Caudal homeobox genes are found across animals, typically linked to two other homeobox genes in what has been called the ParaHox cluster. These genes have been proposed to pattern the anterior-posterior axis of the endoderm ancestrally, but the expression of Caudal in extant groups is varied and often occurs in other germ layers. Here we examine the role of Caudal in the embryo of the mollusc Tritia (Ilyanassa) obsoleta. ToCaudal expression is initially broad, then becomes progressively restricted and is finally only in the developing hindgut (a.k.a. intestine). Knockdown of ToCaudal using morpholino oligonucleotides specifically blocks hindgut development, indicating that despite its initially broad expression, the functional role of ToCaudal is in hindgut patterning. This is the first functional characterization of Caudal in an animal with spiralian development, which is an ancient mode of embryogenesis that arose early in bilaterian animal evolution. These results are consistent with the hypothesis that the ancestral role of the ParaHox genes was anterior-posterior patterning of the endoderm.

在动物中发现尾部同源盒基因，通常与所谓的ParaHox簇中的两个其他同源盒基因连锁。已经提出这些基因在祖先内胚层的前后轴上进行模式化，但是现存组中尾的表达是变化的，并且经常在其他细菌层中发生。在这里，我们检查的作用，尾鳍在软体动物的胚胎Tritia（Ilyanassa）obsoleta。ToCaudal表达最初很宽，然后逐渐受到限制，最后仅在发育中的后肠（又称肠）中表达。击倒ToCaudal使用吗啉代寡核苷酸可特异性阻断后肠的发育，这表明尽管最初表达广泛，但ToC音频的功能仍在后肠模式中。这是尾螺在具有螺旋状发育的动物中的第一个功能性表征，这是一种古老的胚胎发生模式，起源于双语动物进化的早期。这些结果与以下假设相符：ParaHox基因的祖先作用是内胚层的前后模式。