Whitney Laboratory for Marine Bioscience, University of Florida, St. Augustine.
Department of Biology, University of Miami.
Genome Biol Evol. 2019 Apr 1;11(4):1250-1257. doi: 10.1093/gbe/evz052.
Addressing the origin of axial-patterning machinery is essential for understanding the evolution of animal form. Historically, sponges, a lineage that branched off early in animal evolution, were thought to lack Hox and ParaHox genes, suggesting that these critical axial-patterning genes arose after sponges diverged. However, a recent study has challenged this long-held doctrine by claiming to identify ParaHox genes (Cdx family) in two calcareous sponge species, Sycon ciliatum and Leucosolenia complicata. We reanalyzed the main data sets in this paper and analyzed an additional data set that expanded the number of bilaterians represented and removed outgroup homeodomains. As in the previous study, our Neighbor-Joining analyses of the original data sets recovered a clade that included sponge and Cdx genes, whereas Bayesian analyses placed these sponge genes within the NKL subclass of homeodomains. Unlike the original study, only one of our two maximum-likelihood analyses was congruent with Cdx genes in sponges. Our analyses of our additional data set led to the sponge genes consistently being placed within the NKL subclass of homeodomains regardless of method or model. Our results show more support for these sponge genes belonging to the NKL subclass, and therefore imply that Hox and ParaHox genes arose after Porifera diverged from the rest of animals.
阐明轴向模式形成机制对于理解动物形态的进化至关重要。历史上，海绵动物是动物进化早期分支出来的谱系，被认为缺乏 Hox 和 ParaHox 基因，这表明这些关键的轴向模式形成基因是在海绵动物分化之后出现的。然而，最近的一项研究通过声称在两种钙质海绵（Sycon ciliatum 和 Leucosolenia complicata）中鉴定出了 ParaHox 基因（Cdx 家族），对这一长期以来的观点提出了挑战。我们重新分析了本文中的主要数据集，并分析了一个额外的数据集，该数据集扩大了所代表的两侧对称动物的数量，并去除了外群同源域。与之前的研究一样，我们对原始数据集的邻接法分析恢复了一个包括海绵和 Cdx 基因的分支，而贝叶斯分析则将这些海绵基因置于同源域的 NKL 子类内。与原始研究不同的是，我们的两个最大似然分析中只有一个与海绵中的 Cdx 基因一致。我们对额外数据集的分析导致海绵基因始终被置于同源域的 NKL 子类内，无论方法或模型如何。我们的结果表明，这些海绵基因更支持属于 NKL 子类，因此暗示 Hox 和 ParaHox 基因是在多孔动物从其他动物中分化出来之后出现的。
