Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA.
Development. 2022 Jul 1;149(13). doi: 10.1242/dev.200184. Epub 2022 Jul 11.
The pseudobranch is a gill-like epithelial elaboration that sits behind the jaw of most fishes. This structure was classically regarded as a vestige of the ancestral gill arch-like condition of the gnathostome jaw. However, more recently, hypotheses of jaw evolution by transformation of a gill arch have been challenged, and the pseudobranch has alternatively been considered a specialised derivative of the second (hyoid) pharyngeal arch. Here, we demonstrate in the skate (Leucoraja erinacea) that the pseudobranch does, in fact, derive from the mandibular arch, and that it shares gene expression features and cell types with gills. We also show that the skate mandibular arch pseudobranch is supported by a spiracular cartilage that is patterned by a shh-expressing epithelial signalling centre. This closely parallels the condition seen in the gill arches, where cartilaginous appendages called branchial rays, which support the respiratory lamellae of the gills, are patterned by a shh-expressing gill arch epithelial ridge. Together with similar discoveries in zebrafish, our findings support serial homology of the pseudobranch and gills, and an ancestral origin of gill arch-like anatomical features from the gnathostome mandibular arch.
伪鳃是一种位于大多数鱼类下颚后面的鳃状上皮结构。这个结构在经典上被认为是颌口动物下颚的祖先进化的鳃弓状结构的残余。然而，最近，通过鳃弓转化来解释下颚进化的假说受到了挑战，伪鳃被认为是第二（舌骨）咽弓的特化衍生物。在这里，我们在鳐鱼（Leucoraja erinacea）中证明，伪鳃实际上是由下颌弓衍生而来的，并且它与鳃共享基因表达特征和细胞类型。我们还表明，鳐鱼下颌弓伪鳃由一个具有 S hh 表达上皮信号中心的螺旋软骨支撑。这与在鳃弓中看到的情况非常相似，在鳃弓中，称为鳃射线的软骨附属物通过具有 S hh 表达的鳃弓上皮脊来塑造，这些鳃射线支撑着鳃的呼吸薄片。与斑马鱼的类似发现一起，我们的研究结果支持伪鳃和鳃的序列同源性，以及颌口动物下颌弓的鳃弓状解剖特征的祖先起源。
