Schwager, Evelyn (2008) Segmentation of the spider Achaearanea tepidariorum investigated by gene expression and functional analysis of the gap gene hunchback. PhD thesis, Universität zu Köln.
During the last decade, the molecular examination of the spider segmentation process has provided exciting new insights into the question of the origin and evolution of segmentation. Spiders have been shown to utilize two signaling pathways to generate their segmental pattern that are also involved in segmentation in vertebrates. This has been interpreted as an indication that there might be a common origin of segmentation that dates back to the last common ancestor of bilateral symmetrical animals, the �Urbilateria�. However, this data was so far only based on the segmentation process in the posterior body part of the spiders, the opisthosoma. The earlier segmental patterning process in the anterior, the spider prosoma, has remained unstudied. It is therefore the aim of this thesis to provide some insights on the prosomal segmentation mechanism by studying segmentation genes in the early embryo of Achaearanea tepidariorum. The first part of this thesis demonstrates that the anterior patterning mechanism is fundamentally different from the posterior patterning mechanisms, since the prosomal segments are not generated sequentially, but more or less simultaneously. The second part of this thesis takes a closer look at the role of one particular segmentation gene, the gap gene hunchback (hb). In insects, hb is required for the formation of an adjacent set of segments through the regulation of downstream target genes of the pair rule and segment-polarity class. In addition, hb is a major regulator of Hox genes in insects and it has even been suggested that this is the ancestral role of hb. However, to date hb function has only been analyzed in insects. The here presented work surprisingly shows that hb acts like a gap gene during anterior segmentation of the spider, a non-insect arthropod. The leg-bearing segments L1, L2 and L4 are missing after down-regulation of At-hb via RNAi and hb is required for the correct organization of segmentation genes in this region of the embryo. Even more surprisingly, At-hb does not control Hox gene expression in the spider, which thus does not support the assumption that this is the ancestral role of hb. These findings suggest that anterior spider segmentation utilizes a Drosophila-like genetic mode, in that a field of cells is subdivided into segmental units, while a vertebrate-like mechanism involving Wnt8 and Notch/Delta signaling is used to pattern posterior segments. This supports the assumption that short germ arthropods employ two distinct mechanisms to segment their anterior and posterior body parts.
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