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Although the tail is one of the major characteristics of animals of the phylum Chordata, evolutionary aspects of the molecular mechanisms involved in its formation are not clear. To obtain insights into these issues, we have isolated and investigated the caudal gene of an ascidian, one of the lower animal groups among chordates. Ascidian caudal is expressed from the midgastrula stage onward in the lateral walls of the posterior neural tube cell lineage and also in the posterior epidermal cells from the neurula stage. Thus, ascidian caudal expression is restricted to the ectoderm of a tail-forming region throughout embryogenesis. Suppression of caudal function by an antisense oligonucleotide or a dominant negative construct caused inhibition of the cell movement required for tail formation. Overexpression of wild-type caudal mRNA in an ascidian animal cap, an animal half explant prepared at the eight-cell stage, caused elongation of the cap. Furthermore, Xenopus embryos injected with dominant negative ascidian caudal exhibited defects in elongation, suggesting a conserved caudal function among chordates. These results indicate that caudal function is required for chordate tail formation and may play a key role in its evolution.
FIG. 1. Sequence of Hrcad, the ascidian caudal, and comparison with those of other caudal family genes. (A) The composite nucleotide
sequence from cDNA clones and deduced amino acid sequences of the ascidian caudal gene, Hrcad. The homeobox and the homeodomain
are shaded and the hexapeptide is boxed. The nucleotide sequences used for antisense oligonucleotide experiments are indicated by
underlining; asCAD-1 and asCAD-2 sequences are complementary to those indicated by single and double underlining, respectively. (B)
Comparison of the homeodomain and the flanking regions of Hrcad with those of other caudal family genes. Identical residues are indicated
by dashes. (C) A phylogenetic tree of caudal homeodomains. The tree is constructed using the NJ method by a computer program (Genetyx,
Software Development Co. Ltd.). Fly fushi tarazu (ftz) homeodomain is used as an outer group sequence. Numbers on the branches indicate
the values of expected substitution of amino acid residues. Grouping of vertebrate caudal is according to Marom et al. (1997). The database
Accession No. for Hrcad is AB031032.
FIG. 8. Effects of overexpression of the dominant negative and wild-type ascidian caudal in Xenopus embryos. Xenopus embryos were
injected with mRNA coding for either dominant negative Hrcad (dnCAD) (B–F), EnR only (A), or wild-type Hrcad (G,H) and cultured until
the sibling embryos reached the stage 35. (A) A stage 35 embryo injected with 50 ng/ml EnR mRNA, exhibiting normal morphology. (B) An
embryo developed in a batch injected with 12.5 ng/ml dnCAD mRNA. Many of the embryos in the batch developed with normal
morphology, but some embryos had such morphology as shown in this picture (n 5 2/15). (C,D) Embryos developed in a batch injected with
25 ng/ml dnCAD mRNA. Shown are the mildest (C) and severest (D) phenotypes in this batch. (E,F) Embryos developed in a batch injected
with 50 ng/ml dnCAD mRNA. Many of the embryos in this batch developed with the morphology as shown in (E) (n 5 10/11). (F) Shown
is the most severe case in this batch (n 5 1/11). (G,H). Phenotype induced by injection of 25 ng/ml wild-type Hrcad mRNA. In these Xenopus
embryos, the head is eliminated (n 5 8). Scale bar, 1 mm.
