The adult horn fly transcriptome and its complement of transcripts encoding cytochrome P450s, glutathione S-transferases, and esterases

The horn fly, Haematobia irritans, is a blood-feeding parasitic fly with a global distribution that includes Europe, Africa, Asia, and the Americas. The fly has a major detrimental economic impact upon cattle production, with losses estimated at over $800 million annually in the United States and $2.5 billion in Brazil alone. Insecticide resistance in specific horn fly populations has been a problem for many years and there are several mechanisms whereby resistance develops. Little is known about the complement of metabolic enzymes encoded by the horn fly's genome that might provide the fly with detoxification or sequestration pathways to survive insecticide treatments. The cytochrome P450, glutathione S-transferase, and esterase enzyme families contain members that are capable of sequestering and/or detoxifying xenobiotic molecules such as insecticides. We sought to develop a comprehensive dataset of metabolic enzyme-encoding transcript sequences from the adult horn fly, as this is the life stage whose actions directly impose the economic costs to cattle producers. We used an Illumina paired-end read RNA-Seq approach to determine the adult horn fly transcriptomes from laboratory and field populations of horn flies with varying levels of pesticide resistance, including untreated and pyrethroid-treated newly eclosed adult flies. We followed with bioinformatic analyses to discern sequences putatively encoding cytochrome P450, esterase, and GST enzymes. We utilized read-mapping of RNA-Seq data and quantitative real-time polymerase chain reaction (qRT-PCR) to examine gene expression levels of specific P450 transcripts in several fly populations with varying degrees of pesticide resistance.