21Reactive oxygen species (ROS) and downstream products of lipid oxidation are emerging as important 22 secondary messengers in tissue homeostasis. However their regulation and mechanism of action remain 23 poorly studied in vivo during normal development. Here we reveal that the fine regulation of hydrogen 24 peroxide (H2O2) levels at the degradation step by its scavenger Catalase is crucial to mediate the switch 25 from proliferation to differentiation in retinal progenitor cells (RPCs). We further show that altering the 26 levels of downstream products of the Redox signaling can also affect this switch. Indeed, we identify 9-27 hydroxystearic acid (9-HSA), an endogenous downstream lipid peroxidation product, as a mediator of 28 this effect in the zebrafish retina. In fact, RPCs exposed to higher amounts of 9-HSA failed to 29 differentiate and remained proliferative. We found that 9-HSA exerts its biological function in vivo by 30 inhibiting the activity of histone deacetylase 1. We finally show that the local and temporal 31 manipulation of H2O2 levels by catalase overexpression in RPCs was sufficient to trigger their premature 32 differentiation. Therefore the amount of H2O2 in RPCs is instructive of their ability to switch from 33 proliferation to differentiation. We propose a mechanism that acts in RPC and linking H2O2 homeostasis 34 and neuronal differentiation via the modulation of lipid peroxidation. 35 36 37 38 39 40 41 42 43 44 45 46 Key words 47 48 Redox signaling, lipid peroxidation, retinal progenitor cells, stem cell metabolism, neuronal 49 differentiation, H2O2, 9-HSA, HDAC1, zebrafish 50 4 retinogenesis, retinal progenitor cells (RPCs) start differentiating in a central to peripheral manner. By 3 81 dpf, the central retina is fully differentiated and a peripheral stem cell niche, known as the ciliary 82 marginal zone (CMZ), forms to ensure tissue homeostasis throughout the lifelong growth of the fish. The 83 CMZ comprises three main cell domains labeled by the expression of different transcription factors: the 84 rx2 domain, which contains RSCs (retinal stem cells) and RPCs, the ccnd1 (zebrafish equivalent to 85 cyclinD1) domain where cycling progenitors are found and an ath5 domain, which contains cycling and 86 committed RPCs (Agathocleous & Harris, 2009; Ohnuma & Harris, 2003; Reinhardt et al, 2015; Wan et 87 al, 2016). Due to this precise spatial and temporal organization, the zebrafish retina provides an excellent 88 model to assess the effects of ROS and lipid peroxidation downstream products on a stem cell and 89 progenitor cell population in vivo. Here we show that H2O2 levels are dynamically regulated in the 90 developing and in the mature neuro-retina where the expression of the H2O2 scavenger catalase follows a 91 pattern reminiscent of the neurogenic progression. H2O2 levels are further reflected on the lipid 92 peroxidation state in these cells as shown by the enrichment in the end marker of lipid peroxidation 4-93 hydroxynonenal (4-HNE) within the CMZ. Thus, H2O2 and lipid peroxidat...