Abstract
BackgroundThe insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins. The thickness and synthesis of the exocuticle depend on different castes that perform various functions in alates, workers and soldiers. However, it is not clear whether the soft endocuticle is involved in the division of labour in termite colonies. To reveal the structural characteristics of the endocuticle in different castes, we investigated the thickness of endocuticle layers in alates, workers and soldiers of the termite Reticulitermes aculabialis, and then we sequenced their transcriptome and detected the endocuticle protein genes. The differential expression levels of the endocuticular protein genes were confirmed in the three castes.ResultsWe found that there was a great difference in the thickness of the endocuticle among the alates, soldiers and workers. The thickest endocuticle layers were found in the heads of the workers 7.88 ± 1.67 μm. The endocuticle layer in the head of the workers was approximately three-fold and nine-fold thicker than that in the heads of soldiers and alates, respectively. The thinnest endocuticle layers occurred in the head, thorax and abdomen of alates, which were 0.86 ± 0.15, 0.76 ± 0.24 and 0.52 ± 0.17 μm thick, respectively, and had no significant differences. A total of 43,531,650 clean sequencing reads was obtained, and 89,475 unigenes were assembled. Of the 70 identified cuticular protein genes, 10 endocuticular genes that belong to the RR-1 family were selected. qRT-PCR analysis of the five endocuticular genes (SgAbd-2, SgAbd-9, Abd-5, SgAbd-2-like and Abd-4-like) revealed that the endocuticle genes were more highly expressed in workers than in soldiers and alates.ConclusionThese results suggest that SgAbd and Abd are the key components of the endocuticle. We infer that the thicker endocuticle in workers is helpful for them to perform their functions against environmental stress.
Highlights
The insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins
The thinnest endocuticle layers occurred in the head, thorax and abdomen of alates, which were 0.86 ± 0.15, 0.76 ± 0.24 and 0.52 ± 0.17 μm thick, respectively, and had no significant difference
According to the Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis, there were significant differences in 45 pathways among the workers, soldiers and alates indicating that these pathways were related to the physiological functions of different castes
Summary
The insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins. The thickness and synthesis of the exocuticle depend on different castes that perform various functions in alates, workers and soldiers. It is not clear whether the soft endocuticle is involved in the division of labour in termite colonies. The exoskeleton of insect cuticle is an exceptional extracellular structure which is emanate by epidermal cells and covers the outer body of insect. It serves as attachment site for muscles, implement sensory perceptions and coloration, provide protection against parasites, pathogen, mechanical injury and perform other physiological functions. Cuticle at different developmental stages from distinct parts of insect’s anatomy may have different characteristics, their functions based on the thickness, framework or morphology of unique layers, chemical composition and molecular connections in the cuticle [2, 4]
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