Abstract
Ticks modulate their hosts' defense responses by secreting a biopharmacopiea of hundreds to thousands of proteins and bioactive chemicals into the feeding site (tick-host interface). These molecules and their functions evolved over millions of years as ticks adapted to blood-feeding, tick lineages diverged, and host-shifts occurred. The evolution of new proteins with new functions is mainly dependent on gene duplication events. Central questions around this are the rates of gene duplication, when they occurred and how new functions evolve after gene duplication. The current review investigates these questions in the light of tick biology and considers the possibilities of ancient genome duplication, lineage specific expansion events, and the role that positive selection played in the evolution of tick protein function. It contrasts current views in tick biology regarding adaptive evolution with the more general view that neutral evolution may account for the majority of biological innovations observed in ticks.
Highlights
Ticks modulate their hosts’ defense responses by secreting a biopharmacopiea of hundreds to thousands of proteins and bioactive chemicals into the feeding site
All blood-feeders secrete bioactive components from their salivary glands into the feeding site that target similar key host defense mechanisms. This underscores the universality of vertebrate host inflammatory and hemostatic mechanisms, since these evolved (>350 million years ago (MYA)), before most arthropod lineages adapted to blood feeding (Delvaeye and Conway, 2009; Conway, 2015)
These generalizations have predictive value for the study of uncharacterized blood-feeding lineages, where we expect targeting of key host processes, unique salivary gland repertoires and limited numbers of protein families that have been expanded by gene duplication
Summary
Ticks (Ixodida) are obligate blood-feeding ecto-parasites that evolved a blood-feeding lifestyle >250 million years ago (MYA) (Mans et al, 2011, 2012, 2016). Tick-host interactions and lineage specific innovation that occurred after divergence of the main tick families are of interest. This is most apparent in the comparison of the salivary gland repertoires secreted into the host during feeding. The evolution of salivary gland protein families and protein function is mediated by gene duplication and a major issue is whether tick-host evolution is adaptive or neutral. The current review will examine these issues and aim to illuminate the general theories of gene duplication and functional evolution with tick specific examples
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