“Male factors” in ticks: their role in feeding and egg development

A transcriptome and proteome of the tick Rhipicephalus microplus shaped by the genetic composition of its hosts and developmental stage

The cattle tick, Rhipicephalus microplus, is a monoxenous tick that co-evolved with indicine cattle on the Indian subcontinent. It causes massive damage to livestock worldwide. Cattle breeds present heritable, contrasting phenotypes of tick loads, taurine breeds carrying higher loads of the parasite than indicine breeds. Thus, a useful model is available to analyze mechanisms that determine outcomes of parasitism. We sought to gain insights on these mechanisms and used RNA sequencing and Multidimensional Protein Identification Technology (MudPIT) to generate a transcriptome from whole larvae and salivary glands from nymphs, males and females feeding on genetically susceptible and resistant bovine hosts and their corresponding proteomes. 931,698 reads were annotated into 11,676 coding sequences (CDS), which were manually curated into 116 different protein families. Male ticks presented the most diverse armamentarium of mediators of parasitism. In addition, levels of expression of many genes encoding mediators of parasitism were significantly associated with the level and stage of host immunity and/or were temporally restricted to developmental stages of the tick. These insights should assist in developing novel, sustainable technologies for tick control.

Salivary gland degeneration and ovary development in the Rocky Mountain wood tick, Dermacentor andersoni Stiles (Acari: Ixodidae). II. Determination of the 'critical weight'

The feeding cycle of female ixodid ticks is divided into preparatory, slow, and rapid feeding phases. When a female Amblyomma hebraeum is removed from the host after achieving a minimum size of about 10-13× the unfed weight, various physiological and behavioural changes occur: (a) haemolymph ecdysteroid concentration rises dramatically, (b) the tick does not reattach to the host when given the opportunity, (c) its salivary glands degenerate within about 4 days (if mated) or 8 days (if virgin), and (d) oocyte maturation and oviposition occur (Kaufman and Lomas, 1996; Invert. Repr. Devel. 30: 191-198). None of these changes occur if the tick is removed from the host at smaller sizes. This transition, which occurs when the tick enters the rapid phase of engorgement, has been named the 'critical weight'. To date, the critical weight has been determined for A. hebraeum only. The present study established that, in both mated and virgin D. andersoni, the critical weight is similar to that of A. hebraeum. Although a small percentage of virgin A. hebraeum do exceed the critical weight, achieving perhaps 20× the unfed weight, virgin D. andersoni regularly fed well beyond their CW (>50× the unfed weight) and occasionally engorged completely (100× the unfed weight), although they did not detach spontaneously from the host within 21 days of attachment.

An experimental test of host specialization in a ubiquitous polar ectoparasite: a role for adaptation?

The evolution of host specificity is considered to be an essential mechanism driving parasite diversity. It may be governed by adaptive constraints that lead to host-dependent fitness trade-offs. Alternatively, specificity may arise via transmission constraints that isolate parasite populations, without necessarily involving adaptation per se. Here, we ask whether the repeated observation of host-associated genetic races across the worldwide distribution of the seabird ectoparasite Ixodes uriae is associated with host adaptation. We conducted a field-based experiment to test for adaptive specialisation in host races of I. uriae. We transferred unengorged ticks of two life stages (nymphs and adults) originating from three host species (black-legged kittiwake, common guillemot and Atlantic puffin) onto young kittiwake nestlings and followed attraction and attachment rates, engorgement times and feeding success of the transplanted ticks. All ticks were also typed genetically to match exploitation patterns with genetic differences among races. Ticks from atypical hosts were significantly less attracted to nestlings than ticks from the typical host, and showed lower feeding success and higher mortality. The degree of host specificity matched patterns of neutral genetic variation among races, with puffin ticks being more specific than guillemot ticks. Differences in specificity were also apparent among tick life stages, suggesting that nymphal ticks may be less discriminating of host type than adult ticks. Our results indicate that the genetic divergence previously observed among sympatric I. uriae host races is at least partially linked to adaptive specialisation to the host species and not simply to host-mediated transmission. They also suggest that the adaptation process may evolve differently in different life stages based on trade-offs with physiological constraints. The identification of the selective forces acting in host specialization will now be necessary to better characterize these patterns and to understand how transmission interacts with the adaptation process to generate parasite biodiversity.

