An insight into the sialome of the soft tick, Ornithodorus parkeri

While hard ticks (Ixodidae) take several days to feed on their hosts, soft ticks (Argasidae) feed faster, usually taking less than 1h per meal. Saliva assists in the feeding process by providing a cocktail of anti-hemostatic, anti-inflammatory and immunomodullatory compounds. Saliva of hard ticks has been shown to contain several families of genes each having multiple members, while those of soft ticks are relatively unexplored. Analysis of the salivary transcriptome of the soft tick Ornithodorus parkeri, the vector of the relapsing fever agent Borrelia parkeri, indicates that gene duplication events have led to a large expansion of the lipocalin family, as well as of several genes containing Kunitz domains indicative of serine protease inhibitors, and several other gene families also found in hard ticks. Novel protein families with sequence homology to insulin growth factor-binding protein (prostacyclin-stimulating factor), adrenomedulin, serum amyloid A protein precursor and similar to HIV envelope protein were also characterized for the first time in the salivary gland of a blood-sucking arthropod. The sialotranscriptome of O. parkeri confirms that gene duplication events are an important driving force in the creation of salivary cocktails of blood-feeding arthropods, as was observed with hard ticks and mosquitoes. Most of the genes coding for expanded families are homologous to those found in hard ticks, indicating a strong common evolutionary path between the two families. As happens to all genera of blood-sucking arthropods, several new proteins were also found, indicating the process of adaptation to blood feeding still continues to recent times.