Identification of circular RNA in the Bombyx mori silk gland

Bombyx mori is an economically important holometabolous lepidopteran insect. In B. mori endogenous noncoding RNAs such as microRNAs (miRNAs) and Piwi-interacting RNAs play crucial biological functions in metamorphosis and sex determination. In addition, circular RNAs (circRNAs) have been recently identified as noncoding RNAs in most common model organisms and show potential as gene regulators. However, to date, there have been few studies on the circRNAs present in the B. mori genome conducted to date. Here, we identified 3916 circRNAs by deep circular transcriptome sequencing using the silk gland of B. mori. 3155 circRNAs were found to be derived from 1727 parental genes. The circRNAs displayed tissue-specific expression between the middle silk gland (MSG) and posterior silk gland (PSG), with 2532 and 880 being upregulated circRNAs in the MSG and PSG, respectively. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses showed that the parental genes from the MSG and PSG were generally annotated to similar categories and pathways. The interaction network of circRNAs and miRNAs showed that circRNAs might act as miRNA sponges or interact with miRNAs in some other way. Overall, the results revealed the complicated patterns of circRNAs in the B. mori silk gland providing a new angle from which to explore the mechanisms of complex gene regulation and efficient silk protein synthesis.