Insect seminal fluid proteins: identification and function

Seminal fluid proteins (SFPs) produced in reproductive tract tissues of male insects and transferred to females during mating induce numerous physiological and behavioral postmating changes in females. These changes include decreasing receptivity to remating; affecting sperm storage parameters; increasing egg production; and modulating sperm competition, feeding behaviors, and mating plug formation. In addition, SFPs also have antimicrobial functions and induce expression of antimicrobial peptides in at least some insects. Here, we review recent identification of insect SFPs and discuss the multiple roles these proteins play in the postmating processes of female insects.

Cuticular plasticization in the tick, Amblyomma hebraeum (Acari: Ixodidae): possible roles of monoamines and cuticular pH

The degree of plasticization of the alloscutal cuticle of a ‘hard’ (ixodid) tick, Amblyomma hebraeum, and a ‘soft’ (argasid) tick, Ornithodoros moubata, was assessed throughout the blood-feeding period. Cuticle viscosity was calculated from rate of creep of cuticle under constant load using a Maxwell model. Feeding-related plasticization (i.e. increased rate of extension under a constant load) occurred in A. hebraeum but not in O. moubata. Maxwell viscosity of unfed A. hebraeum cuticle was relatively high (~720 GPa s) but was significantly lower in feeding ticks. Small partially fed ticks displayed a viscosity of ~108 GPa s. Still lower values (42 GPa s) were observed in the largest of the engorged ticks. Following cessation of feeding, there was a significant but limited reversal in viscosity back to ~100 GPa s. The water content of cuticle of unfed A. hebraeum (23.4% of wet mass) rose sharply after the onset of feeding and reached a plateau value of 34.0% at a fed/unfed weight ratio of 3 and beyond. Ixodid ticks lay down new endocuticle during the feeding period. The observed increase in cuticle hydration suggests that both old and new cuticles are hydrated during feeding. Monoamines may play an important role in controlling cuticle viscosity. Dopamine (DA) injected into partially fed A. hebraeum caused plasticization. 5-Hydroxytryptamine (serotonin, 5-HT), which induces plasticization in the blood-sucking insect Rhodnius prolixus, had no statistically significant effect on tick cuticle. Octopamine (OA) and tyramine both caused cuticle stiffening (i.e. opposed plasticization). This suggests a possible inhibitory effect but co-injection of OA with DA did not reduce DA-induced plasticization. The mechanism leading to plasticization of tick cuticle may involve a change in cuticular pH. The viscosity of tick cuticle loops was highest at pH 8.0 (389 GPa s) and fell precipitously in the acidic range to a low value of 2.2 GPa s at pH 5.5–5.7. A cuticular pH of ~6.5 would account for the lowest viscosity observed under physiological conditions (42.4 GPa s for large, day 0, engorged ticks). The V-ATPase inhibitor, concanamycin A, was a potent inhibitor of DA-induced plasticization. These results are consistent with a model in which DA acts to cause plasticization through transport of H+ ions into the cuticle. Measurement of cuticular ion (Na+, K+, Ca2+, Mg2+) content did not suggest that plasticization is caused by any of these ions. Taken together, our results suggest that the mechanism of cuticular plasticization in feeding A. hebraeum is related to hydration, and involves the transport of H+ ions into the sub-cuticular space by cells in the hypodermis. Feeding-induced plasticization was not observed in the rapid feeding tick, O. moubata.

