Abstract
The fish swimbladder is a unique organ in vertebrate evolution and it functions for regulating buoyancy in most teleost species. It has long been postulated as a homolog of the tetrapod lung, but the molecular evidence is scarce. In order to understand the molecular function of swimbladder as well as its relationship with lungs in tetrapods, transcriptomic analyses of zebrafish swimbladder were carried out by RNA-seq. Gene ontology classification showed that genes in cytoskeleton and endoplasmic reticulum were enriched in the swimbladder. Further analyses depicted gene sets and pathways closely related to cytoskeleton constitution and regulation, cell adhesion, and extracellular matrix. Several prominent transcription factor genes in the swimbladder including hoxc4a, hoxc6a, hoxc8a and foxf1 were identified and their expressions in developing swimbladder during embryogenesis were confirmed. By comparison of enriched transcripts in the swimbladder with those in human and mouse lungs, we established the resemblance of transcriptome of the zebrafish swimbladder and mammalian lungs. Based on the transcriptomic data of zebrafish swimbladder, the predominant functions of swimbladder are in its epithelial and muscular tissues. Our comparative analyses also provide molecular evidence of the relatedness of the fish swimbladder and mammalian lung.
Highlights
The swimbladder is a specialized organ in teleosts that regulates buoyancy
All sequence tags were mapped to known transcripts in ZGC (Zebrafish Gene Collection) in order to reveal the molecular characteristics of the swimbladder transcriptome
Real-time PCR was carried out to verify relative abundance of several selected transcripts determined by RNA-seq and the result indicated a good correlation of the two methods (Figure 2)
Summary
The swimbladder is a specialized organ in teleosts that regulates buoyancy. It is a sac filled by several types of gas, mainly oxygen and carbon dioxide [1,2], and is located between the peritoneum and the vertebral column in the dorsal part of the body. It has been noted that both the swimbladder and lung are originated from the same ancestral organ, namely the respiratory pharynx, which is the posterior region of the pharynx [4,5]. All ray-finned fish except the Polypteriformes develop the dorsal part directly from the ancestral respiratory pharynx as a pulmonoid swimbladder, which has a homologous blood supply with the lung. The homology of the lung and swimbladder has been well recognized based on morphological and embryological evidence, molecular evidence is still lacking [6,7]
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