Washitani-Nemoto S, Saitoh C, Nemoto S
Biological Laboratory, Hitotsubashi University, Tokyo, Japan.
Dev Biol. 1994 Jun;163(2):293-301. doi: 10.1006/dbio.1994.1148.
Artificial parthenogenesis in starfish requires the activation of oocytes and the suppression of the polar body (PB) extrusion. To induce parthenogenesis we have employed a combination of calcium ionophore A23187 for activating oocytes and cytochalasin B (CB) for the suppression of PB extrusions. The treatment of activated oocytes with CB during meiosis I produced eggs lacking polar bodies, i.e., during meiosis I produced eggs lacking polar bodies, i.e., 0pb eggs, and treatment during meiosis II induced eggs bearing the first PB only, i.e., 1pb eggs. About 90% of both 0pb and 1pb eggs developed parthenogenetically with strong synchrony of cell division among eggs. After meiotic chromosomes in CB-treated oocytes separated finally into monads, they came together to form a single nucleus. The nucleus was found to be tetraploid in the 0pb eggs and diploid in the 1pb eggs. Regardless of the difference in ploidy, both 0pb and 1pb eggs developed as tetraploid embryos. Observations of the behavior of meiotic chromosomes and nuclei revealed that in the 0pb eggs, the first round of chromosomal replication was followed by the first cleavage, as in normally fertilized eggs. In the 1pb eggs, on the other hand, two rounds of chromosomal replication were found to precede the first cleavage, indicating the occurrence of one (first) round of chromosomal replication that is not accompanied with cytokinesis. In the first round of mitosis, a bipolar spindle did not appear, but only a half spindle was formed, resulting in the failure of both karyokinesis and cytokinesis. Thus, both types of parthenogenetic eggs became tetraploid before the first cleavage. At the first cleavage, both types of eggs formed a bipolar spindle and divided into a pair of blastomeres. Based on these observations, we suggest that the meiotic centrosomes remaining in these eggs by the failure of PB extrusion are diverted into mitosis-organizing centers in the mitotic spindle, and this results in parthenogenetic development.
海星的人工孤雌生殖需要激活卵母细胞并抑制极体（PB）排出。为了诱导孤雌生殖，我们采用了钙离子载体A23187激活卵母细胞和细胞松弛素B（CB）抑制PB排出相结合的方法。在减数分裂I期间用CB处理激活的卵母细胞产生了没有极体的卵，即0pb卵，而在减数分裂II期间处理则诱导产生了仅带有第一极体的卵，即1pb卵。大约90%的0pb卵和1pb卵都进行了孤雌生殖发育，卵之间细胞分裂同步性很强。在经CB处理的卵母细胞中，减数分裂染色体最终分离成单体后，它们聚集在一起形成一个单核。发现0pb卵中的核是四倍体，1pb卵中的核是二倍体。尽管倍性不同，但0pb卵和1pb卵都发育成了四倍体胚胎。对减数分裂染色体和细胞核行为的观察表明，在0pb卵中，第一轮染色体复制后紧接着第一次卵裂，就像正常受精的卵一样。另一方面，在1pb卵中，发现两轮染色体复制先于第一次卵裂，这表明发生了一轮（第一次）不伴随胞质分裂的染色体复制。在第一轮有丝分裂中，没有出现双极纺锤体，只形成了半个纺锤体，导致核分裂和胞质分裂都失败。因此，两种类型的孤雌生殖卵在第一次卵裂前都变成了四倍体。在第一次卵裂时，两种类型的卵都形成了双极纺锤体并分裂成一对卵裂球。基于这些观察结果，我们认为由于PB排出失败而残留在这些卵中的减数分裂中心体被转移到有丝分裂纺锤体中的有丝分裂组织中心，这导致了孤雌生殖发育。