Salivary gland degeneration and vitellogenesis in the ixodid tick Amblyomma hebraeum: Surpassing a critical weight is the prerequisite and detachment from the host is the trigger

The normal engorged body weight of female ixodid ticks (Acari: Ixodidae) is about 100x the unfed weight. Virgin female Amblyomma hebraeum normally do not feed beyond 10x the unfed weight. However, about 10-20% of a population of virgins will feed to perhaps 20x the unfed weight, but not much beyond that. In A. hebraeum, when females surpass about 10x the unfed weight, the following changes in physiology occur if they are removed from the host: (a) they will not reattach if given the opportunity, (b) their salivary glands (SGs) will undergo autolysis within 4 days if they are mated or 8 days if they are virgin, and (c) egg maturation and oviposition will occur in due course. Mated or virgin female ticks removed from the host below about 10x the unfed weight do not experience the latter changes (Kaufman, W.R., Lomas, L., 1996. 'Male Factors' in ticks: their role in feeding and egg development. Invertebrate Reproduction and Development 30, 191-198). In 1984 we named this transitional weight, the 'critical weight' (CW). Its absolute value is probably a species-specific characteristic (Kaufman, W.R., 2007. Gluttony and sex in female ixodid ticks: how do they compare to other blood-sucking arthropods? Journal of Insect Physiology 53, 264-273). Although mated females tend to engorge within a day of surpassing the CW, virgin females surpassing the CW can remain attached to the host for at least several weeks more. It is not known whether the physiological changes in the SGs and ovaries listed above occur in those large virgins that remain attached, although we suppose that this would be maladaptive. Instead, we hypothesize in this study that surpassing the CW is only a prerequisite for inducing these changes, and that detachment is the actual trigger. We support our hypothesis by demonstrating that large virgins, remaining attached to a host for 8 days, did not undergo SG degeneration nor complete egg maturation during the attachment period. Those changes occurred only within 8 days following detachment. So some type of sensory information associated with attachment to the host, and still undefined, inhibits expression of the physiological changes hitherto associated merely with surpassing the CW.

Male engorgement factor: Role in stimulating engorgement to repletion in the ixodid tick, Dermacentor variabilis

Mating in ticks results in profound physiological changes that eventually results in egg production. In the American dog tick, Dermacentor variabilis, mating causes partially blood-fed female ticks to commence rapid engorgement to repletion and eventual detachment from the host and egg laying. The peptidic male pheromone (engorgement factor alpha/beta) transferred to the female during mating is known only from a single tick species, Amblyomma hebraeum, and was shown to consist of two peptides produced in the testis/vas deferens (TVD) and not in the male accessory gland (MAG). In the current study, we obtained 2704bp of sequence data for efalpha from D. variabilis, of 7kb as determined by Northern blot, and show that it is also present in the Southern cattle tick, Rhipicephalus microplus and the deer tick, Ixodes scapularis. Analysis of the male gonad transcriptome by pyrosequencing produced 563,093 reads of which 636 matched with efalpha; none matched with efbeta. No evidence of efbeta orthologs could be found in any publicly available database including the I. scapularis genome. Silencing efalpha in male ticks failed to significantly reduce the engorgement weight of females compared to controls. Injection of sephadex beads, replete female synganglia, fed male MAG, fed male TVD, or replete female vagina/seminal receptacle (VA/SR), separately, failed to initiate feeding to repletion like that found in normally mated females. However, a small percentage of females injected with VA/SR that fed beyond the arbitrary weight for repletion of 300mg, produced brown eggs (an indication of vitellogenin uptake by the oocytes). The greatest effect was observed in female ticks injected with a suspension of MAG and TVD combined; 50% fed to repletion and all of these dropped off from the host and laid brown eggs. The effect was abolished if the aqueous fraction of the MAG/TVD homogenate only was injected suggesting that EF in ticks is a non-secreted membrane-bound or intracellular protein. Overall, these data suggest that EFalpha in D. variabilis is not an engorgement factor.

