The origin of vertebrate-specific characters and functional systems is one of the most important questions in the evolutionary biology. Amphioxus, the most basal chordate, is considered as the sister of vertebrates based on the recent molecular phylogenetic evidences, and it is used as an ideal model organism to study the origin and evolution of vertebrate organ systems including immune system. Three digestive organs, gill slits, hepatic caecum and intestine in amphioxus were also considered as primitive immune organs of amphioxus.

To reveal the expression patterns of miRNAs in these organs, researchers from Nanjing University utilized a μParaflo-microfluidic chip-microarray screen performed by LC Sciences. In two independent replicates, we calculate the number of miRNAs with microarray signal intensity greater than 50 in each organ based on 272 target probes from B. belcheri and B. floridae, and identified 138 miRNAs uniquely expressed in the gill, 127 miRNAs in the intestine and 107 miRNAs in the hepatic caecum, 114 miRNAs in both gill and intestine, 53 miRNAs in both gill and hepatic caecum, 45 miRNAs in both intestine and hepatic caecum, and 42 miRNAs in all three organs. To overview the global expression patterns of miRNAs in these three organs, a heatmap was generated including all miRNAs from the microarray chip. Further analysis of the heatmap showed eight different expression trends of miRNAs in these three organs. These different expression trends of miRNAs suggest a dynamic expression profile of miRNAs in distinct organs. Interestingly, the hierarchical cluster and statistical analysis showed that the miRNA expression pattern of gill and intestine is close to each other as compared to the expression pattern of hepatic caecum and intestine or the expression pattern of hepatic caecum and gill.

In summary, this study provides a systemic analysis of the expression of miRNAs in three different digestive organs that are associated with immune system evolution, and identifies a set of miRNAs with tissue-specific expression patterns. These results form a basis to further study the function of miRNAs in specific tissues and during evolution.

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Reference
X. Liao, L. Yang, X. Chen, et al. (2017) Identification of microRNA expression profiles in the gill, intestine and hepatic caecum of Branchiostoma belcheri Protein Cell 8: 302. doi:10.1007/s13238-016-0365-3 [article]

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