Comparative analysis of the cardiac structure and transcriptome of scallop and snail, perspectives to heart evolution
As a vital organ to pump blood in the circulatory system, the heart has undergone gradual refinement in its structure and functional efficiency along animal evolution. The occurrence of cardiac chambers, the atrium and the ventricle, is an important event during heart evolution.The two-chambered heart, consisting of a single atrium and a single ventricle, first appeared among mollusks, prior to the evolution of multi-chambered hearts in higher vertebrates. The molecular bases driving the evolution of cardiac chambers, however, are largely unknown. In this study, we studied two common mollusks, the giant African snail (Lissachatina fulica) and scallops (Mizuhopecten yessoensis), to explore the gene profiles of atrium vs. ventricle in mollusks. By examining their heart structures, we found that the snail heart possesses a more compact muscle layer than the scallop heart. Then, RNA sequencing and differentially expressed genes (DEGs) analysis of the atrium and ventricle revealed that these two chambers are distinct in gene expression preference and functional features. Moreover, by comparing with the zebrafish heart, a typical two-chambered heart in vertebrates, mollusk ventricles share similar gene function enrichments with the zebrafish ventricle, but their atria exhibit obvious variation in functional clusters from the zebrafish atrium. Interestingly, we also found that the scallop hearts contain a high amount of collagens, which could be beneficial for heart repair after damage. Together, our work presented novel gene profiling data in the mollusk atria and ventricles, thereby providing new insights into the evolution of cardiac chambers. Overall design: To systematically profile the cardiac gene expression in adult scallop and snail hearts, their atria and ventricles were separately  collected, and RNA sequencing for these samples was performed.