Analyse der evolutionären Modifikationen der Neurogenese innerhalb der Arthropoden
Final Report Abstract
The aim of the proposed project was to uncover similarities and differences in (1) the establishment of the identity of neural precursor and (2) the formation of the axonal scaffold of euarthropods. Arthropods are a diverse group and can be found in marine, freshwater, terrestrial, and even aerial environments. The arthropod nervous systems must therefore be adjusted to the highly diverse behaviour and requirements of the individual arthropod species. This raises the question how the underlying patterning mechanisms have changed during arthropod evolution to produce the characteristic axonal scaffold on the one hand and allow for variations in neuronal networks on the other hand. Euarthropods consist of four groups: insects, crustaceans, chelicerates and myriapods. We have shown recently that neurogenesis in chelicerates and myriapods is significantly different from insects and crustaceans. While in insects and higher crustaceans (malacostracans) the nervous system is generated by single stem‐cell like cells (neuroblasts), in chelicerates and myriapods groups of neural precursors (NPGs) are specified for the neural fate, which directly differentiate into neural cells. In this project we have shown that regardless of the different modes of neurogenesis, the overall number of NPGs/neuroblasts as well as their spatial arrangement in rows and columns is similar in all four euarthropod groups indicating a common origin of this pattern. We suggest that the stereotyped arrangement of NPGs/neuroblasts is required for the generation of the highly conserved spatial pattern of the axonal scaffold in arthropods. Furthermore, we could demonstrate differences in the spatial and temporal expression of genes required for the identity of neural precursors in insect, myriapods and chelicerates. While the spatial changes in the expression pattern of the columnar gene muscle segment homeobox (msh) in chelicerates and myriapods only result in subtle changes in relation to the NPG pattern, the temporal and spatial changes together with the modifications in neural precursor formation in insects has led to a significant divergence of msh expression relative to the neuroblast/NPG arrangement. In addition, we detected differences in the expression and regulation of the motor‐ and interneuron specific genes even‐skipped and islet in chelicerates. These modifications might have allowed for evolutionary variations in neural identity in the individual arthropod groups. In the second part of the project we analysed the development of the precheliceral lobe of C. salei. We could show that the head neuroectoderm that gives rise to the precheliceral lobe can be subdivided into two areas: (1) the peripheral primordium which produces the optic lobe, the mushroom bodies and the arcuate body, and (2) the central area which develops into the central protocerebrum. The central domain, similar to the neuroectoderm that forms the ventral ganglia, gives rise to a stereotyped area of NPGs that later dissociate into tightly packed clusters of neurons. By contrast, in the peripheral primordium of the developing precheliceral lobe large invaginations foreshadow the emergence of the arcuate body, mushroom bodies and optic ganglia; many NPGs are incorporated into the large invaginations and remain visible during further development. This suggests that the NPGs represent functional elements that are responsible for the formation of discrete parts of the brain centres. The areas of the developing visual brain centres – the optic ganglia, the mushroom bodies and the arcuate body ‐ express the proneural gene CsASH1 followed by the expression of the neural differentiation marker Prospero. Furthermore, the transcription factor dachshund, which is strongly enriched in the mushroom bodies and the outer optic ganglion of Drosophila, is expressed in the optic anlagen and the mushroom bodies of the spider. We could show that several molecular and morphological aspects of the development of the optic ganglia and the mushroom bodies are similar in the spider and in insects. In the last part of the project we analysed the formation of the axonal scaffold in the two spider species Cupiennius salei and Achaearanea tepidariorum. In both spider species axonal tract formation seems to follow the same sequence as in insects and crustaceans. First, segmental neuropiles are established which then become connected by the longitudinal fascicles. The commissures are established at the same time as the longitudinal tracts despite the large gap between the corresponding hemi‐neuromeres which results from the lateral movement of the germband halves during inversion. Furthermore, we identified two nerve roots, the ISN and SN, exiting each hemi‐neuromere in both spider species. We then analysed the function of the axonal guidance cue Netrin and could show that Netrin has a conserved function in commissural axon guidance. This function is achieved by an adaptation of netrin expression to the specific movements of the germband that occur during spider embryogenesis. Furthermore, we describe a novel function of Netrin in the formation of glial sheath cells that enwrap the neural precursor groups in Cupiennius salei. Loss of Netrin function leads to the absence of glial sheath cells which in turn leads to premature segregation of neural precursors and overexpression of the early motor‐ and interneuronal marker islet. Both the conserved and novel functions of Netrin seem to be required for the proper formation of the axonal scaffold. Taken together the data show that conserved genes are required for the different steps of neurogenesis in euarthropods. Conserved functions are maintained by an adaptation of the expression patterns of these genes to the specific morphology of the individual arthropod groups. On the other hand, variations in neural networks seem to be achieved by subtle changes of conserved expression domains in relation to the underlying morphological structures and changes in the regulation of downstream genes.
Publications
- (2010) Compartmentalization of the precheliceral neuroectoderm in the spider Cupiennius salei: Development of the arcuate body, optic ganglia, and mushroom body. J Comp Neurol 518(13), 2612‐32
Doeffinger, C., Hartenstein, V., Stollewerk, A.
- (2010) How can conserved gene expression allow for variation? Lessons from the dorso‐ventral patterning gene muscle segment homeobox. Dev Biol 345, 105‐16
Doeffinger, C. and Stollewerk, A.
