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Research published online this week in PNAS suggests that a peptide cocktail transferred from male ticks to females during copulation not only stimulates the females to gorge on food, but also stimulates the development of the ovary and the degeneration of salivary glands, preparing females for the reproductive tasks ahead.Previous work has shown that virgin female Amblyomma hebraeum (African cattle ticks) will not feed beyond a “critical weight” of around 10 times their unfed body mass, but mated ticks will. W. Reuben Kaufman, professor of biological sciences at the University of Alberta, and his colleague Brian Weiss identified this “engorgement factor”—which they have termed “voraxin”—and show it to be a combination of two peptides that are upregulated in the testis and vas deferens of fed, but not unfed, males. “If an effective vaccine can be derived from voraxin, the anticipated results from reduced feeding would include less salivation, hence reduced pathogen transmission to the host and a reduction in oocyte development,” Kaufman told The Scientist.Weiss and Kaufman identified 35 genes that are upregulated in the testis and vas deferens of feeding males. Moth cells transfected with cDNA encoding these gene products expressed 28 recombinant proteins, which were then injected into the hemocoel of virgin female ticks in different combinations. This revealed two peptides that have no effect on female feeding if given alone, but if given together, stimulated virgins latch onto their host and feed as if they had copulated. Dissection of engorged virgin ticks showed that voraxin induced salivary gland degeneration and stimulated the development of the ovary—classical post-copulation morphological changes.This research adds to the evidence that components of seminal fluid can have important behavioral and physiologic consequences for females, said Tracey Chapman, an evolutionary biologist at University College London, who has produced clear evidence for seminal fluid proteins in insects. It is interesting that sex proteins also occur in noninsect species like ticks because this suggests that these male factors could be quite ubiquitous, she said.In the case of the tick, it appears that these sex peptides are acting in the reproductive interests of both sexes. “It's a kind of switch effect to tell females that they've been properly mated,” said Chapman, who was not involved in the study. “It's only worth starting to produce eggs and develop your ovaries once you have been mated.”In addition, Weiss and Kaufman showed that when normal mated females were put on a rabbit immunized against voraxin, 74% showed little interest in feeding, but all mated ticks put on a control, nonimmunized rabbit engorged normally. So, a vaccine that blocks the action of voraxin should reduce feeding, argued Kaufman, and hence transmission of tick-borne pathogens such as the bacterium Ehrlichia ruminantium, which causes a Rickettsia-like infection in African cattle.José De La Fuente, a veterinary pathobiologist at Oklahoma State University, who was not involved in the study, agreed that voraxin could make a good target for a vaccine. However, he disputed the suggestion that a voraxin-based vaccine would achieve better control of ticks than existing vaccines based on antigens from the southern cattle tick Boophilus microplus. “At this point, it's difficult to determine if voraxin would be as effective as the B. microplus antigens… or if the combination with these and other tick antigens would result in more effective tick vaccines. Pen and field trials in several hosts would be required to evaluate these possibilities,” De La Fuente said.

Why sex makes female ticks fat

Male to female peptide transfer during tick copulation stimulates female engorgement

Apr 5, 2004

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