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Smed-egfr-4 is required for planarian eye regeneration

Planarians are amazing animals that can regenerate a whole body from a tiny piece of them thanks to their pluripotent stem cells, the neoblasts. Planarian neoblasts include both pluripotent stem cells and specialized lineage-committed progenitors that give rise to all the mature cell types during regeneration and homeostatic cell turnover in these plastic animals. Little is known, however, about the mechanisms that regulate neoblast differentiation. Recently, it has been shown that Smed-egfr-1, a homologue of the epidermal growth factor receptor (EGFR) family is required for the final differentiation of the gut progenitors into mature cells but not for their specification. As planarians have several EGFR homologues it has been proposed that they could have diverged functionally to regulate the differentiation of the different cell types found in these animals. Here, we report on the function of Smed-egfr-4 on eye regeneration. The silencing of this gene by RNAi results in animals regenerating smaller eyes compared to controls. The numbers of both eye mature cell types, photoreceptor neurons and eye-cup pigment cells, are significantly decreased in the Smed-egfr-4(RNAi) animals. In contrast, the number of eye progenitor cells expressing the specific markers Smed-ovo and Smed-sp6-9 is increased. These results suggest that Smed-egfr-4 would be required not for the specification of eye progenitor cells but rather for their final differentiation and support the idea that in planarians the EGFR pathway could play a general role regulating the differentiation of lineage-committed progenitors.