Carrillo-Baltodano Allan M, Boyle Michael J, Rice Mary E, Meyer Néva P
Biology Department, Clark University, Worcester, Massachusetts.
Smithsonian Institution, Smithsonian Marine Station at Fort Pierce, Fort Pierce, Florida.
J Morphol. 2019 Nov;280(11):1628-1650. doi: 10.1002/jmor.21054. Epub 2019 Sep 5.
Sipuncula is a clade of unsegmented marine worms that are currently placed among the basal radiation of conspicuously segmented Annelida. Their new location provides a unique opportunity to reinvestigate the evolution and development of segmented body plans. Neural segmentation is clearly evident during ganglionic ventral nerve cord (VNC) formation across Sedentaria and Errantia, which includes the majority of annelids. However, recent studies show that some annelid taxa outside of Sedentaria and Errantia have a medullary cord, without ganglia, as adults. Importantly, neural development in these taxa is understudied and interpretation can vary widely. For example, reports in sipunculans range from no evidence of segmentation to vestigial segmentation as inferred from a few pairs of serially repeated neuronal cell bodies along the VNC. We investigated patterns of pan-neuronal, neuronal subtype, and axonal markers using immunohistochemistry and whole mount in situ hybridization (WMISH) during neural development in an indirect-developing sipunculan, Themiste lageniformis. Confocal imaging revealed two clusters of 5HT neurons, two pairs of FMRF neurons, and Tubulin peripheral neurites that appear to be serially positioned along the VNC, similar to other sipunculans, to other annelids, and to spiralian taxa outside of Annelida. WMISH of a synaptotagmin1 ortholog in T. lageniformis (Tl-syt1) showed expression throughout the centralized nervous system (CNS), including the VNC where it appears to correlate with mature 5HT and FMRF neurons. An ortholog of elav1 (Tl-elav1) showed expression in differentiated neurons of the CNS with continuous expression in the VNC, supporting evidence of a medullary cord, and refuting evidence of ontogenetic segmentation during formation of the nervous system. Thus, we conclude that sipunculans do not exhibit any signs of morphological segmentation during development.
星虫动物门是一类不分节的海洋蠕虫，目前被置于明显分节的环节动物门的基部辐射类群之中。它们的新位置为重新研究分节身体结构的进化和发育提供了独特的机会。在包括大多数环节动物的定居亚纲和游走亚纲的神经节腹神经索（VNC）形成过程中，神经分节明显可见。然而，最近的研究表明，定居亚纲和游走亚纲之外的一些环节动物类群在成年后具有无神经节的髓索。重要的是，这些类群的神经发育研究不足，解释差异很大。例如，关于星虫动物的报道从没有分节证据到从沿着腹神经索的几对连续重复的神经元细胞体推断出的残留分节。我们在间接发育的星虫动物拉氏阔沙蚕的神经发育过程中，使用免疫组织化学和全胚胎原位杂交（WMISH）研究了泛神经元、神经元亚型和轴突标记的模式。共聚焦成像显示了两簇5-羟色胺（5HT）神经元、两对FMRF神经元和微管蛋白外周神经突，它们似乎沿着腹神经索依次排列，类似于其他星虫动物、其他环节动物以及环节动物门之外的螺旋动物类群。拉氏阔沙蚕（Tl-syt1）中突触结合蛋白1直系同源物的WMISH显示其在整个中枢神经系统（CNS）中表达，包括腹神经索，在那里它似乎与成熟的5HT和FMRF神经元相关。elav1直系同源物（Tl-elav1）在中枢神经系统的分化神经元中表达，并在腹神经索中持续表达，支持了髓索的证据，并反驳了神经系统形成过程中个体发育分节的证据。因此，我们得出结论，星虫动物在发育过程中没有表现出任何形态分节的迹象。
