the majority of them are also labelled with lacZ ( Supplementary Fig. 1d). Seventy-two hours after irradiation, we observed a smaller number of large LacZ patches, but none of them expresses GFPDbox ( Supplementary Fig. 1e), indicating that JNK has been turned off, consistent with the results above (Fig. 1f). The large size of those patches suggests that the mature disc has been reconstituted by few cells, many of which are descendants from those in which JNK was activated by the irradiation. This is proof that X-rays can activate JNK to sub-lethal levels.
The same lineage analysis after irradiation of dronc − discs yields different results. The principal difference is that 72 h after the irradiation, we still observed many groups of cells expressing GFPDbox ( Supplementary Fig. 1g), again indicating the persistence of JNK expression in apoptotic defective cells and reinforcing the results above (Fig. 1g). In these discs, the size of the lacZ patches is smaller than in the dronc + discs, which is expected because the number of cells is greater as there is no cell death after the irradiation.
To explore the possibility that just an initiation event is sufficient to generate persistent JNK activity in apoptosisdefective cells, we carried out two experiments in which we checked JNK expression several days after a short pulse of induction (see Methods for details). In the first experiment, a 16 h pulse of the JNK activator p53 28 in the posterior compartment causes persistent JNK activity, visualised 96 h after the pulse (Fig. 1i). In the second experiment, we obtain a similar result after a 16 h pulse of JNK itself (Fig. 1k), achieved by the expression of a form of hemipterous (hep CA , a JNKK 4 ), which causes constitutive activity of the pathway. In control discs, in which apoptosis is not prevented, the brief expression of JNK ( Supplementary Fig. 1h) rapidly subsides (Fig. 1h, j).
ROS and moladietz are required for JNK persistent activity.
The preceding experiments establish that events leading to JNK activation trigger a genetic operation that causes its sustained adventitious function. This phenomenon is of little consequence when cells are open to apoptosis because they die shortly after JNK induction or recover and the JNK pathway is turned off, but becomes prominent when cells are refractory to apoptosis. This persistent activity cannot be due to the apoptotic amplification loop 28 because in the absence of the function of dronc or the pro-apoptotic genes hid/rpr/grim (DfH99 deletion), the loop cannot be completed.
We sought to identify the mechanism(s) responsible for the persistence of JNK activity after a transient initiation event. It has been shown that cellular damage to the wing disc induces a burst of reactive oxygen species (ROS), which in turn activate JNK 29 .   a Simplified representation of the apoptotic pathway in Drosophila. JNK/p53 are activated after irradiation and induce the activation of the pro-apoptotic genes that block DIAP1, inducing the activation of the apical caspase Dronc and effector caspases Dcp1 and Drice. Besides, Dronc reinforces JNK activity generating an amplification loop that augments the apoptotic response 26 . As a side event, JNK activates the Dpp, Wg and JAK/STAT pathways. b-g Wing discs showing the effects of X-rays at different time points. JNK activity is monitored by the expression of a LacZ insert at the JNK target puckered (puc, red). The blue background reflects TOPRO staining. b, c puc expression in non-irradiated dronc + control (n = 10) and null dronc− mutant (n = 17) discs. Note that the label is restricted in both cases to a small region that corresponds to the most proximal zone of the disc. d, e Twenty-four hours after irradiation, both dronc + (n = 22) and dronc − (n = 18) discs show high expression levels of puc, especially in the region corresponding to the wing pouch. f, g puc expression 96 h after irradiation in dronc + (n = 28) and dronc− (n = 31) discs. Whereas puc levels have returned to normal in dronc + (f), they remain high in many zones of the dronc− discs (g). Wing discs of genotype: h UAS-p53/UAS-GFPtubGal80 ts ;hh-Gal4dronc i29 /+ (n = 10), i UAS-p53/UAS-GFPtubGal80 ts ;hh-Gal4dronc i29 /dronc i29 (n = 14), j UAS-hep CA /UAS-GFPtubGal80 ts ;hhGal4dronc i29 /+ (n = 10) and k UAS-hep CA /UAS-GFPtubGal80 ts ;hh-Gal4dronc i29 /dronc i24 (n = 11). Discs are stained with Cubitus interruptus, Ci (green) to mark the anterior compartment and Mmp1 (red) to monitor JNK activity. The short pulse of p53 (h, i) or of hep CA (j, k), at the posterior compartment is administered by temperature shift, detailed in Methods section; 96 h after the change to 17°C, no activity of JNK can be detected by Mmp1 staining (red) in the posterior compartment (lack of green) in p53 dronc + (h) and hep CA dronc + discs (j). In contrast, there is JNK activity (red) at the posterior compartment (lack of green) of p53 dronc − (i) and in hep CA dronc − discs (k). Scale bars are 100 μm Furthermore, recent work on wing disc regeneration 30 has identified the gene moladietz (mol), transcriptionally activated by JNK, which encodes a Duox maturation factor necessary for production of ROS, with subsequent induction of JNK. JNK and mol establish a positive amplification loop that ensures prolonged JNK function during the regeneration process 30 .
