Transcriptomic analysis of the testicular fusion in Spodoptera litura

Background Lepidoptera is one group of the largest plant-feeding insects and Spodoptera litura (Lepidoptera: Noctuidae) is one of the most serious agricultural pests in Asia countries. An interesting and unique phenomenon for gonad development of Lepidoptera is the testicular fusion. Two separated testes fused into a single one during the larva-to-pupa metamorphosis, which is believed to contribute to sperm production and the prevalence in field. To study the molecular mechanism of the testicular fusion, RNA sequencing (RNA-seq) experiments of the testes from 4-day-old sixth instar larvae (L6D4) (before fusion), 6-day-old sixth instar larvae (L6D6, prepupae) (on fusing) and 4-day-old pupae (P4D) (after fusion) of S. litura were performed. Results RNA-seq data of the testes showed that totally 12,339 transcripts were expressed at L6D4, L6D6 and P4D stages. A large number of differentially expressed genes (DEGs) were up-regulated from L6D4 to L6D6, and then more genes were down-regulated from L6D6 to P4D. The DEGs mainly belongs to the genes related to the 20E signal transduction pathway, transcription factors, chitin metabolism related enzymes, the families of cytoskeleton proteins, extracellular matrix (ECM) components, ECM-related protein, its receptor integrins and ECM-remodeling enzymes. The expression levels of these genes that were up-regulated significantly during the testicular fusion were verified by qRT-PCR. The matrix metalloproteinases (MMPs) were found to be the main enzymes related to the ECM degradation and contribute to the testicular fusion. The testis was not able to fuse if MMPs inhibitor GM6001 was injected into the 5th abdomen region at L6D6 early stage. Conclusions The transcriptome and DEGs analysis of the testes at L6D4, L6D6 and P4D stages provided genes expression information related to the testicular fusion in S. litura . These results indicated that cytoskeleton proteins, ECM-integrin interaction genes and ECM-related proteins were involved in cell migration, adhesion and fusion during the testicular fusion. The ECM degradation enzymes MMPs probably play a critical role in the fusion of testis.