Abstract

Evolutionary innovations are biological revolutions: new organs are critically associated with the emergence of new species and their exploitation of new niches. Despite their importance in the history of life, how morphological novelty arises and evolves is a long-standing question in Evolutionary Biology. How the genetic network associated to the new structure appears? How this new structure is functionally and anatomically integrated into the pre-existing body plan? One of the most striking examples of a sexually dimorphic novel structure occurs in males of the mayfly species Cloeon dipterum. Cloeon males develop, in addition to the compound eyes (shared by males and females), an extra pair of extremely large dorsal, turban-shaped eyes. Thus, by comparing males versus females, this mayfly species provides a privileged system to understand the origin and integration of new structures. To answer these questions, we describe the development of the eye and its integration with the optic lobes of male and female Cloeon nymphs using confocal and electronic microscopy. Furthermore, we compare sex-specific gene expression in nymphal heads at single cell resolution, to show how the highly conserved Retinal Determination Network (RDN) could have evolved to play a role in the origin of this novel sexually dimorphic visual organ.