The cleavage pattern of calanoid copepods-a case study.

Many crustacean groups show stereotyped cleavage patterns during early ontogeny. However, these patterns differ between the various major crustacean taxa, and a general mode is difficult to extract. Previous studies suggested that also copepods undergo an early cleavage with a more or less stereotyped pattern of blastomere divisions and fates. Yet, copepod embryology has been largely neglected. The last investigation of this kind dates back more than a century and the results are somewhat contradictory when compared with those of other researchers. To overcome these problems, we studied the early development of a so far undescribed calanoid copepod species, Skistodiaptomus sp., applying histochemical staining, confocal laser scanning microscopy, and bifocal 4D microscopy. The blastomere arrangement of the four-cell stage of this species varies to a large degree. It can either form a typical radial pattern with the four blastomeres lying in one plane or a tilted orientation of the axes connecting the sister cells of the previous division. In both cases, a stereotyped division pattern is maintained inside each quadrant during subsequent cleavages. In addition, we found two types of blastomere arrangements with a mirror symmetry. Most divisions within the quadrants follow the perpendicularity rule until the eighth cleavage. Deviations from this rule occur only in the narrow regions where the different quadrants touch and near the site of gastrulation. Gastrulation is initiated around the descendants of one individually identifiable blastomere of the 16-cell stage. This cell divides in a specific manner forming a characteristic cell arrangement, the gastrulation triangle. This gastrulation triangle initiates the internalization process of the gastrulation and it is encircled by another characteristic cell type, the crown cells. Our observations reveal several similarities to the early development of Calanus finmarchicus, another calanoid species. These relate to blastomere arrangements and divisions and the pattern of gastrulation. As Calanoida represent a basal or near basal branch of the copepod tree, this description will provide the ground for reconstruction of the cleavage pattern of the last common ancestor of Copepoda.

BioCell2XML: a novel tool for converting cell lineage data from SIMI BioCell to MaMuT (Fiji).

Computer-assisted 4D manual cell tracking has been a valuable method for understanding spatial-temporal dynamics of embryogenesis (e.g., Stach & Anselmi BMC Biol, 13(113), 1-11 2015; Vellutini et al. BMC Biol, 15(33), 1-28 2017; Wolff et al. eLife, 7, e34410 2018) since the method was introduced in the late 1990s. Since two decades SIMI® BioCell (Schnabel et al. Dev Biol, 184, 234-265 1997), a software which initially was developed for analyzing data coming from the, at that time new technique of 4D microscopy, is in use. Many laboratories around the world use SIMI BioCell for the manual tracing of cells in embryonic development of various species to reconstruct cell genealogies with high precision. However, the software has several disadvantages: limits in handling very large data sets, the virtually no maintenance over the last 10 years (bound to older Windows versions), the difficulty to access the created cell lineage data for analyses outside SIMI BioCell, and the high cost of the program. Recently, bioinformatics, in close collaboration with biologists, developed new lineaging tools that are freely available through the open source image processing platform Fiji. Here we introduce a software tool that allows conversion of SIMI BioCell lineage data to a format that is compatible with the Fiji plugin MaMuT (Wolff et al. eLife, 7, e34410 2018). Hereby we intend to maintain the usability of SIMI BioCell created cell lineage data for the future and, for investigators who wish to do so, facilitate the transition from this software to a more convenient program.