Jeffery W R, Swalla B J, Ewing N, Kusakabe T
Department of Biology, Pennsylvania State University, University Park, USA.
Mol Biol Evol. 1999 May;16(5):646-54. doi: 10.1093/oxfordjournals.molbev.a026147.
Most ascidians pass through a tadpole (urodele) larval stage, although some species have derived a tailless (anural) larva. New insights into the evolution of anural larvae in the Roscovita clade of molgulid ascidians were obtained from studing embryonic development of the transitional anural species Molgula bleizi and from phylogenetic analysis based on muscle and cytoskeletal actin gene sequences. By observing in vitro fertilized eggs, we found that M. bleizi, previously described as a typical anural developer, actually forms a short immotile tail during embryogenesis. The short tail contains notochord lineage cells, which undergo abbreviated morphogenetic movements but eventually arrest in development. Molgula bleizi tail muscle lineage cells produce the muscle enzyme acetylcholinesterase (AChE) but do not express muscle actin genes. The presence of a short tail, a vestigial notochord, and AChE-positive muscle cells suggest that M. bleizi is a recently derived anural species. An M. bleizi larval muscle actin gene (MbMA1) was isolated, sequenced, and shown to be a pseudogene based on critical deletions in its coding region that would result in a nonfunctional actin protein. The mutations in MbMA1 are distinct from and have evolved independent of the larval muscle actin pseudogenes MoccMA1a and MoccMA1b in Molgula occulta, another anural developer in the Roscovita clade. Pseudogene formation explains the absence of muscle actin mRNA in M. bleizi embryos. The 3' untranslated region of an M. bleizi cytoskeletal actin gene was also isolated and sequenced. Phylogenetic trees reconstructed using muscle and cytoskeletal actin sequences suggest that the anural developer M. bleizi evolved prior to the divergence of the urodele developer Molgula oculata and the anural developer M. occulta in the Roscovita clade. Since M. bleizi lives attached to hard substrata in the tidal zone, whereas M. oculata and M. occulta live buried in subtidal sand flats, our results suggest that the anural larva evolved at least twice in the Roscovita clade of molgulid ascidians as an adaptation to different habitats.
大多数海鞘会经历蝌蚪（有尾）幼虫阶段，不过有些物种演化出了无尾（无尾）幼虫。通过研究过渡性无尾物种布莱兹海鞘（Molgula bleizi）的胚胎发育以及基于肌肉和细胞骨架肌动蛋白基因序列的系统发育分析，我们对莫氏海鞘科罗斯科维塔分支中无尾幼虫的进化有了新的认识。通过观察体外受精卵，我们发现之前被描述为典型无尾发育者的布莱兹海鞘在胚胎发生过程中实际上会形成一条短的、不能活动的尾巴。这条短尾巴包含脊索谱系细胞，这些细胞经历了简化的形态发生运动，但最终发育停滞。布莱兹海鞘的尾肌谱系细胞会产生肌肉酶乙酰胆碱酯酶（AChE），但不表达肌肉肌动蛋白基因。短尾巴、退化的脊索以及AChE阳性肌肉细胞的存在表明布莱兹海鞘是一个最近演化出的无尾物种。我们分离并测序了布莱兹海鞘的幼虫肌肉肌动蛋白基因（MbMA1），结果表明它是一个假基因，其编码区存在关键缺失，这会导致产生无功能的肌动蛋白蛋白。MbMA1中的突变与莫氏海鞘（Molgula occulta）中的幼虫肌肉肌动蛋白假基因MoccMA1a和MoccMA1b不同，并且是独立进化的，莫氏海鞘是罗斯科维塔分支中的另一个无尾发育者。假基因的形成解释了布莱兹海鞘胚胎中肌肉肌动蛋白mRNA的缺失。我们还分离并测序了布莱兹海鞘细胞骨架肌动蛋白基因的3'非翻译区。使用肌肉和细胞骨架肌动蛋白序列重建的系统发育树表明，无尾发育者布莱兹海鞘在有尾发育者眼斑海鞘（Molgula oculata）和无尾发育者莫氏海鞘在罗斯科维塔分支分化之前就已经进化出来了。由于布莱兹海鞘附着生活在潮间带的硬基质上，而眼斑海鞘和莫氏海鞘则埋栖在潮下沙质浅滩中，我们的结果表明，在莫氏海鞘科罗斯科维塔分支中，无尾幼虫至少独立进化了两次，以适应不同的栖息地。
