Neurogenesis in the chilopod Lithobius forficatus suggests more similarities to chelicerates than to insects.

In a recent comparative study on neurogenesis in the diplopod Glomeris marginata we have shown that the millipede and the spider share several features that cannot be found in homologous form in insects and crustaceans. The most distinctive difference is that groups of neural precursors are singled out from the neuroectoderm of the spider and the diplopod, rather than individual cells (i.e. neuroblasts) as in insects or crustacean. This observation constitutes the first morphological indication for a close myriapod/chelicerate relationship that has otherwise only been suggested by molecular phylogenetic analysis. To see whether the pattern of neurogenesis described for the diplopod is representative for myriapods, we analysed neurogenesis in the basal chilopod Lithobius forficatus. We show here that groups of cells invaginate from the chilopod neuroectoderm at strikingly similar positions as the invaginating cell groups of the diplopod and the spider. Furthermore, the expression patterns of the proneural and neurogenic genes reveal more similarities to the chelicerate and the diplopod than to insects. Thus, chelicerates and myriapods share the developmental mechanism for neurogenesis, either because they are true sister groups, or because this reflects the ancestral state of neurogenesis in arthropods.