The Zebrafish as a Model for the Evolution and Development of Breeding Tubercles in Fishes

Breeding tubercles are multicellular, keratinized epidermal projections that are used in intraspecific competition and maintaining contact during spawning in fishes. The development of these novel structures has been little studied, despite their presence in the Zebrafish (Danio rerio) model system. The aim of the present study was to help establish the Zebrafish as a model for the development and evolution of breeding tubercles in fishes. The appearance of breeding tubercles during development was documented for this species with special focus on tubercles of a lateral flap of the lower jaw, which represents a synapomorphy (shared derived feature) of the genus Danio. As this fleshy flap is supported by a lateral extension of the dentary bone, the possibility of induction of tubercles by this structure was investigated by comparing tubercle and projection development in the Zebrafish. Breeding tubercles, the fleshy jaw flap, and dentary projection were also characterized in eight additional species of the subfamily Danioninae. This study revealed the coordinated appearance of the three structures as well as loss of tubercles in one clade. Finally, the Ectodysplasin signaling pathway was determined to be necessary and sufficient for tubercle development, establishing variation in this pathway as a candidate cause of variation in breeding tubercles of fishes.