Spam and the evolution of the fly's eye

The open rhabdoms of the fly's eye enhance absolute sensitivity but to avoid compromising spatial acuity they require precise optical geometry and neural connections.1 This neural superposition system evolved from the ancestral insect eye, which has fused rhabdoms. A recent paper by Zelhof and co-workers2 shows that the Drosophila gene spacemaker (spam) is necessary for development of open rhabdoms, and suggests that mutants revert to an ancestral state. Here I outline how open rhabdoms and neural superposition may have evolved via nocturnal intermediates, and discuss the implications for the role of spam in insect phylogeny.