Abstract

Fitting pieces and further puzzles – What ‘nobody’ can tell about the evolution of arthropod neurogenesis

Pycnogonida (sea spiders or ‘nobodies’) are a group of bizarre marine arthropods being now generally considered as basal-most chelicerate branch or alternatively even discussed as earliest extant offshoot from the euarthropod lineage. Given this interesting position, investigation of pycnogonid development promises to contribute important data for the unravelling of arthropod evolution. During the last two decades, new data on early neurogenesis in different arthropod lineages has led to a vivid discussion about the evolution of arthropod nervous system development. However, a re-investigation of sea spider neurogenesis is still lacking. We studied early neurogenesis in Pseudopallene sp., a pycnogonid with prolonged embryonic development. It is characterised by immigration of cells and cell clusters lacking mitotic activity, similar to euchelicerates and myriapods. In contrast to the latter two lineages, only part of the immigrating cell groups can be shown to arise in a stereotyped arrangement, which is potentially due to spatial restrictions of the neuroectoderm. Intriguingly, big flask-shaped cells with high mitotic activity are encountered in paired neuroectodermal invaginations during later embryonic development. These cells represent morphologically distinct neural precursor cells, a feature unknown for all euchelicerates investigated so far and only tentatively discussed for some myriapod representatives. In terms of cell size, position and proliferation activity, the pycnogonid neural precursors show some correspondences to tetraconate neuroblasts. However, for the assessment of more detailed similarities or differences cell lineage studies and gene expression data are indispensable. During postembryonic development, the neural precursor cells detach from the ectoderm and form small segmental clusters on the ventral side of the growing ganglion anlagen. A persisting connection between ganglia and ventral cell clusters via ‘cell-streams’, coupled to mitosis patterns and ganglion cell counts indicates the clusters to act as source for additional neuronal cell material during postembryonic life. The ‘cluster/cell-stream/ganglion’ composition shows structural similarities to the life-long active neurogenic niche in the olfactory system of some crustaceans. A similar function of the pycnogonid ventral clusters is discussed.