Therefore, we checked the involvement of ROS and mol on the phenomenon of persistent JNK function observed in our experiments. First, using dihydroethidium (DHE) staining 31 to monitor ROS production, we found high DHE levels in dronc − discs 96 h after irradiation (Fig. 2b), whereas DHE is not detected in dronc + discs (Fig. 2a). Second, mol activity, visualised with a mol-LacZ insert, is also detected in dronc − discs 96 h after irradiation (Fig. 2e), but not in controls (Fig. 2d).
We checked the functional requirements of ROS and mol activity for the sustenance of JNK function. The reduction of ROS levels in the posterior compartment by overexpression of Superoxide dismutase (SOD) and Catalase (Cat) 29 , or the suppression of mol function using a molRNAi construct 30 , resulted in reduced JNK activity in both cases 96 h after the irradiation (Fig. 2c,   The preceding results establish the involvement of ROS and mol in the continuity of JNK function after transient activation. Next, we examined whether the persistent JNK function in apoptosis-deficient cells causes continuous mis-expression of pathways downstream JNK like JAK/STAT 23,32 , Dpp and Wg 17,33 . Indeed, JAK/STAT, Wg and Dpp are induced after irradiation in territories outside their normal domains, and their expression is associated with the zones of JNK ectopic activity (Fig. 3b, e, g). Moreover, the ectopic expression of JAK/ STAT and Wg depends on JNK, for compromising JNK activity with a dominant negative form of Bsk (JNK itself) suppresses their expression (Fig. 3c, h).
Sustained JNK induces tissue overgrowth. We also observed a correlation between the continuous expression of JNK and an increase of cell proliferation, the latter monitored by EdU incorporation, which results in the formation of overgrowths (Fig. 4a, b, d). We also find that the development of the   (Fig. 4c, d).
