Cañestro Cristian, Postlethwait John H, Gonzàlez-Duarte Roser, Albalat Ricard
Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA.
Evol Dev. 2006 Sep-Oct;8(5):394-406. doi: 10.1111/j.1525-142X.2006.00113.x.
Development of many chordate features depends on retinoic acid (RA). Because the action of RA during development seems to be restricted to chordates, it had been previously proposed that the "invention" of RA genetic machinery, including RA-binding nuclear hormone receptors (Rars), and the RA-synthesizing and RA-degrading enzymes Aldh1a (Raldh) and Cyp26, respectively, was an important step for the origin of developmental mechanisms leading to the chordate body plan. We tested this hypothesis by conducting an exhaustive survey of the RA machinery in genomic databases for twelve deuterostomes. We reconstructed the evolution of these genes in deuterostomes and showed for the first time that RA genetic machinery--that is Aldh1a, Cyp26, and Rar orthologs--is present in nonchordate deuterostomes. This finding implies that RA genetic machinery was already present during early deuterostome evolution, and therefore, is not a chordate innovation. This new evolutionary viewpoint argues against the hypothesis that the acquisition of gene families underlying RA metabolism and signaling was a key event for the origin of chordates. We propose a new hypothesis in which lineage-specific duplication and loss of RA machinery genes could be related to the morphological radiation of deuterostomes.
许多脊索动物特征的发育依赖于视黄酸（RA）。由于RA在发育过程中的作用似乎仅限于脊索动物，此前有人提出，RA遗传机制的“发明”，包括RA结合核激素受体（Rars）以及分别负责合成RA和降解RA的酶Aldh1a（Raldh）和Cyp26，是导致脊索动物身体结构发育机制起源的重要一步。我们通过对12种后口动物的基因组数据库中RA机制进行详尽调查来验证这一假设。我们重建了这些基因在后口动物中的进化过程，并首次表明RA遗传机制——即Aldh1a、Cyp26和Rar直系同源基因——存在于非脊索动物的后口动物中。这一发现意味着RA遗传机制在早期后口动物进化过程中就已存在，因此并非脊索动物所特有的。这一新的进化观点与以下假设相悖：即获得RA代谢和信号传导相关的基因家族是脊索动物起源的关键事件。我们提出了一个新的假设，即RA机制基因的谱系特异性复制和丢失可能与后口动物的形态辐射有关。
