Jeffery W R, Swalla B J
Department of Zoology, University of California, Davis, Bodega Bay 94923.
Dev Biol. 1992 Oct;153(2):194-205. doi: 10.1016/0012-1606(92)90105-p.
The ascidian Molgula oculata has a tailed (or urodele) larva, whereas Molgula occulta develops directly via a tailless (or anural) embryo. Interspecific hybrid embryos produced by fertilizing M. occulta eggs with M. oculata sperm (M. occulta x M. oculata hybrids) can develop urodele larval structures, including a brain pigment cell and a short tail containing a small notochord. Development of larval features differs in individual M. occulta clutches: some eggs develop into hybrids with both a brain pigment cell and a tail, some into hybrids with either a brain pigment cell or a tail, and others into hybrids without urodele features. The expression of a 58-kDa protein (p58), which is present in eggs and embryos of urodele ascidians but lacking in those of most anural species, also varies in expression between different clutches of M. occulta eggs. Western blot and immunofluorescence studies show that p58 expression is correlated with the ability of hybrid embryos to express urodele features. For example, clutches of M. occulta eggs containing relatively high levels of p58 produce many hybrids with both a brain pigment cell and a tail. Differential expression of p58 occurs during oogenesis in M. occulta individuals: p58 is found at similar levels in previtellogenic oocytes, but in some animals it disappears during vitellogenesis, while in others it persists throughout oogenesis and is present in mature eggs. When M. occulta eggs are extracted with Triton X-100, p58 remains in the detergent-insoluble fraction, suggesting that it is associated with the cytoskeleton. In most unfertilized M. occulta eggs, p58 is uniformly distributed, but after fertilization it is localized in the uncleaved zygote and then concentrated in embryonic ectoderm, notochord, and muscle lineage cells. Despite containing high levels of p58, gynogenetic hybrid embryos, produced by fertilizing M. occulta eggs with uv-irradiated M. oculata sperm, develop into hybrids without a brain pigment cell or a tail. The results suggest that both a functional paternal genome and p58 are necessary for restoration of larval features in M. occulta x M. oculata hybrids. The cytoskeletal complex containing p58 may mediate the localization of key maternal factors in the egg or may be involved in cellular interactions during embryogenesis which are responsible for development of urodele cell fates.
海鞘眼状海鞘有带尾（或有尾目）幼虫，而隐匿海鞘则通过无尾（或无尾型）胚胎直接发育。用眼状海鞘精子使隐匿海鞘卵子受精产生的种间杂交胚胎（隐匿海鞘×眼状海鞘杂交种）可发育出有尾幼虫结构，包括脑色素细胞和含有小脊索的短尾。隐匿海鞘不同卵块中幼虫特征的发育情况有所不同：一些卵发育成同时具有脑色素细胞和尾巴的杂交种，一些发育成只有脑色素细胞或只有尾巴的杂交种，还有一些发育成没有有尾特征的杂交种。一种58 kDa蛋白（p58）在有尾海鞘的卵和胚胎中存在，但在大多数无尾物种中不存在，其表达在隐匿海鞘不同卵块之间也有所变化。蛋白质免疫印迹和免疫荧光研究表明，p58的表达与杂交胚胎表达有尾特征的能力相关。例如，含有相对高水平p58的隐匿海鞘卵块会产生许多同时具有脑色素细胞和尾巴的杂交种。隐匿海鞘个体在卵子发生过程中p58存在差异表达：在卵黄生成前的卵母细胞中p58水平相似，但在一些动物中它在卵黄生成过程中消失，而在另一些动物中它在整个卵子发生过程中持续存在并存在于成熟卵中。当用Triton X - 100提取隐匿海鞘卵时，p58保留在去污剂不溶性部分，这表明它与细胞骨架相关。在大多数未受精的隐匿海鞘卵中，p58均匀分布，但受精后它定位于未分裂的合子中，然后集中在胚胎外胚层、脊索和肌肉谱系细胞中。尽管含有高水平的p58，但用紫外线照射的眼状海鞘精子使隐匿海鞘卵子受精产生的雌核发育杂交胚胎发育成没有脑色素细胞或尾巴的杂交种。结果表明，功能性父本基因组和p58对于隐匿海鞘×眼状海鞘杂交种中幼虫特征的恢复都是必需的。含有p58的细胞骨架复合体可能介导关键母体因子在卵中的定位，或者可能参与胚胎发生过程中的细胞相互作用，这些相互作用决定了有尾细胞命运的发育。
