Bates William R
Bamfield Marine Science Centre, Bamfield, British Columbia V0R 1B0, Canada.
Evol Dev. 2004 May-Jun;6(3):180-6. doi: 10.1111/j.1525-142X.2004.04023.x.
Gastrulation in the maximum direct developing ascidian Molgula pacifica is highly modified compared with commonly studied "model" ascidians in that endoderm cells situated in the vegetal pole region do not undergo typical invagination and due to the absence of a typical blastopore the involution of mesoderm cells is highly modified. At the gastrula stage, embryos are comprised of a central cluster of large yolky cells that are surrounded by a single layer of ectoderm cells in which there is only a slight indication of an inward movement of cells at the vegetal pole. As a consequence, these embryos do not form an archenteron. In the present study, ultraviolet (UV) irradiation of fertilized eggs tested the possibility that cortical cytoplasmic factors are required for gastrulation, and blastomere isolation experiments tested the possibility that cell signaling beginning at the two-cell stage may be required for the development of the gastrula. Irradiation of unoriented fertilized eggs with UV light resulted in late cleavage stage embryos that failed to undergo gastrulation. When blastomeres were isolated from two-cell embryos, they developed into late cleavage stage embryos; however, they did not undergo gastrulation and subsequently develop into juveniles. These results suggest that cytoplasmic factors required for gastrulation are localized in the egg cortex, but in contrast to previously studied indirect developers, these factors are not exclusively localized in the vegetal pole region at the first stage of ooplasmic segregation. Furthermore, the inability of embryos derived from blastomeres isolated at the two-cell stage to undergo gastrulation and develop into juveniles suggests that important cell signaling begins as early as the two-cell stage in M. pacifica. These results are discussed in terms of the evolution of maximum direct development in ascidians.
与常见的被研究“模式”海鞘相比，最大直接发育的太平洋海鞘的原肠胚形成有很大改变，因为位于植物极区域的内胚层细胞不经历典型的内陷，并且由于没有典型的胚孔，中胚层细胞的内卷也有很大改变。在原肠胚阶段，胚胎由一群位于中央的大卵黄细胞组成，这些细胞被单层外胚层细胞包围，在植物极处只有轻微的细胞向内运动迹象。因此，这些胚胎不形成原肠。在本研究中，对受精卵进行紫外线（UV）照射，测试了原肠胚形成是否需要皮质细胞质因子的可能性，而卵裂球分离实验则测试了从两细胞阶段开始的细胞信号传导对于原肠胚发育是否可能是必需的可能性。用紫外线照射未定向的受精卵会导致胚胎在晚期卵裂阶段无法进行原肠胚形成。当从两细胞胚胎中分离出卵裂球时，它们发育成晚期卵裂阶段的胚胎；然而，它们没有进行原肠胚形成，随后也没有发育成幼体。这些结果表明，原肠胚形成所需的细胞质因子定位于卵皮质中，但与之前研究的间接发育者不同，这些因子在卵质分离的第一阶段并非仅定位于植物极区域。此外，两细胞阶段分离出的卵裂球所产生的胚胎无法进行原肠胚形成并发育成幼体，这表明在太平洋海鞘中，重要细胞信号传导早在两细胞阶段就开始了。本文根据海鞘最大直接发育的进化对这些结果进行了讨论。