To test if the overgrowth observed in the RHGmiRNA experiment is induced by the sustained expression of JNK, we made use of the tub-Gal80 ts transgene to suppress the JNK pathway with Bsk DN   (red) to monitor JNK activity and GFP (green) to identify the P-compartment; TOPRO staining delimits the discs. The disc in e shows Mmp1 label in both compartments, but expression of the Bsk DN blocks JNK activity (Mmp1, red) as shown in f. g Quantification of the posterior compartment volume over the total disc volume for the genotypes: UAS-GFPtubGal80 ts /+; dronc i24 /hh-Gal4 dronc i29 (column: hh>GFPdronc − X-rays, n = 79), UAS-Bsk DN ; UAS-GFPtubGal80 ts /+; dronc i24 /hh-Gal4dronc i29 expressing Bsk DN 24 h (column: hh>Bsk DN dronc − 24 h after X-rays, n = 43) or 48 h after irradiation (column: hh>Bsk DN dronc − 48 h after X-rays, n = 18). There is a significant size decrease when Bsk DN is expressed 24 h and 48 h after irradiation (***p-value < 0.0001), indicating that inhibiting JNK after irradiation suppresses overgrowth. Quantification for the genotypes: UAS-GFPtubGal80 ts /UAS-SOD:UAS-Cat;dronc i24 /hh-Gal4dronc i29 (column: hh>SOD:Cat dronc − 24 h after X-rays, n = 14) inducing the expression of SOD:Cat 24 h after irradiation and UAS-GFPtubGal80 ts /UAS-molRNAi; dronc i24 /hh-Gal4dronc i29 (column: hh>molRNAidronc − 24 h after X-rays, n = 7) inducing the expression of molRNAi 24 h after irradiation. There is a significant size decrease after the expression of SOD:Cat and the inhibition of mol (**p-value < 0.001 and *p-value < 0.02, respectively), indicating that ROS levels are important to maintain JNK overgrowth. Columns in the graphs represent mean ± s.d. Statistical test used: Mann-Whitney U-test. Scale bars are 100 μm posterior compartment of dronc − mutants. In the 24 h experiment, we already found a significant reduction of the overgrowth in the posterior compartment (Fig. 4f, g), which is also observed in the 48 h experiment (Fig. 4g). Along the same lines, reduction of ROS levels or of mol function 24 h after the irradiation also results in suppression of the overgrowths (Fig. 4g). Overall, these results indicate that the overgrowths observed are dependent on persistent JNK activity.
Lack of apoptosis in ras V12 tissue leads to tumorigenesis.
Overexpression of the Ras-MAPK pathway is associated with tumour development in vertebrates and Drosophila 34-38 . Since it renders cells refractory to stress-induced apoptosis 24-26 , we surmised that this property might confer a tumorigenic potential to cells with elevated levels of Ras activity. In Drosophila, the overexpression of Ras does not generate significant overgrowth, although it up regulates the oncogene dMyc 39,40 . In our experiments, the growth of clones or body regions with constitutive Ras function (induced by the ras V12 transgene) is like in the wildtype: the size of the Sal domain (a region covering 15% of the wing disc) is similar in sal Epv >GFP (control) and sal Epv >ras V12 discs ( Supplementary Fig. 2 a-d), and the growth rate of clones containing ras V12 is also similar to that of the surrounding tissue ( Supplementary Fig. 2e). We confirmed that ras V12 -expressing cells show high levels of dMyc 41 ( Supplementary Fig. 2d, e). We also confirmed that those cells exhibit a reduced apoptotic response to X-rays: the apoptotic levels after irradiation of the posterior compartment or the Sal domain containing Ras V12 are much lower than in controls ( Supplementary Fig. 3a-c). We then checked whether the lack of apoptotic response of ras V12 -expressing cells also causes sustained JNK activity after irradiation. Indeed, in sal Epv >ras V12 (Supplementary Fig. 3d) and in hh>ras V12 discs (Fig. 5b), JNK remained active in the Sal domain and the posterior compartment, respectively, 72 and 96 h after irradiation. Irradiated (Fig. 5a) and non-irradiated (Fig. 5d, e) controls show no JNK activity. Moreover, Ras activity is associated with ectopic activity of the JAK/STAT and Wg pathways (Fig. 5g, h) and with overgrowth of the posterior compartment (Fig. 5f). These overgrowths are likely the result of the inactivation of the Hippo pathway 42 and require JNK activity, for in irradiated hh>ras V12 UAS-puc discs the overgrowth is much reduced (Fig. 5c, f). This observation links the tumorigenic property of the Ras pathway with JNK activity. Interestingly, Ras-induced oncogenic transformations in vertebrates also require c-JNK activity 43 .
