Bridging and mirroring registrations for Drosophila neuroanatomy

The fly brain is a highly stereotyped 3D structure. A number of groups have now published 3D confocal image datasets where all images have been registered to a specific template brain. In order to co-visualise image data from different studies, data registered against different template brains must be brought into the same 3D coordinate space. This can be achieved by applying a <strong>bridging registration</strong> which maps one template brain onto another. For almost all purposes the fly brain should be considered symmetric about its mid-sagittal plane (i.e. the plane perpendicular to the medio-lateral axis – typically the x-axis in image data). However, while the platonic fly brain may be symmetric, individual fly brains that have been fixed, stained and imaged are often significantly asymmetric and not placed exactly in the centre of the image with respect to their mid-sagittal plane. While it is trivial to make template brains that are centred and possible to make symmetric template brains if one wishes, this has rarely been done in practice. It is therefore necessary to do more than simply mirror along the medio- lateral axis if one desires to map neurons/structures in the left brain hemisphere onto the right hemisphere. <strong>Mirroring registrations</strong> account for this asymmetry and allow for comparisons across brain hemispheres. This collection contains bridging and mirroring registrations for a number of common Drosophila template brains, including those used by the FlyLight GAL4 collection, FlyCircuit, the Vienna Tiles collection, and the new systematic nomenclature for the insect brain.