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The proPO system regulates melanization in arthropods. However, the genes that are involved in the proPO system in housefly Musca domestica remain unclear. Thus, this study analyzed the combined transcriptome obtained from M. domestica larvae, pupae, and adults that were either normal or bacteria-challenged by an Escherichia coli and Staphylococcus aureus mixture. A total of 54,821,138 clean reads (4.93 Gb) were yielded by Illumina sequencing, which were de novo assembled into 89,842 unigenes. Of the 89,842 unigenes, based on a similarity search with known genes in other insects, 24 putative genes related to the proPO system were identified. Eight of the identified genes encoded for peptidoglycan recognition receptors, two encoded for prophenoloxidases, three encoded for prophenoloxidase-activating enzymes, and 11 encoded for serine proteinase inhibitors. The expression levels of these identified genes were investigated by qRT-PCR assay, which were consistent with expected activation process of the proPO system, and their activation functions were confirmed by the measurement of phenoloxidase activity in bacteria-infected larvae after proPO antibody blockage, suggesting these candidate genes might have potentially different roles in the activation of proPO system. Collectively, this study has provided the comprehensive transcriptomic data of an insect and some fundamental basis toward achieving understanding of the activation mechanisms and immune functions of the proPO system in M. domestica.

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With a novel and universal strategy for the cloning of multiple DNA fragments, a complex synthetic vector (pVEC100), harboring the target DNA fragments in conventional 100-bp DNA ladder, was constructed for efficient and large-scale production of 100-bp DNA marker through bacteria fermentation, plasmid extraction and restrictive digestion. Since the restrictive digestion of complex vectors yields insufficient small DNA fragments, an innovative PCR model was developed as an alternative. The PCR model comprised a specially designed template vector and a unit amplification model for producing groups of small DNA fragments. The unit amplification model improved the efficiency of the PCR protocol and made it more economical and easier for small DNA fragment amplification. The approach presented in this paper--a unit cloning model for constructing complex synthetic vectors combined with the modular design of unit amplification by PCR--is a powerful method for preparing small DNA fragments of DNA molecular weight standards.