Fins from scales? - An evo-devo approach to the developmental basis and shared evolutionary origin of scales, scutes, fulcra and fin rays.

• Applicant: Dr. Joost Woltering
• Subject Area: Evolutionary Cell and Developmental Biology (Zoology)
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404363160

The vertebrate dermal skeleton can be subdivided into two main components, fin rays and scales. A long-standing hypothesis predicts that scales and fin-rays are serially homologous and share a common evolutionary origin, whereby fin rays evolved out of scales. This hypothesis suggests the existence of an ancestral genetic module for scales, which is also active to pattern fin rays, as well as the existence of a “novel” genetic component that evolved to individuate fin rays. The question of scale/fin ray homology has been mostly addressed at the histological level and no broad-scale genetic profiling has been performed to systematically identify the shared and divergent genetic pathways underlying scale and fin ray development. In this project, we will leverage RNAseq, together with downstream validation using in situ hybridization and CRISPR/Cas9, with the aim of identifying the shared and divergent genetic modules underlying scale and fin ray development during embryogenesis of cichlids and sturgeons. In a comparison with the fin fold of shark embryos, (which do not possess fin-rays but only the more ancestral actinotrichia), we will investigate the hypothesis that fin-rays have evolved via co-option of an ancestral scale module into the fin fold. Under this transformational scenario we predict that the transcriptional program driving fin ray development can be interpreted as an amalgamation of ancestral scale and fin fold programs. In addition, we will investigate the transcriptional relationships between fin-rays, scales and enigmatic “fin-associated scale-like elements”. The latter concern the fulcra and scutes that are present in sturgeons and the procurrent fin-rays in teleost fish. Fulcra are scale-like elements present on the anterior caudal fin margin in basal ray-finned fish which transformed into the homologous procurrent fin rays in teleost. Scutes are modified scales present in sturgeons, which in the dorsal fin show significant ontogenetic similarity with fin-rays. Therefore, scutes and fulcra are scale-like elements with an intricate link to fin development and will be able par excellence to clarify the essential differences between the genetic individuation of fin-rays and scales. Altogether, this project will further our understanding of the natural history of the vertebrate dermal skeleton and inform on the evolutionary and developmental interrelationships of its two primary components: fin-rays and scales.

• DFG Programme: Research Grants