This scenario raises the following questions: It has been suggested that ticks shared this trait before adaptation to a blood-feeding environment ( Walter and Proctor 1998 ). Holothyrida is a group of free-living scavenging mites, which mainly live on body fluids of dead arthropods. The Holothyrida, with the Ixodida and Mesostigmata, forms the superorder Parasitiformes, and it was indicated that the Holothyrida rather than Mesostigmata are a sister-group to ticks ( Dobson and Barker 1999 ). This indicates that ticks had already diverged by ∼92 MYA into the main tick families, as well as the argasid genus level. Phylogenetic analysis indicates that the Ixodida are monophyletic ( Black, Klompen, and Keirans 1997 ), whereas the oldest tick fossil to date is an argasid ( Carios jerseyi) found in New Jersey amber (90–94 MYA Klompen and Grimaldi 2001 ). Their origins have been estimated to be in the Late Cretaceous, ∼120 MYA, which was the last time that Australia was part of Gondwanaland, indicating that this period played an important role in the origin of the Australian tick lineages, and by extension, the entire tick family ( Klompen et al. Ticks (suborder Ixodida) are obligate hematophagous organisms that comprise three families, the Ixodidae (hard ticks), Argasidae (soft ticks), and Nuttalliellidae (monotypic Hoogstraal 1956 ). Understanding the evolution of antihemostatic strategies of blood-feeding arthropods could shed light on the scope of diversity exhibited by these arthropods and allow the development of new control strategies by identification of novel and shared targets. Questions that remain to be answered are the nature of the nonhematophagous ancestors and how these antihemostatic mechanisms evolved. Evolutionary mechanisms of adaptation to a blood-feeding environment can be studied by the identification and characterization of antihemostatic components that are secreted during the feeding of hematophagous organisms. It is interesting to note that similar mechanisms for the inhibition of the host's hemostatic system have evolved several times ( Law, Ribeiro, and Wells 1992 ). Adaptation of hematophagous arthropods to a blood-feeding environment thus entails specific adaptation to an efficient existing hemostatic system. In contrast, the vertebrate blood coagulation cascade has been evolving since ∼400 MYA and was in place in its present form by ∼200 MYA ( Doolittle and Feng 1987 ). Hematophagy (blood-feeding behavior) evolved independently at least six times in the approximately 15,000 species and 400 genera of hematophagous arthropods during the Jurassic and Cretaceous eras, 145–65 MYA ( Balashov 1984 Ribeiro 1995 ). This coincides with current molecular phylogeny views on the early divergence of modern birds and placental mammals in the Late Cretaceous, which suggests that this event might have been a driving force in the evolution of hematophagy in ticks. Independent evolution of these mechanisms in ticks points to a rapid divergence between tick families that could be dated between 120 and 92 MYA. Comparison with hemostatic inhibitors of hard ticks suggests that the two main tick families have independently evolved novel antihemostatic mechanisms. Higher nonsynonymous-to-synonymous substitution rates indicate positive Darwinian selection for the fXa and PAIs. This evolutionary model explains why the tick serine protease inhibitors have inhibition mechanisms that differ from that of the canonical bovine pancreatic trypsin inhibitor (BPTI)-like inhibitors. In this scenario, the thrombin inhibitors preceded the fXa and PAIs. Maximum parsimony analysis and a phylogeny based on root mean square deviation values of α-carbon backbone structures suggest a novel evolutionary pathway by which different antihemostatic functions have evolved through a series of paralogous gene duplication events. Neighbor-joining analysis indicates that fXa, thrombin, and PAIs share a common ancestor. This study describes common origins of both blood coagulation inhibitors and platelet aggregation inhibitors (PAIs) from soft ticks of the genus Ornithodoros. Although many bioactive components involved in the regulation of the host's hemostatic system have been described, the evolutionary mechanisms of how arthropods adapted to a blood-feeding environment have not been elucidated. Identification and characterization of antihemostatic components from hematophagous organisms are useful for the elucidation of the evolutionary mechanisms involved in adaptation to a highly complex host hemostatic system.
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