Migration and motility of spermatozoa in the female reproductive tract of the soft tick Ornithodoros moubata (Acari, Argasidae)

The spermatozoa of ticks are anomalous in many respects: they are very large, cytoplasm-rich cells which lack a flagellum but move with a peculiar gliding motility. Their metamorphosis after deposition in the female has been well documented, but many of the subsequent events in the career of the spermatozoa are controversial or poorly documented. Our observations of motility imply that the many types of motility that have been reported (up to 5 different types in several reports) can be reduced to 2 apparently independent types of active motility: (1) gliding motility generated along the whole spermatozoon and (2) contortions of the anterior tip of the head. These types of motility appear as a consequence of sperm maturation after transfer to the female, but only become pronounced if the female has taken a recent bloodmeal. A consequence of this enhanced gliding motility after feeding is the movement of the spermatozoa out of the naturally ruptured neck of the spermatophore and up the female genital tract. This occurs without any apparent assistance from the female's musculature and likely is the prime mechanism of movement of the spermatozoa to the site of fertilization.

The impact of RNA interference of the subolesin and voraxin genes in male Amblyomma hebraeum (Acari: Ixodidae) on female engorgement and oviposition

Reducing or replacing the use of chemical pesticides for tick control is a desirable goal. The most promising approach would be to develop vaccines that protect hosts against tick infestation. Antigens suitable for the development of anti-tick vaccines will likely be those essential for vital physiological processes, and in particular those directly involved in feeding and reproduction. In this study genes from Amblyomma hebraeum Koch that encode for subolesin and voraxin were studied in male ticks by RNA interference (RNAi). Males (unfed or fed) were injected with dsRNA of (1) subolesin, (2) voraxin, (3) subolesin plus voraxin or (4) injection buffer, after which they were held off-host overnight and then allowed to feed on rabbits together with normal female A. hebraeum. Females that fed together with male ticks injected with subolesin or subolesin + voraxin dsRNA had a higher rate of mortality, weighed substantially less and produced a smaller egg mass than the controls. However, females feeding with males injected with voraxin dsRNA alone were not significantly different from the controls with respect to mortality, engorged weight or fecundity. However, as assessed by semi-quantitative RT-PCR, voraxin was not silenced in this study, the reasons for which remain unknown. The results of this study suggest that A. hebraeum subolesin is worthy of further testing as a candidate tick vaccine antigen.

Effect of 20-hydroxyecdysone and haemolymph on oogenesis in the ixodid tick Amblyomma hebraeum

