Developmental Studies of Appendage Patterning and Formation in Spiders
by Natascha Zhang née Turetzek
Date of Examination:2016-03-17
Date of issue:2016-11-21
Advisor:Dr. Nikola-Michael Prpic-Schäper
Referee:Dr. Nikola-Michael Prpic-Schäper
Referee:Prof. Dr. Martin Göpfert
Persistent Address:
http://dx.doi.org/10.53846/goediss-5988
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Abstract
English
In the animal kingdom arthropods are unparalleled in species number and diversity. Especially their appendages show a great morphological diversity and are adapted for different purposes like feeding, walking, flying, swimming, breathing and courtship. The segmentation of the appendages into podomers is one characteristic trait shared by all extant arthropod groups and, at least in part, might be the key to their evolutionary success. Comparative studies of the genetic mechanisms involved in the formation and patterning of the appendages in different representatives of the four major arthropod clades is important to understand the evolution of this diversity. After the specification of the appendage primordia, appendage patterning in general requires the establishment of an additional axis, the proximo-distal axis. The formation of this axis is highly conserved in the arthropods, including homothorax/extradenticle defining the proximal, dachshund the medial and Distal-less the distal leg area. Further mechanisms of proximo- distal patterning involved in tarsus patterning are largely conserved in the insects, but not much is known for arthropods outside the insect clade. To study these processes in non-insect arthropods, I used the main spider model Parasteatoda tepidariorum which has an undivided tarsus and the long-legged cellar spider Pholcus phalangioides, which shows tarsus segmentation that is morphologically similar to the insect tarsus. The conserved leg gap genes were already studied in two other spider species and homothorax and extradenticle were found to be present as duplicates. I studied homologs of all leg gap genes in Parasteatoda tepidariorum and Pholcus phalangioides to clarify the origin of the duplication event and their impact on appendage patterning differences leading to morphological diversification. My data suggest that all proximal and medial leg gap genes were duplicated in the lineage leading to the spiders. Whereas the initial leg gap genes all show the conserved expression as found for other panarthropods and thus might be a synapomorphic trait of the entire phylum, the expression of the paralogs differs indicating that the duplicates have undergone neofunctionalization. However, the origin of duplication as well as the conservation of the new function differs for these genes. The duplicate of dachshund is arachnid specific. It is required for patella formation, which is a morphologically novel walking leg segment only present in spiders and allies. The analysis of candidate patterning genes involved in tarsus segmentation of insects shows that these genes have some conserved functions within the phylum of the arthropods, but that their tarsus segmentation function is not conserved between spiders and insects. These data show that the initial steps of proximo-distal axis formation are highly conserved, but that more downstream patterning networks can be altered to drive the development new morphologies. In addition the presence of gene duplications is beneficial for functional gene diversification leading to novel traits whereas the fundamental developmental role of one paralog remains conserved to ensure proper development.
Keywords: Appendage Development; Spiders; Gene Duplications; Patella; Neofunctionalization; Morphological Novelty; Evolution; Parasteatoda tepidariorum; Pholcus phalangioides