Gastrulation is a fundamental morphogenetic process in development. In echinoderms with ancestral‐type development through feeding larvae, gastrulation involves radially symmetrical invagination of cells around the blastopore. Gastrulation in the seastar Parvulastra exigua, a species with non‐feeding larvae deviates from this pattern. Microinjection of cells with fluorescent lineage tracer dye revealed that early blastomeres contribute unequally to ectoderm and endoderm. In embryos injected at the two‐cell stage, asymmetry was evident in the fluorescence at the top of the archenteron and animal pole ectoderm. Archenteron elongation is driven by asymmetrical involution of cells with more cells crossing the blastopore on one side. Lineages of cells injected at the four‐cell stage also differed in allocation to endoderm and ectoderm. In embryos injected at the eight‐cell stage ectodermal and endodermal fates were evident reflecting the animal and vegetal fates determined by third cleavage as typical of echinoderms. Modification of gastrulation associated with evolution of development in P. exigua shows that this foundational morphogenetic process can be altered despite its importance for subsequent development. However, observations of slight asymmetry in the lineage fates of blastomeres in asterinids with planktotrophic development indicates that gastrulation by asymmetrical involution in P. exigua may be a hypertrophic elaboration of a pre‐existing state in ancestral‐type development. As for echinoids with lecithotrophic development, involution as a mechanism to contribute to archenteron elongation may be associated with the impact of extensive maternal nutritive reserves on the mechanics of cell movement and a novel innovation to facilitate early development of the adult rudiment.

中文翻译：

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Parvulastra exigua 原肠胚形成的进化修饰，一种具有全底性嗜卵发育的海星

原肠胚形成是发育中的基本形态发生过程。在通过喂养幼虫进行祖先型发育的棘皮动物中，原肠胚形成涉及胚孔周围细胞的径向对称内陷。海星Parvulastra exgua的原肠胚形成，具有不摄食幼虫的物种偏离了这种模式。用荧光谱系示踪染料显微注射细胞显示早期卵裂球对外胚层和内胚层的贡献不均等。在双细胞阶段注射的胚胎中，在原肠胚和动物极外胚层顶部的荧光不对称很明显。Archenteron伸长是由细胞的不对称退化驱动的，更多的细胞在一侧穿过胚孔。在四细胞阶段注射的细胞谱系在分配到内胚层和外胚层方面也不同。在八细胞阶段注射的胚胎中，外胚层和内胚层的命运很明显，反映了由第三次卵裂决定的动物和植物的命运，这是典型的棘皮动物。与发育进化相关的原肠胚形成的改变P. exigua表明，尽管它对后续发展很重要，但这种基本的形态发生过程可以改变。然而，对具有浮游营养发育的 asterinids 卵裂球谱系命运的轻微不对称的观察表明，P. exigua中不对称退化的原肠胚形成可能是祖先型发育中预先存在的状态的肥大阐述。至于具有促卵泡发育的海胆类动物，退化作为促进原肠延长的机制可能与母体丰富的营养储备对细胞运动机制的影响以及促进成年雏形早期发育的新创新有关。