Earlier work from our laboratory indicated that injection of 20-hydroxyecdysone (20E) into non-vitellogenic female Amblyomma hebraeum ticks stimulates the synthesis of vitellogenin (Vg), but not its uptake into oocytes [Friesen, K., Kaufman, W.R., 2004. Effects of 20-hydroxyecdysone and other hormones on egg development, and identification of a vitellin-binding protein in the ovary of the tick, Amblyomma hebraeum. Journal of Insect Physiology 50, 519-529]. In contrast, Thompson et al. [Thompson, D.M., Khalil, S.M.S., Jeffers, L.A., Ananthapadmanaban, U., Sonenshine, D.E., Mitchell, R.D., Osgood, C.J., Apperson, C.S., Roe, M.R., 2005. In vivo role of 20-hydroxyecdysone in the regulation of the vitellogenin mRNA and egg development in the American dog tick, Dermacentor variabilis (Say). Journal of Insect Physiology 51, 1105-1116] demonstrated that injection of 20E into virgin female Dermacentor variabilis ticks stimulated both vitellogenesis and Vg uptake into oocytes. In addition to the species difference in the two studies there were substantially different methods for injecting 20E. In our earlier work we injected small partially fed ticks after removing them from the host. Thompson et al. injected the females while they remained attached to the host. So in this study we repeated our earlier experiments on A. hebraeum using on-host injection. We also injected 20E into off-host ticks with or without haemolymph collected from engorged ticks (days 2-10 post-engorgement), or from large partially fed mated ticks in the rapid phase of engorgement, to see whether we might detect a 'vitellogenin uptake factor' (VUF) in haemolymph. Off-host injection of 20E (0.45microg/g body weight (bw)) did not induce ovary development beyond that of vehicle-injected controls. But ticks in this study, receiving 20E plus haemolymph from engorged ticks, showed a significant increase in ovary weight beyond that of 20E alone (1.31+/-0.05% bw; 34 for 20E plus haemolymph and 1.03+/-0.05% bw; 25 for 20E alone). However, in normal engorged A. hebraeum, the ovary exceeds 7% bw at the onset of oviposition. As in our earlier work, in this study 20E stimulated Vg-synthesis (3.9+/-0.5mgVt-equivalents/ml) beyond that occurring in vehicle-injected ticks (0.76+/-0.14mgVt-equivalents/ml), and there was a further increase in ticks injected with 20E plus haemolymph from engorged ticks (8.9+/-1.0mgVt-equivalents/ml). On-host injection of 20E alone (6microg20E/g bw) did not produce a statistically significant increase in oocyte length over that of vehicle-injected controls, whereas on-host injection of 20E plus engorged haemolymph resulted in significantly larger oocytes (261+/-57microm) compared to vehicle-injected controls (132+/-11microm), compared to 20E alone (131+/-12microm), or haemolymph alone (124+/-24microm). There was a marked stimulation of Vg-synthesis by 31microg20E/g bw (6.0+/-1.5mgVt-equivalents/ml) compared to vehicle-injected controls (1.02+/-33mgVt-equivalents/ml). Vt accumulation by ovaries was significantly greater in ticks treated with haemolymph (12+/-3microgVt/mg ovary) or 20E plus haemolymph (56+/-26microgVt/mg ovary) compared to vehicle-injected controls (5.1+/-1.5microgVt/mg ovary). There was also a significant effect of 6microg20E/g bw plus engorged haemolymph on ovary weight (1.74+/-0.29% bw) compared to vehicle-injected ticks (0.95+/-0.10% bw), but not compared to ticks injected with 20E alone (1.25+/-0.19% bw). We conclude that at least some of the differences observed between the two laboratories relate to the species difference, and that there is some evidence that the engorged haemolymph of A. hebraeum contains a VUF.

Prostaglandin E2 modulates the expression of antimicrobial peptides in the fat body and midgut of Anopheles albimanus

Prostaglandins (PGs) participate in the regulation of vertebrate and in at least six insect orders' immune responses. We identified PGE2 in midgut, fat body, Malpighian tubules, and ovarioles of Anopheles albimanus (Aa) mosquitoes. Our data indicate that PGE2 synthesis in cultured midguts responds to the presence of two bacterial species, Micrococcus luteus and Klebsiella pneumoniae. The production of mRNA coding for antimicrobial peptides Aa-Attacin, Aa-Cecropin, and Aa-Gambicin was observed in cultured fat bodies and midguts. The production of these messengers was reduced in the presence of dexamethasone, and this effect was reversed by arachidonic acid. Adding PGE2 to cultures resulted in increased Aa-cecropin mRNA and decreased Aa-attacin and Aa-gambicin mRNAs.

Gluttony and sex in female ixodid ticks: how do they compare to other blood-sucking arthropods?

The central issue dealt with here is the role of copulation in the control of feeding behaviour in ticks and some haematophagous insects. Female ticks of the family Ixodidae normally engorge to approximately 100 x their unfed body weight, and then drop from the host, produce and lay eggs, and die. Virgins, on the other hand, normally do not exceed 5-40% (depending on species) of the normal engorged body weight. But instead of detaching voluntarily at that point most virgins remain fixed to the host for extended periods, waiting for males to find them so they can complete engorgement. Virgin haematophagous insects, and virgin ticks of the family Argasidae display little, if any, reduction in blood meal size compared to mated females, at least not during the first ovarian cycle. During subsequent ovarian cycles, meal size in some virgin insects may be somewhat reduced depending on how many eggs are retained in the reproductive tract, but the reduction is not nearly to the same extent as that observed for virgin ixodid females. The stimulatory effect of copulation on engorgement in the latter is caused by a pair of proteins (voraxin alpha and beta) produced in the testis and transferred to the female with the spermatophore. Here, I propose why it might be adaptive for an ixodid female to remain small until mated. The hypothesis is suggested from the facts that ixodid ticks remain attached to the host for days (rather than minutes), and that virgin ticks, above a certain critical weight, lose all opportunity for producing viable offspring should they be groomed off the host prematurely, or should the host die while ticks are still attached.

