Relative Timing of Stiffening with Various Combinations of Nucleate and Enucleated Egg Fragments of Xenopus laevis: (egg stiffening/Xenopus laevis/cell cycle/timing system).

Previously, I found that in Xenopus eggs, the surface contraction wave (SCW) can arise at a point close to a male, a female or a zygote nucleus or a top portion of the egg (9). This finding suggested that all types of nucleus and cytoplasm of a Xenopus egg have the potential to determine the point of initiation of the SCW. Since stiffening is closely associated with the SCW, to determine the factors governing this phenomenon, I compared the times of egg stiffening of pairs of fragments containing different types of nuclei or cytoplasm. I found that the stiffening occurs earlier in fragments containing any type of nucleus than in those containing no nucleus, and earlier in fragments containing a male nucleus than in those containing a female nucleus. These results are consistent with the notion that either a nucleus or cytoplasm that induces the SCW earliest determines the point of initiation of the SCW close to itself. I also found that DNA replication is essential for the earlier occurrence of stiffening in a fragment containing a nucleus.