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Abstract
IntroductionWithin arthropods, several crustacean groups are unique in their early development due to their stereotyped cell division patterns and cell lineages. However, it is still unclear whether these cell division patterns are homologous between the various crustacean groups and whether they could indicate the ground pattern of Tetraconata (Crustacea and Hexapoda). In this study we describe the early development of the raptorial water flea Bythotrephes longimanus as a representative of the Cladocera within branchiopods.ResultsIn B. longimanus the early cell lineage and the cell division pattern are stereotyped up to the fifth cell division cycle. As a morphological marker a nurse cell remnant (ncr) identifies the cell lineage of the smallest and division delayed blastomere up to the 16-cell stage. This marker might be indicative of the germ line. By combining histology, confocal laser scanning microscopy, and 4D microscopy, we reconstruct the early cell lineage and cell division pattern and follow transient formations of cell morphological structures in their temporal and spatial behavior up to gastrulation.ConclusionsCorrespondences to the early cleavage pattern of other Cladocera suggest that the described pattern can be assumed to be ancestral for either the entire Cladocera or for the majority of the Cladocera comprising Anomopoda, Ctenopoda and Onychopoda. The comparison to the cell division patterns of other crustacean groups such as Malacostraca, Ostracoda, and Copepoda reveals similarities that allow for a discussion of a common pattern for the crustacean groups and a ground pattern for the Tetraconata.
Highlights
Within arthropods, several crustacean groups are unique in their early development due to their stereotyped cell division patterns and cell lineages
In this study we provide a detailed description of the early development and cell lineage onychopod cladoceran Bythotrephes longimanus using recent methods
We can show that the early development of B. longimanus is very similar to that of the polyphemid onychopod Polyphemus pediculus
Summary
Several crustacean groups are unique in their early development due to their stereotyped cell division patterns and cell lineages. Cleavage modes range from holoblastic to superficial over mixed cleavage, in which holoblastic cleavage turns into superficial cleavage and vice versa This is even true within the major crustacean groups such as Malacostraca or Branchiopoda [1]. Until the late 1990′s the prevailing idea was that Arthropoda and the spirally cleaving Annelida are closely related, forming the Articulata (see [25]) This view implied the interpretation of stereotyped crustacean cleavage as modified spiral The current interpretation of Arthropoda being more closely related to the Cycloneuralia forming Ecdysozoa renders the assumption of crustacean cleavage as spiral obsolete It has instead provoked the resumption of studies on the early cell lineage pattern of other holoblastically cleaving arthropod groups like pycnogonids [31]. Since no stereotyped cleavage pattern is detectable in pycnogonids or other arthropods apart from crustaceans, there is the question as to whether the various examples of stereotyped cleavage in crustaceans follow a corresponding pattern and whether stereotypy of cleavage evolved once in the tetraconate lineage or independently in the various crustacean lineages
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