Characterization of the peptidase and the non-peptidase homologs from the transcriptome of Callosobruchus maculatus

The cowpea bruchid, Callosobruchus maculatus is a major pest of the cowpea, which is an important crop in many areas of Africa and is also consumed in other parts of the world. The cowpea utilizes serine peptidase inhibitors as a defense against herbivorous insects. The cowpea bruchid in turn employs cysteine peptidases as its major digestive peptidase to avoid the plant’s defense. How the use of digestive cysteine peptidases over serine peptidases affects the peptidase expression in the transcriptome is the central question addressed by this study.

A de novo transcriptome assembly was built from 840,766 Roche 454 reads using CLC Genomics workbench. The assembly resulted in 35,111 contigs that were analyzed using Blast2GO. The peptidase and non-peptidase homologs were annotated using a local BLAST search against sequences downloaded from the MEROPS peptidase database. This resulted in 264 putative peptidase and non-peptidase homologs for C. maculatus. Illumina sequencing from the same sample produced 295,690,882 reads which were mapped back to the assembly using BWA software. While C. maculatus does not appear to have an increased number of expressed cysteine peptidases compared with other insects it does appear have more highly expressed cysteine peptidases compared to serine peptidases.