Stach Thomas, Anselmi Chiara
Institut für Biologie, Kompetenzzentrum Elektronenmikroskopie, Humboldt-Universität zu Berlin, Philippstrasse 13, Haus 14, 10115, Berlin, Germany.
Dipartimento di Biologia, Università degli Studi di Padova, Via Ugo Bassi 58/B, 35131, Padova, Italy.
BMC Biol. 2015 Dec 23;13:113. doi: 10.1186/s12915-015-0218-1.
Understanding the evolution of divergent developmental trajectories requires detailed comparisons of embryologies at appropriate levels. Cell lineages, the accurate visualization of cleavage patterns, tissue fate restrictions, and morphogenetic movements that occur during the development of individual embryos are currently available for few disparate animal taxa, encumbering evolutionarily meaningful comparisons. Tunicates, considered to be close relatives of vertebrates, are marine invertebrates whose fossil record dates back to 525 million years ago. Life-history strategies across this subphylum are radically different, and include biphasic ascidians with free swimming larvae and a sessile adult stage, and the holoplanktonic larvaceans. Despite considerable progress, notably on the molecular level, the exact extent of evolutionary conservation and innovation during embryology remain obscure.
Here, using the innovative technique of bifocal 4D-microscopy, we demonstrate exactly which characteristics in the cell lineages of the ascidian Phallusia mammillata and the larvacean Oikopleura dioica were conserved and which were altered during evolution. Our accurate cell lineage trees in combination with detailed three-dimensional representations clearly identify conserved correspondence in relative cell position, cell identity, and fate restriction in several lines from all prospective larval tissues. At the same time, we precisely pinpoint differences observable at all levels of development. These differences comprise fate restrictions, tissue types, complex morphogenetic movement patterns, numerous cases of heterochronous acceleration in the larvacean embryo, and differences in bilateral symmetry.
Our results demonstrate in extraordinary detail the multitude of developmental levels amenable to evolutionary innovation, including subtle changes in the timing of fate restrictions as well as dramatic alterations in complex morphogenetic movements. We anticipate that the precise spatial and temporal cell lineage data will moreover serve as a high-precision guide to devise experimental investigations of other levels, such as molecular interactions between cells or changes in gene expression underlying the documented structural evolutionary changes. Finally, the quantitative amount of digital high-precision morphological data will enable and necessitate software-based similarity assessments as the basis of homology hypotheses.
了解不同发育轨迹的演变需要在适当层面详细比较胚胎学。目前，只有少数不同的动物类群有细胞谱系、卵裂模式的精确可视化、组织命运限制以及单个胚胎发育过程中发生的形态发生运动，这阻碍了具有进化意义的比较。被认为是脊椎动物近亲的被囊动物是海洋无脊椎动物，其化石记录可追溯到5.25亿年前。该亚门的生活史策略截然不同，包括具有自由游动幼虫和固着成年阶段的双相海鞘，以及全浮游的幼形类。尽管取得了相当大的进展，尤其是在分子层面，但胚胎学中进化保守和创新的确切程度仍不清楚。
在这里，我们使用双焦点4D显微镜的创新技术，准确展示了海鞘乳头海鞘和幼形类住囊虫细胞谱系中的哪些特征在进化过程中得到了保留，哪些发生了改变。我们精确的细胞谱系树与详细的三维表示相结合，清楚地确定了来自所有预期幼虫组织的几条谱系中相对细胞位置、细胞身份和命运限制的保守对应关系。同时，我们精确指出了在发育的各个层面都能观察到的差异。这些差异包括命运限制、组织类型、复杂的形态发生运动模式、幼形类胚胎中大量异时加速的情况，以及双侧对称性的差异。
我们的结果极其详细地展示了适合进化创新的众多发育层面，包括命运限制时间的细微变化以及复杂形态发生运动的巨大改变。我们预计，精确的时空细胞谱系数据还将作为一个高精度指南，用于设计对其他层面的实验研究，比如细胞之间的分子相互作用或记录的结构进化变化背后的基因表达变化。最后，数字高精度形态学数据的定量将使基于软件的相似性评估成为同源性假设的基础，并使其成为必要。
