Sato S, Masuya H, Numakunai T, Satoh N, Ikeo K, Gojobori T, Tamura K, Ide H, Takeuchi T, Yamamoto H
Biological Institute, Graduate School of Science, Tohoku University, Aoba-yama, Sendai, Japan.
Dev Dyn. 1997 Mar;208(3):363-74. doi: 10.1002/(SICI)1097-0177(199703)208:3<363::AID-AJA7>3.0.CO;2-G.
Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.
海鞘的蝌蚪幼虫在大脑中有两种感觉色素细胞。一种是作为重力感受器起作用的耳石细胞，另一种色素细胞是被称为眼点的原始光感结构的一部分。这些感觉细胞，与脊椎动物的色素细胞一样，含有膜结合的黑色素颗粒，被认为在这种细胞类型的进化过程中占据关键地位。为了研究伴随色素细胞进化的分子变化，我们从柄海鞘中分离出了一个编码酪氨酸酶的基因，酪氨酸酶是黑色素生物合成中的关键酶。该cDNA有一个596个氨基酸的开放阅读框（ORF），其氨基酸序列与脊椎动物酪氨酸酶的同源性为36 - 39%。此外，对cDNA和基因组克隆的序列分析揭示了酪氨酸酶基因的异常组织、转录本的一个与编码序列具有显著同源性的非同寻常的3'非翻译区，以及在编码细胞质结构域的序列中有一个短内含子。该基因首先在早期神经胚神经板中的两个色素前体细胞中被检测到表达，随后在晚期尾芽胚胎大脑中的两个含黑色素的色素细胞中表达。其表达模式与发育中的大脑中酪氨酸酶活性的出现密切相关。这些结果首次在分子水平上描述了海鞘胚胎中色素细胞的分化，也将有助于更好地理解脊索动物色素细胞的进化。
