Boyle Michael J, Yamaguchi Emi, Seaver Elaine C
Naos Island Laboratory, Smithsonian Tropical Research Institute, Apartado, 0843-03089 Panamá, República de Panamá
Kewalo Marine Laboratory, PBRC/University of Hawaii, 41 Ahui Street, Honolulu, HI 96813 USA.
Evodevo. 2014 Oct 29;5:39. doi: 10.1186/2041-9139-5-39. eCollection 2014.
Metazoan digestive systems develop from derivatives of ectoderm, endoderm and mesoderm, and vary in the relative contribution of each germ layer across taxa and between gut regions. In a small number of well-studied model systems, gene regulatory networks specify endoderm and mesoderm of the gut within a bipotential germ layer precursor, the endomesoderm. Few studies have examined expression of endomesoderm genes outside of those models, and thus, it is unknown whether molecular specification of gut formation is broadly conserved. In this study, we utilize a sequenced genome and comprehensive fate map to correlate the expression patterns of six transcription factors with embryonic germ layers and gut subregions during early development in Capitella teleta.
The genome of C. teleta contains the five core genes of the sea urchin endomesoderm specification network. Here, we extend a previous study and characterize expression patterns of three network orthologs and three additional genes by in situ hybridization during cleavage and gastrulation stages and during formation of distinct gut subregions. In cleavage stage embryos, Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a are expressed in all four macromeres, the endoderm precursors. Ct-otx, Ct-blimp1, and Ct-nkx2.1a are also expressed in presumptive endoderm of gastrulae and later during midgut development. Additional gut-specific expression patterns include Ct-otx, Ct-bra, Ct-foxAB and Ct-gsc in oral ectoderm; Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a in the foregut; and both Ct-bra and Ct-nkx2.1a in the hindgut.
Identification of core sea urchin endomesoderm genes in C. teleta indicates they are present in all three bilaterian superclades. Expression of Ct-otx, Ct-blimp1 and Ct-bra, combined with previously published Ct-foxA and Ct-gataB1 patterns, provide the most comprehensive comparison of these five orthologs from a single species within Spiralia. Each ortholog is likely involved in endoderm specification and midgut development, and several may be essential for establishment of the oral ectoderm, foregut and hindgut, including specification of ectodermal and mesodermal contributions. When the five core genes are compared across the Metazoa, their conserved expression patterns suggest that 'gut gene' networks evolved to specify distinct digestive system subregions, regardless of species-specific differences in gut architecture or germ layer contributions within each subregion.
后生动物的消化系统由外胚层、内胚层和中胚层的衍生物发育而来，各胚层在不同分类群以及肠道不同区域的相对贡献有所不同。在少数经过充分研究的模型系统中，基因调控网络在双潜能胚层前体——内中胚层中指定肠道的内胚层和中胚层。很少有研究考察这些模型之外的内中胚层基因的表达情况，因此，肠道形成的分子指定是否广泛保守尚不清楚。在本研究中，我们利用已测序的基因组和全面的命运图谱，将六种转录因子的表达模式与小头虫早期发育过程中的胚胎胚层和肠道亚区域相关联。
小头虫的基因组包含海胆内中胚层指定网络的五个核心基因。在此，我们扩展了之前的一项研究，并通过原位杂交在卵裂期、原肠胚形成期以及不同肠道亚区域形成期间，对三个网络直系同源基因和另外三个基因的表达模式进行了表征。在卵裂期胚胎中，Ct-otx、Ct-blimp1、Ct-bra和Ct-nkx2.1a在所有四个大卵裂球（内胚层前体）中表达。Ct-otx、Ct-blimp1和Ct-nkx2.1a在原肠胚的推定内胚层以及中肠发育后期也有表达。其他肠道特异性表达模式包括Ct-otx、Ct-bra、Ct-foxAB和Ct-gsc在口外胚层中的表达；Ct-otx、Ct-blimp1、Ct-bra和Ct-nkx2.1a在前肠中的表达；以及Ct-bra和Ct-nkx2.1a在后肠中的表达。
在小头虫中鉴定出海胆内中胚层核心基因表明它们存在于所有三个两侧对称动物超群中。Ct-otx、Ct-blimp1和Ct-bra的表达，结合先前发表的Ct-foxA和Ct-gataB1模式，提供了来自螺旋动物门中单个物种的这五个直系同源基因的最全面比较。每个直系同源基因可能都参与内胚层指定和中肠发育，其中几个可能对于口外胚层、前肠和后肠的建立至关重要，包括外胚层和中胚层贡献的指定。当在整个后生动物中比较这五个核心基因时，它们保守的表达模式表明“肠道基因”网络进化而来以指定不同的消化系统亚区域，而不管每个亚区域内肠道结构或胚层贡献的物种特异性差异。
