Phylogenetic comparison of serotonin-immunoreactive neurons in representatives of the Chilopoda, Diplopoda, and Chelicerata: implications for arthropod relationships.

The phylogenetic relationships within the Arthropoda have been discussed controversially for more than a century. Comparative studies on structure and development of the nervous system have contributed important arguments to this discussion. Arthropods have individually identifiable neurons that can be used as characters in phylogenetic studies. In the present report, the arrangement of serotonin-immunoreactive neurons in the ventral nerve cord was examined in seven representatives of the Chelicerata, Chilopoda, and Diplopoda. The goal of this analysis was to determine whether number, arrangement, and axonal morphology of the serotonergic neurons in these groups are similar to the pattern found in representatives of the Hexapoda and Crustacea, as explored in a previous study. The results indicate that the pattern in the seven species examined here does not correspond to that present in the Hexapoda and Crustacea. In particular, the pattern in Chilopoda and Diplopoda is clearly different from that of the Hexapoda. The hexapodan pattern most closely resembles that of the Crustacea. These findings are discussed with regard to recent reports on the mechanisms of neurogenesis in these taxa. Furthermore, the proposed ground patterns of the various groups are reconstructed and the characters are plotted on two competing hypotheses of arthropod phylogeny, the traditional Tracheata hypothesis and an alternative hypothesis derived from molecular and recent morphological data, the Tetraconata concept. The data discussed in this article moderately support the Tetraconata hypothesis.