Developmental Studies on Eye Types and Pedipalps in Parasteatoda tepidariorum

Abstract

Arthropods are by far the most species rich phylum in the animal kingdom and their evolutionary success is at least in part rooted in their diverse morphology. Despite these morphological differences, arthropods share a common segmented body plan. These two factors make the arthropods an ideal group for comparative studies. Furthermore, arthropods, and especially chelicerates, show many lineage-specific evolutionary novelties. Parts of both these aspects were addressed in the spider Parasteatoda tepidariorum in the four parts of this thesis. Molecular characterization and embryonic origin of the eyes in P tepidariorum: Two visual systems are present in most arthropod groups: median and lateral eyes. Most of our current knowledge about the developmental and molecular mechanisms involved in eye formation in arthropods comes from research in the model system Drosophila melanogaster. By contrast, not much is known about the development of the up-to-eight eyes present in spiders. Therefore, the embryonic expression of core retinal determination genes in the common house spider Parasteatoda tepidariorum was analyzed. The anlagen of the median and lateral eyes in P. tepidariorum originate from different regions of the non-neurogenic ectoderm in the embryonic head. The median eyes are specified as two individual anlagen in an anterior median position in the developing head and subsequently move to their final position following extensive morphogenetic movements of the non-neurogenic ectoderm. The lateral eyes develop from a more lateral position. Using gene expression analyses, a unique combination of determination gene expression in the anlagen of the lateral and median eyes, respectively were identified. Phylogenetic analysis of forkhead transcription factors: The forkhead family of transcription factors is present in the entire animal kingdom and has roles in various aspects of development and physiological processes. Although homologs of family members have been identified in almost all animals, the relationship between the different groups, especially arthropods and vertebrates is not entirely clear. Therefore, homologs of forkhead transcription factors were identified from the official gene sets of animals with sequenced genomes from all major phylogenetic groups available. This resulted in the categorization of forkhead domain genes into 25 different classes. The roles of these classes correlated with the evolutionary novelties of animal groups, sharing these genes. Moreover, the genomic organization of the forkhead genes was analyzed, adding further evidence to the presence of a conserved cluster from a subset of forkhead genes in different animal lineages. Embryonic development of the pedipalp in P. tepidariorum: The pedipalp is the appendage of the tritocerebral segment in chelicerates. Its development is dependent on the Hox gene labial, which gives identity to this segment. Apart from that, the genetic framework, which leads to the development of this appendage and its specific morphology, different from the walking legs is unknown. Therefore, homologs of genes were identified and analyzed, which were known from Drosophila to be expressed in the tritocerebral segment, or which interact with labial, since they might have retained ancient functions in P. tepidariorum. None of the genes analyzed here showed exclusive expression in the pedipalps, or differential expression patterns between the pedipalps and the legs, but they gave insights into various aspects of shared and diverged developmental processes among arthropods. Development of the bulbus organ in P. tepidariorum: The bulbus organ is a specialized outgrowth of the male spiders’ pedipalp tarsus, which is used as a copulatory organ. It is developed between the last juvenile stage and the adult in a process called pediapalp metamorphosis. Nothing is known of the developmental processes and genetic basis during this process. Therefore, the transcriptomes of juvenile, subadult male, subadult female and adult male spiders were used to find genes, which might be involved in the development of this structure. The analysis resulted in a set of candidate genes, which showed a specific and high expression in the developing subadult male pedipalps. Apart from a large number of genes, which appeared to be specific to spiders, or even only P. tepidariorum, candidate genes included a set of transcription factors with known roles in appendage or sex-specific development.