Cleavage and gastrulation in Pycnogonum litorale (Arthropoda, Pycnogonida): morphological support for the Ecdysozoa?

摘要

The early cleavage and gastrulation of the pyc-nogonid Pycnogonum litorale is investigated in detail by fluorescence microscopy, confocal laser scanning microscopy, and histology. The cleavage is holoblastic with equally sized blastomeres and an irregular radial pattern. There is no stereotypic cell lineage, and timing and spindle directions of individual mitoses vary to a high degree. Gastrulation begins at the 63-cell stage with the retardation and enlargement of a cell which adopts the form of a bottle and fills the interior of the egg, followed by immigration and epiboly of smaller cells surrounding the large bottle-shaped cell. The gastrulation site marks the dorsal side of the embryo and the stomodaeum forms adjacent to the area of gastrulation. The pattern of the early development of Pycnogonum is compared with that of other Pycnogonida resulting in a putative ground pattern of pycnogonid development. Furthermore, our results are discussed in the wider framework of putative arthropod and cycloneuralian relationships. This comparison implies morphological support for the Ecdysozoa.