The evolution and development of functional units in chelicerates

• Applicant: Dr. Carolin Haug
• Subject Area: Palaeontology
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 312904847

The evolutionary success of arthropods seems to be (at least partly) based on the subdivision of the segmented body into functional units, the tagmata. In chelicerates (scorpions, spiders, mites and alike) the tagmosis pattern is generally considered to be very strict, with an anterior prosoma and a posterior opisthosoma. However, this seemingly stereotypic body organisation is broken up in different chelicerate groups. Three types of tagmosis divergencies can be recognised, which will be studied in the course of this project: 1) a subdivision of the prosoma (e.g., into pro-, meso-, metapeltidium); 2) the different location of the prosoma-opisthosoma boundary; 3) the further subdivision of the opisthosoma (also termed meso- and metasoma or pre- and postabdomen). Based on these observations, three general questions arise: How strictly conservative is the pattern of tagmosis within chelicerates? How flexible is the evolution of functional units in chelicerates? Which functional constraints trigger or hamper the evolution of new subdivisions of tagmata? To address these questions, extant and fossil representatives of the different chelicerate groups will be investigated; especially developmental data will be included, also of fossil representatives as far as available. Different up-to-date imaging techniques such as polarised or fluorescence imaging (with and without staining), serial sections with 3D reconstruction, or micro-computed tomography (microCT) allow to make even minute structures visible. Via cooperations with different collections, researchers and commercial sellers worldwide I have access to all kinds of chelicerates, extant and fossil, adult and earlier developmental stages. The combination of developmental biology and palaeontology, both with modern imaging techniques, is seen as the central advantage of the presented project to reveal the evolution of body organisation of chelicerates. Identifying homologous structures is often hampered by their highly specialised appearance; this difficulty can be resolved by the inclusion of developmental data. While representatives of different chelicerate lineages, for example, Limulus polyphemus, are often treated as 'living fossils', they indeed possess many apomorphic features (evolutionary novelties). With the aid of well preserved fossil representatives these can be identified and 'subtracted'. Only with this interdisciplinary approach, it will be possible to elucidate the underlying mechanisms for evolutionary changes in tagmatisation patterns, e.g., changes in developmental timing (heterochrony) and reconstruct a reliable scenario for chelicerate evolution. Ultimately, I expect to gain important new insights into the impact the tagmatisation as 'division of labour' had on the evolutionary success of chelicerates and finally also on all sclerotised arthropods.

• DFG Programme: Research Grants