Origin of the novel chemoreceptor Aesthetasc "Y" in Ostracoda: morphogenetical thresholds and evolutionary innovation

SUMMARY The morphology and developmental processes of the two types of ostracod chemoreceptors, the Aesthetasc "Y" and the "Grouped setae," were compared. Cypridoidea and Pontocypridoidea, belonging to Cypridocopina, have a large baseball bat-like seta as an autapomorphic character on the second antenna, whereas most ostracod taxa with plesiomorphic characters bear "Grouped setae" consisting of multiple setae on the second antenna. Their budding positions, morphology, and ontogenetic changes were compared, and our deduction is that the Aesthetasc "Y" originated from "Grouped setae-like" organ in the Paleozoic. The morphogenetic processes in the molting period of these chemoreceptors were compared at the cellular level. The observations suggest that the "Grouped setae" are formed by hypodermal cells and share sheath cells corresponding to those of the Aesthetasc "Y" as a common constraint in the molting process of setae. We conclude that modification of the morphogenetic processes in the molting period of the "Grouped setae" gave rise to the Aesthetasc "Y" as a novel organ in the evolutionary pathway of the Ostracoda.