Establishment of Axial Properties in the Arrow Worm Embryo, Paraspadella gotoi (Chaetognatha): Developmental Fate of the First Two Blastomeres

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Abstract
Chaetognaths are bilateral animals totally symmetrical on both sides of their body. To elucidate the manner in which cell fates are established, single blastomeres of the two-cell stage embryos of the benthic arrow worm, Paraspadella gotoi, were injected with a fluorescent lineage-tracing dye. The distribution of labels was observed in the hatchlings by using a confocal laser scanning microscope. A total of four different labeling patterns was observed: 1) the dorsal epidermis, the right half of the ventral epidermis, and the right half of the dorsal longitudinal muscles (DR pattern); 2) the dorsal epidermis, the left half of the ventral epidermis, and the left half of the dorsal longitudinal muscles (DL pattern); 3) the right half of the ventral epidermis, the ventral longitudinal muscles, the right half of the dorsal body muscles, and the primordial germ cells (PGCs) (VR pattern); and 4) the left half of the ventral epidermis, the ventral longitudinal muscles, the left half of the dorsal body muscles, and the PGCs (VL pattern). Thus, one blastomere of the two-cell stage largely contributes the dorsal epidermis and the other contributes the ventral longitudinal muscles and the PGCs. Bilateral halves of the dorsal longitudinal muscles and the ventral epidermis were labeled as complementary pairs. These labeling patterns indicate that the first cleavage plane runs oblique to the bilateral and dorsoventral axes. In addition, the occurrence of complementary “DL and VR” and “DR and VL” labeling patterns indicates that the first cleavage plane bears one of two different angular relationships relative to the future body axes