Sea spiders show a surprising pattern in the early development of their central nervous system, which has not been seen previously in any other arthropod, shows research published in EvoDevo by Georg Brenneis from Humboldt University and colleagues.
Arthropods comprise a diverse group of invertebrate animals that share traits including an exoskeleton, jointed limbs, and a segmented body. This includes all insects, crustaceans, spiders, scorpions as well as numerous other smaller groups of animals, such as sea spiders, or Pycnogonida. In order to infer the phylogeny (evolutionary relatedness) of this diverse group, a number of morphological, developmental and molecular techniques are nowadays employed.
Using these techniques, arthropods were divided neatly into those that exhibit neural stem cells, the tetraconates (Hexapoda and ‘Crustacea’), and those that don’t, which was presumed to be all other arthropods. As Hexapoda and crustaceans are close relatives, it was thought that they had evolved neural stem cells as a derived feature, exclusively seen in these arthropods. Research by Brenneis and colleagues now questions this established concept.
Included in this recent article is an annotated phylogenetic tree of arthropods (see simplified schematic, left), which shows a separation of two major groups, Chelicerata (which includes Pycnogonida and Euchelicerata) and Mandibulata (including Myriapoda, and the more closely related Hexapoda and ‘Crustacea’ (Tetraconata)), and an outgroup of Onychophora, more commonly known as velvet worms, which are the sister group to all arthropods.
From previous analyses of embryonic neurogenesis in these groups, it was believed that since splitting from the rest of Arthropoda, the Tetraconata evolved a new process of embryonic neurogenesis. In this new process, the nervous system is formed by a type of neural stem cell known as a neuroblast, from which neurons and glial cells are derived through multiple divisions. However, in other arthropods, the Euchelicerata and Myriapoda, neuroblasts are lacking, and neural cells develop directly from immigrating neuroectodermal cells. As a derived feature, scientists therefore would only expect to see these neuroblasts in Tetraconata, and not in any other, distantly related arthropod groups.
However, when studying the non-model Pycnogonida, Brenneis and colleagues were surprised to find that these sea spiders share some features of neurogenesis with crustaceans and hexapods, as well as showing other neurogenesis features characteristic of Euchelicerata. The sea spiders display a two phase pattern of nervous system development, including the development of neural stem cells, which show similarities with the neuroblasts found in Tetraconata. As Pycnogonida is not closely related to Tetraconata, this finding suggests that neural stem cells may in fact be an ancestral feature of Arthropoda, which may have been subsequently lost in some clades. The authors call for a thorough reinvestigation of embryonic neurogenesis across all arthropods, with a particular focus on Myriapoda, as a sister taxon to the tetraconates.
This discovery could have important implications on our understanding of the neurogenesis of the theoretical ancestor of all arthropods. Previous assumptions on which our current model of arthropod neurogenesis is based may no longer be valid. Further research will play a vital role in resolving of our understanding of the evolution of the arthropods.