Since in ras V12 cells the apoptotic response is not completely suppressed, we studied the growth of ras V12 tissue also mutant for dronc. After irradiation, sal Epv >ras V12 dronc − discs showed very large overgrowths in comparison with non-irradiated discs of the same genotype ( Supplementary Fig. 3e, f). In this experiment, we introduced a lineage-tracing cassette (see Methods) to follow the progeny of ras V12 -expressing cells, which expand to the surrounding tissue. It is worth emphasising that these tumorigenic features are triggered by a single 3000 R irradiation administered 96 h before. In these discs, we compared the relative growth of the ras V12 tissue with that of the rest of the disc. As shown in Supplementary Fig. 3F, the ras V12 cells generate a disproportionate part of the overgrowth. We believe that the strong tumorigenic character of irradiated ras V12 cells defective in apoptosis is caused by the sum of two factors: (1) persistent tumorigenic JNK activity and (2) the high metabolic activity conferred by the high dMyc levels 40,44,45 .

Discussion
JNK is associated in vertebrates and in Drosophila both with anti-tumorigenic and pro-tumorigenic roles. The anti-tumour function of the single Jun kinase of Drosophila is achieved by its ability to induce apoptosis in oncogenic cells 3,4 . In our experiments, we have characterised the pro-tumorigenic function of JNK in apoptosis-suppressed cells by examining the response to a brief induction of JNK caused by X-rays or by a short pulse of p53, or of JNK itself. We find that it causes sustained activity of JNK and of genetic factors, e.g. the JAK/STAT, Wg and Dpp pathways, which function downstream of JNK. Furthermore, it gives rise to the appearance of tumorous overgrowths caused by over-proliferation of the apoptosis-defective cells. We emphasise the differences between these experiments and those reported after blocking cell death in apoptotic cells with the baculovirus protein P35 9,33 . Those "undead" cells persistently express JNK, Dpp and Wg, which generate tissue overgrowths. However, in undead cells, the apoptosis program remains fully active, except for the blocking of the effector caspases, and the continuous expression of Dronc generates persistent JNK function 28,46 . In contrast, in the present experiments, the apoptosis program is completely suppressed.
The fact that a short-term activation event is sufficient to induce persistent JNK activity in apoptosis-defective tissues defines an important property of the pathway: it includes a selfsustenance mechanism. In normal tissues open to apoptosis, this feature is largely irrelevant because the cells die soon after JNK activation, but in cells refractory to apoptosis, the function of JNK and of subsidiary pathways is maintained for the rest of the development of the disc. The result is the production of tumour outgrowths. A key aspect of this process is the mechanism of JNK sustenance after the initial event. Based on the results of Khan et al. 30 and our own, we propose a simple model: the activation of JNK (by X-rays, or P53 or Hep itself) induces mol function, which generated high ROS levels that in turn induce JNK function. As mol is induced downstream of JNK 30 , it generates an amplification loop that maintains indefinite activity of JNK.
Our results also bear on the tumorigenic properties of the Ras pathway, whose deregulated expression is associated with cancer development in mammals 21 . We show that the principal tumorigenic feature of overexpressing Ras in Drosophila is that it makes cells refractory to apoptosis; these cells can acquire irrepressible JNK activity after an irradiation event that would have eliminated them but for the overexpression of Ras. It is of interest that Ras activity is known to have anti-apoptotic activity in mammalian cells 26 , although the apoptotic response to irradiation has not been studied. It is also worth noting that oncogenic transformations associated with de-repressed function of Ras in vertebrates require activity of the c-Jun pathway 43 ; thus both in Drosophila and in vertebrates, tumour development appears to require combined activities of the Ras and JNK pathways.
DHE staining was performed as in Owusu-Ansah et al. 31 with modifications. Larvae were dissected in 1× PBS and incubated with DHE (ThermoFisher Scientific, catalogue number D1168) 30 μM for 5 min at room temperature followed by three washes of 5 min in 1× PBS. Discs were mounted in 1× PBS and images were captured with Leica (Solms, Germany) DB5500 B confocal microscope.
All the n numbers stated in the text represent individual discs of the mentioned genotypes. All experiments are replicated at least three times.
