de novo Assembly of an Expressed Gene Catalog from the Tick, Rhipicephalus appendiculatus

Ticks are blood-feeding ectoparasites that serve as vectors for a variety of human and veterinary diseases worldwide. Ticks evolved highly adaptive mechanisms to feed unnoticed on their host for an extended period of time. The proteins involved in feeding modulate the host’s haemostasis, inflammation and immune response and are attractive candidates for the development of recombinant anti-tick vaccines. The tick lineage is taxonomically isolated, in that the closest completed genome is that of the mite Tetranychus urticae which diverged from ticks ±400 million years ago. This taxonomic isolation makes it difficult to annotate tick proteins based on similarity searches. Next generation sequencing can bridge this gap by increasing the number of available tick transcriptomes. To this end, we sequenced the salivary glands of Rhipicephalus appendiculatus using the Illumina® HiSeq2000. Over 300 million Illumina paired end sequences were generated, quality filtered and de novo assembled using the Trinity assembly software package. Nearly 58K transcripts longer than 300 bp were assembled (median length 825 bp; N50 2445 bp). Thirty two thousand putative open reading frames were predicted using TransDecoder. Annotation was done by BLAST alignments against the Nr, UniProtKB, Blast2GO and KEGG databases. Proteins were further characterised by similarity searches against the CDD, Pfam, SignalP and tmHMM databases. The completed gene catalog revealed metabolic pathways involved in blood-feeding and protein families that are expanded in tick salivary glands. These proteins are good candidates to use in future vaccine developments.