Initial observation of potential factors involved in the specification process of oral‐aboral axis in the sand dollar Scaphechinus mirabilis

To elucidate factors involved in the oral‐aboral axis specification, several observations and experiments were undertaken using the sand dollar Scaphechinus mirabilis . Unlike in Strongylcentrotus purpuratus , localization of mitochondria was not detected in unfertilized eggs. After fertilization, however, the bulk of mitochondria became localized to the opposite side of sperm entry. The first cleavage divided this mitochondrial cluster into daughter blastomeres. On the other hand, a second cleavage produced daughter blastomeres containing quite different amounts of mitochondria. To know whether such mitochondrial localization affects the oral‐aboral axis specification, 4‐cell‐stage embryos were separated along the second cleavage plane. Although both half embryos developed into morphologically normal plutei, some differences, such as the number of pigment cells, were noticed between the siblings. In contrast, cell tracing revealed that the first cleavage separated the oral from the aboral part in most cases, indicating that the unequal distribution of mitochondria is not critical for the oral‐aboral axis specification. Further, stained and non‐stained half embryo fragments were combined. Such combined embryos developed into normal plutei with a single oral‐aboral axis. The plane dividing labeled and non‐labeled parts were incident, oblique or perpendicular to the median plane of the combined embryo, and the appearance frequencies of those labeling patterns were similar to those obtained by cell tracing in intact embryos. Interestingly, the half fragments derived from embryos inseminated earlier showed a tendency to form the oral part. These suggest that several factors as well as the localized cytoplasmic components would be involved in the specification process of oral‐aboral axis.