MBE Advance Access published online on May 30, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg126
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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1 Department of Biochemistry, University of Pretoria, Pretoria 0002, South Africa
* To whom correspondence should be addressed. E-mail: albert.neitz{at}bioagric.up.ac.za.
The origins of tick toxicoses are still a subject of controversy as no molecular data are yet available to study the evolution of tick-derived toxins. In this study we describe the molecular structure of toxins from the soft tick, Ornithodoros savignyi. The TSGPs are four highly abundant proteins, proposed to play a role in salivary gland granule biogenesis of the soft tick, O. savignyi, of which the toxins (TSGP2 and TSGP4) are part off. They were assigned to the lipocalin family based on sequence similarity to known tick lipocalins. Several other tick lipocalins were also identified using Smith-Waterman database searches, bringing the tick lipocalin family up to 20. Phylogenetic analysis showed that most tick lipocalins group within genus-specific clades, suggesting that gene duplication and divergence of tick lipocalin function occurred after tick speciation, most probably during the evolution of an hematophagous lifestyle. TSGP2 and TSGP3 show high sequence identity and group terminal to moubatin, an inhibitor of collagen-induced platelet aggregation from the tick, O. moubata. However, no platelet aggregation inhibitory activity is associated with the TSGPs using ADP or collagen as agonists, suggesting that TSGP2 and TSGP3 duplicated after divergence of O. savignyi and O. moubata. This is supported by the absence of TSGP2-4 in the salivary gland extracts of O. moubata. The absence of TSGP2 and TSGP4 in salivary gland extracts from O. moubata correlates with the non-toxicity of this tick species. The implications of this study are that the various forms of tick toxicoses do not have a common origin, but must have evolved independently in those tick species that cause pathogenesis. Key Words:
evolution, gene duplication, lipocalins, tick toxicoses
© 2003 Society for Molecular Biology and Evolution
Original Articles
The Major Tick Salivary Gland Proteins and Toxins from the Soft Tick, Ornithodoros savignyi, Are Part of the Tick Lipocalin Family: Implications for the Origins of Tick Toxicoses
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