Collapse of compartment boundaries and induced identity changes after massive damage in the imaginal discs of Drosophila. Salvador C. Herrera, Ginés Morata. Centro de Biología Molecular (CSIC-UAM), Madrid, Madrid, Spain.

   One of the major questions in regenerative biology is how determined or differentiated cells can acquire pluripotency to repair a missing part of the organism. The wing disc compartments are established very early in development as lineage blocks and act as independent developmental units, as its cells never mix with other compartments. In our experiments we have inflicted massive cell death to either the posterior or the dorsal wing disc compartments and then have studied the regenerative response to the damage and the stability of the A/P and the D/V compartment boundaries. Strikingly, we have discovered that after damage both the A/P and the D/V compartment boundaries very frequently collapse and are later re-established. We detect cells crossing them in both directions and changing the activity of the identity genes (engrailed, cubitus interruptus or apterous) according to their new compartmental determination. We have found that this process is associated with loss of tension at the boundary and with alterations in the activities of factors involved in epigenetic regulation such as the Polycomb and trithorax genes. We believe that after the damage some cells close to the A/P or the D/V borders descend to a naïve determination state and are later reprogrammed. We are trying to identify the mechanisms behind the acquisition of the new identities. By genetic manipulation, we are generating situations in which isolated wild type cells are surrounded by cells overexpressing an ectopic identity gene, e.g. the posterior identity gene engrailed in the anterior compartment. We observe that the isolated anterior cells acquire activity of their endogenous engrailed gene, indicating that it is induced by some signal/interaction with the surrounding cells. This result may open new questions on how developmental genes are regulated.