Experiments to suppress JNK. To block JNK activity, the UAS-Bsk DN transgene was expressed in larvae of the following genotypes: UAS-Bsk DN ; tub-Gal80 ts , UAS-GFP; dronc i29 hh-Gal4/dronc i24 and UAS-Bsk DN ; nub-Gal4/STATGFP; dronc i29 / dronc i24 pucLacZ. Larvae were raised at 25°C and changed to 29°C 24 h before 4000 X-rays dose to enable the expression of Bsk DN . Irradiation was performed with an X-ray machine Phillips MG102. Larvae were kept at 29°C for 72 h and then dissected and stained with the appropriate antibodies.
We have also used the UAS-puc 2A transgene in the following genotype: tub-Gal80 ts /UAS-Ras V12 ; hh-Gal4/UAS-puc 2A . The high levels of the negative regulator puc suppress most of JNK function. Larvae were raised at 25°C and changed to 29°C 24 h before 3000 X-rays dose to enable the expression of Ras V12 and puc 2A . Larvae were kept at 29°C for 72 h and then dissected and stained with the appropriate antibodies.
Quantification of the overgrowths. No statistical methods were used to estimate the sample size, as we use Drosophila melanogaster, and because it has a high fecundity, we can use a large number of animals for our experiments. No randomisation or blind experiments were performed.
For quantification in the RHGmiRNA experiments, we used larvae of the genotypes: UAS-RHG-miRNA/+; hh-Gal4 UAS-GFP/+and UAS-Bsk DN ; UAS-RHG-miRNA/+; hh-Gal4 UAS-GFP/+. Larvae were raised at 25°C and changed to 29°C 24 h before X-rays (4000 R) to induce expression of the Bsk DN and RHG-miRNA transgenes. Control non-irradiated UAS-RHG-miRNA/+; hh-Gal4 UAS-GFP/+larvae were raised in the same conditions. All larvae were kept at 29°C for 72 h and then dissected and stained with GFP and TO-PRO3. Stacks were captured with a Leica (Solms, Germany) DB5500 B confocal microscope, and analysed with Fiji 52 voxel counter plugin to obtain the volume of the samples.
To test if the sustained activity of JNK is responsible for the overgrowths, we blocked JNK activity after irradiation as follows. Larvae of the following genotypes: tub-Gal80 ts , UAS-GFP; dronc i29 hh-Gal4/dronc i24 and UAS-Bsk DN ; tub-Gal80 ts , UAS-GFP; dronc i29 hh-Gal4/dronc i24 were raised at 17°C and transferred to 29°C 24 h before irradiation (4000 R) or transferred to 29°C 24 h or 48 h after irradiation. All larvae were kept at 29°C for 72 h and then dissected and stained with GFP and TO-PRO3. Stacks were captured with a Leica confocal microscope, and analysed with Fiji 52 voxel counter plugin to obtain the volume of the samples. Some samples were also stained with Mmp1 to check the JNK activity.
To test if ROS and mol are involved in promoting overgrowth, we also quantified the volumes of discs overexpressing SOD:Cat and molRNAi, as in the previous experiment for the Bsk DN .
Statistical analysis was performed with the software Prism5 (Graphpad).
EdU incorporation. Wing discs were cultured in 1 mL of EdU labelling solution for 20 min at room temperature and subsequently fixed in 4% paraformaldehyde for 30 min at room temperature. Rabbit anti-GFP (Invitrogen) 1:200 antibody was used overnight at 4°C before EdU detection to protect GFP fluorescence. EdU detection was performed according to the manufacturer instructions (Click-iT EdU Alexa Fluor 555 Imaging Kit, ThermoFisher Scientific).
In the experiments in which we overexpressed the Ras pathway in the Sal domain of dronc mutant larvae, e.g. sal Epv -Gal4, UAS-GFP/act>stop>lacZ UAS-Flp; dronc i29 UAS-Ras V12 /dronci 24 , the flippase activity directed by the sal Epv -Gal4 driver induces recombination of the act>stop>lacZ cassette in all the cells of the Sal domain. These cells become irreversibly labelled with lacZ, thus allowing the identification of the cellular progeny of the domain.
Data availability. The authors declare that the data supporting the findings reported in this manuscript are available within the article and the Supplementary Information files or from the corresponding author upon request.