脊椎动物和有尾索动物是演化过程中形成的两个成功的脊索动物类别。有尾索动物在解剖上要比脊椎动物简单得多，所以达尔文和海克尔将它们看作是蠕虫与脊椎动物之间的原始联系。Hee-Chan Seo等人通过研究有尾索动物Oikopleura dioica （一种微小的、蝌蚪一样的海洋生物）的起调节作用的Hox基因，对这一思想重新进行了分析。Hox基因之所以让研究人员感兴趣，是由于它们在调节胚胎发育中所起的中心作用以及它们在演化过程中保留下来的在基因组中一个位置上的聚集现象。令人吃惊的是，Oikopleura已经失去全部中央Hox 基因，保留下来的散布在基因组中。这一极端现象表明，有尾索动物之所以简单，不是因为它们是原始的，而是因为它们是简化的。
Hox cluster disintegration with persistent anteroposterior order of expression in Oikopleura dioica
Tunicate embryos and larvae have small cell numbers and simple anatomical features in comparison with other chordates, including vertebrates. Although they branch near the base of chordate phylogenetic trees1, their degree of divergence from the common chordate ancestor remains difficult to evaluate. Here we show that the tunicate Oikopleura dioica has a complement of nine Hox genes in which all central genes are lacking but a full vertebrate-like set of posterior genes is present. In contrast to all bilaterians studied so far, Hox genes are not clustered in the Oikopleura genome. Their expression occurs mostly in the tail, with some tissue preference, and a strong partition of expression domains in the nerve cord, in the notochord and in the muscle. In each tissue of the tail, the anteroposterior order of Hox gene expression evokes spatial collinearity, with several alterations. We propose a relationship between the Hox cluster breakdown, the separation of Hox expression domains, and a transition to a determinative mode of development.
Figure 1 Evolutionary relationship of the Hox and ParaHox homeodomain sequences inferred by the neighbour-joining method. The neighbour-joining tree including a 1000 replicate bootstrap was inferred by PAUP26. O. dioica and C. intestinalis gene names are highlighted in red and yellow, respectively. These include the Oikopleura ParaHox genes Gsx and Cdx and its unique Evx gene, for which genomic and cDNA sequences have been isolated. Other genes are from human (Hs), amphioxus (Bf) and D. melanogaster. The same data set was used for maximum-parsimony (MP; PAUP) and quartet-puzzling-likelihood (QP; TREE-PUZZLE27) analyses (not shown), leading to the same overall topology. MP and QP methods placed the O. dioica Hox4 gene together with the other Hox4 sequences as a monophyletic group.
Figure 2 Expression patterns of Oikopleura Hox genes at 4 h after fertilization. a, The sites of Hox gene expression were identified by detection with both alkaline phosphatase and tyramide signal amplification (for confocal microscopy) and through comparisons with signals obtained with three marker genes (-tubulin A (-TubA) for neurons, -tubulin K (-TubK) for muscle cells, and Brachyury for notochord28). The expression domains are identified with coloured bars or arrowheads (epidermis in white, notochord in red, nerve cord in yellow, and muscle cells in brown). b, The schematic organization of each tissue is drawn in blue, from the posterior end of the trunk (left) to the tail tip (right). Hox gene expression domains are represented in red for anterior genes and in green for posterior genes. The ISH protocol was adapted from ref. 25.
Figure 3 Genomic organization of the Oikopleura Hox genes, indicating total Hox cluster breakdown. a, A sperm BAC library (15 –20 coverage) was screened at high stringency with all nine Hox cDNA probes labelled with digoxigenin, and also with a cDNA probe of Evx. The inserts of two positive BAC clones were end-sequenced for a genomic walk (black lines). One of each Hox-containing BAC clone (grey lines) was fully sequenced. The Hox2 clone could not be consistently assembled, but included no other Hox gene. b, The sequence of each BAC clone was annotated using BLASTX (dark grey rectangles) and other gene prediction tools (light grey rectangles)11. Each Hox gene was isolated in its BAC insert except Hox4, which was partly duplicated (see Supplementary Fig. S2). Blue bars represent short repeats conserved within or between BAC clones. A single transposable element related to Gypsy elements was identified in the Hox1 BAC (black rectangle).
Figure 4 Discrete changes of Hox gene complements in chordates. The chordate ancestor gained a rich set of posterior genes, which were inherited in the three subphyla but partly lost in ascidians. Central genes were gradually lost in tunicates, with larvaceans keeping anterior and posterior genes only. Whereas the Hox cluster was multiplied in vertebrates (with subsequent losses of a few paralogues in some clusters), the cluster degenerated in tunicates, and ultimately disappeared in larvaceans. The loss of central genes and of the Hox cluster coincides with a partition of Hox expression domains, which largely overlap in cephalochordates and vertebrates (ascidian data are still lacking). The motor of both events might be the decrease in size and transition to determinative development.
