Bielen Holger, Oberleitner Sabine, Marcellini Sylvain, Gee Lydia, Lemaire Patrick, Bode Hans R, Rupp Ralph, Technau Ulrich
Sars International Centre for Marine Molecular Biology, University of Bergen, Thormøhlensgt. 55, 5008 Bergen, Norway.
Development. 2007 Dec;134(23):4187-97. doi: 10.1242/dev.010173.
Homologues of the T-box gene Brachyury play important roles in mesoderm differentiation and other aspects of early development in all bilaterians. In the diploblast Hydra, the Brachyury homologue HyBra1 acts early in the formation of the hypostome, the location of the organiser in adult Hydra. We now report the isolation and characterisation of a second Brachyury gene, HyBra2. Sequence analysis suggests that HyBra1 and HyBra2 are paralogues, resulting from an ancient lineage-specific gene duplication. We show that both paralogues acquired novel functions, both at the level of their cis-regulation as well as through significant divergence of the coding sequence. Both genes are expressed in the hypostome, but HyBra1 is predominantly endodermal, whereas HyBra2 transcripts are found primarily in the ectoderm. During bud formation, both genes are activated before any sign of evagination, suggesting an early role in head formation. During regeneration, HyBra1 is an immediate-early response gene and is insensitive to protein synthesis inhibition, whereas the onset of expression of HyBra2 is delayed and requires protein synthesis. The functional consequence of HyBra1/2 protein divergence on cell fate decisions was tested in Xenopus. HyBra1 induces mesoderm, like vertebrate Brachyury proteins. By contrast, HyBra2 shows a strong cement-gland and neural-inducing activity. Domain-swapping experiments show that the C-terminal domain of HyBra2 is responsible for this specific phenotype. Our data support the concept of sub- and neofunctionalisation upon gene duplication and show that divergence of cis-regulation and coding sequence in paralogues can lead to dramatic changes in structure and function.
T 盒基因短尾基因（Brachyury）的同源物在所有两侧对称动物的中胚层分化及早期发育的其他方面发挥着重要作用。在双胚层动物水螅中，短尾基因同源物 HyBra1 在口道的形成过程中早期发挥作用，口道是成年水螅中组织者的位置。我们现在报告第二个短尾基因 HyBra2 的分离和特征。序列分析表明，HyBra1 和 HyBra2 是旁系同源物，源于古老的谱系特异性基因复制。我们表明，这两个旁系同源物在顺式调控水平以及编码序列的显著分化方面都获得了新功能。这两个基因都在口道中表达，但 HyBra1 主要在内胚层表达，而 HyBra2 的转录本主要在外胚层中发现。在芽形成过程中，这两个基因在任何外翻迹象出现之前就被激活，表明在头部形成中起早期作用。在再生过程中，HyBra1 是一个立即早期反应基因，对蛋白质合成抑制不敏感，而 HyBra2 的表达开始延迟且需要蛋白质合成。在非洲爪蟾中测试了 HyBra1/2 蛋白分化对细胞命运决定的功能后果。HyBra1 诱导中胚层，类似于脊椎动物的短尾基因蛋白。相比之下，HyBra2 表现出很强的黏腺和神经诱导活性。结构域交换实验表明，HyBra2 的 C 末端结构域负责这种特定表型。我们的数据支持基因复制后亚功能化和新功能化的概念，并表明旁系同源物中顺式调控和编码序列的分化可导致结构和功能的巨大变化。
