http://www.bbc.co.uk/news/science-environment-10664362

The thing to note is the comment about the wingtip improving performance.  The wing tip is unusual !

well according to me the requirements of minimum vortex drag (in other words induced drag)and minimum structure weight are almost directly opposed. To minimize the vortex drag, the wing system must have either large lateral or large vertical dimensions -- leading to a heavy structure.
that is what happened in this case inverted winglets with polydihedral wing.