Timing of early developmental events in embryos of a tropical sea urchin Echinometra mathaei.

Egg volume of a tropical sea urchin Echinometra mathaei is about one half that of other well-known species. We asked whether such a small size of eggs affected the timings of early developmental events or not. Cleavages became asynchronous from the 7th cleavage onward, and embryos hatched out before completion of the 9th cleavage. These timings were one cell cycle earlier than those in well-known sea urchins, raising the possibility that much earlier events, such as the increase in adhesiveness of blastomeres or the specification of dorso-ventral axis (DV-axis), would also occur earlier by one cell cycle. By examining the pseudopodia formation in dissociated blastomeres, it was elucidated that blastomeres in meso- and macromere lineages became adhesive after the 4th and 5th cleavages, respectively. From cell trace experiments, it was found that the first or second cleavage plane was preferentially employed as the median plane of embryo; the DV-axis was specified mainly at the 16-cell stage. Timings of these events were also one cell cycle earlier than those in Hemicentrotus pulcherrimus. The obtained results suggest that most of the early developmental events in sea urchin embryos do not depend on cleavage cycles, but on other factors, such as the nucleo-cytoplasmic ratio.