Heteroplasmic conjugates formed by the fusion of starfish oocyte pairs with a 12 minute time difference in the maturation phase

摘要

Two starfish oocytes with a 12 min time difference in the maturation phase were fused together with electric pulses to make a heteroplasmic conjugate. The starfish used were Asterina pectinifera. The emergence of the first meiotic spindle and the extrusion of the polar bodies in the conjugate were timed. Under polarization microscopy two meiotic spindles emerged with a time difference of 10-11 min, which is close to the time difference in the maturation phase between the original oocytes before fusion. In contrast, subsequent formation of the first two polar bodies occurred successively with a short time lag of 1-3 min between them. Times for the formation of both polar bodies were midway between the anticipated times for polar body formation in respective non-fused contro oocytes. Thus, in one nucleus the meiotic division was delayed, while in another nucleus it was accelerated, in a single heteroplasmic conjugate. These two sets of observations indicate the presence of a certain control systemthat regulates progression of the cell cycle at a point during the period from the entry into metaphase through to late anaphase of meiosis I in starfish oocytes. This type of cell cycle control in starfish oocytes is obviously distinct from the currently accepted view of the cell cycle control by the spindle assembly checkpoint that monitors unattached kinetochores of mitotic chromosomes.