Hilbrant, Maarten (2008) Development and Evolution of the Spider Silk Producing System. PhD thesis, Universität zu Köln.
The spider silk producing system comprises a complex set of different gland types ending in specialized appendages called spinnerets. Using these organs, all spiders produce different types of silk that they employ for a wide range of tasks, which is a unique trait in the animal kingdom. Unfortunately, the evolutionary origin of the silk producing system is currently poorly understood. Available hypotheses proposed that the different components of the system are homologous to particular other spider organs. If this is true, then it is likely that conserved features, such as parts of the genetic program that regulates their development, are shared between different elements of the silk producing system and their postulated homologs. Examining the morphological and molecular basis of silk gland and spinneret development could therefore provide evidence for their evolutionary origin. The goals of this thesis thus were to describe the embryonic and post-embryonic development of the silk glands and spinnerets in our model species Cupiennius salei, to localize the position of the primordia of the silk glands and to investigate the genetic patterning of the developing spinneret limb buds. In order to facilitate these goals, the late embryonic and early post-embryonic staging system of C. salei was reassessed and modified. The silk glands of the first free foraging stage were described in detail and were found to be already remarkably similar to the adult glands, including a system that allows for the production of silk during molting. It was confirmed that the silk glands first appear inside the spinnerets, supporting their supposed epidermal origin. Detailed investigation of the epidermis of the spinneret limb buds showed invaginations that possibly are the earliest morphological indications of the primordia of the silk glands, and provide a lead to investigating which molecular factors are involved in their early developmental origin. Finally, the study of expression patterns of leg patterning genes in the spinneret limb buds provided a way of comparing these structures with other spider appendages. Taken together, this thesis provides a solid basis for further research to shed more light on the evolution of the spider silk producing system.
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