The Daphnia carapace and the origin of novel structures

Understanding how novel structures arise is a central question in evolution. Novel structures are often defined as structures that are not derived from (homologous to) any structure in the ancestor1. The carapace of the water flea Daphnia magna is a bivalved "cape" of exoskeleton that has been proposed to be one of many novel arthropod structures that arose through repeated co-option of genes that also pattern insect wings2-4. To determine whether the Daphnia carapace is a novel structure, we compare the expression of pannier, araucan, and vestigial between Daphnia, Parhyale, and Tribolium. Our results suggest that the Daphnia carapace did not arise by co-option, but instead derives from an exite (lateral lobe) that emerges from an ancestral proximal leg segment that was incorporated into the Daphnia body wall. The Daphnia carapace therefore appears to be homologous to the Parhyale tergal plate and the insect wing5. Remarkably, the vestigial-positive region that gives rise to the Daphnia carapace appears to be present in Parhyale6 and Tribolium as a small, inconspicuous protrusion. Similarly, the vestigial-positive regions that form thoracic tergal plates in Parhyale appear to be present in Daphnia, even though Daphnia does not form thoracic tergal plates. Thus, rather than a novel structure resulting from gene co-option, the Daphnia carapace appears to have arisen from a shared, ancestral tissue (morphogenetic field) that persists in a cryptic state in other arthropod lineages. Cryptic persistence of unrecognized serial homologs may thus be a general solution for the origin of novel structures.