Structural and cytochemical changes in the salivary glands of the Rhipicephalus (Boophilus) microplus (CANESTRINI, 1887) (Acari: Ixodidae) tick female during feeding

This study describes the morphology of salivary glands of Rhipicephalus (Boophilus) microplus female ticks at beginning of feeding (24-48 h of attachment) and semi-engorged (4-5 days of attachment) to verify the degenerative characteristics of these organs and the secretory phase in which the process begins. At the beginning of feeding, secretion granules had been observed only in the cytoplasm of cells b, c1, c2, c4 (type II acinus) and d (type III acinus), as well as large nuclei with regular and preserved morphology. In the semi-engorged females the acini presented few normal cells, few partially preserved ones, and the remaining ones in several stages of degeneration, that is, with retraction and cytoplasmic vacuolization, and nuclei with chromatin in several stages of condensation, picnotic and/or in fragmentation. In type I acinus and in the excretory ducts of the studied glands, at both feeding stages, no degenerative characteristic was observed. In females of R. (B.) microplus, the salivary glands degenerate asynchronically and precociously when compared with those of others tick's species.

Prostaglandins and other eicosanoids in insects: biological significance

Prostaglandins and other eicosanoids are oxygenated metabolites of certain polyunsaturated fatty acids. These compounds are well known for their important actions in mammalian physiology and disease. Recent work has revealed the presence and biological actions of eicosanoids in insects and many other invertebrate animals. In insects, eicosanoids mediate cellular immunity to microbial and metazoan challenge. Notably, some infectious organisms secrete factors responsible for impairing host insect immune reactions by inhibiting biosynthesis of eicosanoids. Eicosanoids also act in insect reproductive biology, in ion transport physiology, and in fever response to infection as well as in protein exocytosis in tick salivary glands. Aside from ongoing actions in homeostasis, certain eicosanoid actions occur at crucial points in insect life histories, such as during infectious challenge and important events in reproduction.

Assuring paternity in a promiscuous world: are there lessons for ticks among the insects?

In this article I begin with a few current ideas on some physiological factors that influence mating choice in insects. Emphasis is placed on those proteins produced by the male reproductive accessory glands which increase female fecundity and reduce her receptivity to subsequent males. Strategies used by late-arriving males to favour their paternity are also mentioned. With a number of insect models as background, I then review what is currently known about several male factors in ticks (a capacitation factor, a male factor, an engorgement factor and a vitellogenesis stimulating factor) and suggest where we might focus our experimental activities in the future.

Effects of 20-hydroxyecdysone and other hormones on egg development, and identification of a vitellin-binding protein in the ovary of the tick, Amblyomma hebraeum

Partially fed adult female Amblyomma hebraeum ticks were injected with 20-hydroxyecdysone (20E; up to 43 microg/g body weight (bw)), juvenile hormone III (JH III; up to 100 microg/g bw), bovine insulin (up to 2000 mU/g bw), or triiodothyronine (up to 200 ng/g bw) in an attempt to stimulate vitellogenesis. Of these, only 20E stimulated synthesis and release of vitellogenin (Vg). Immunoblot analysis revealed that Vg-synthesis occurred in the fat body. However, consistent with earlier observations suggesting that a distinct signal may be required for Vg-uptake, there was no significant Vg-uptake by oocytes of partially fed, 20E-treated ticks. Because Vg-uptake commonly occurs via receptor-mediated endocytosis (i.e., a specific Vg-receptor), we attempted to identify a vitellin (Vt)-binding protein in ovaries of engorged female ticks. A single 86 kDa Vt-binding protein was identified, even under reducing conditions (2-mercaptoethanol), by a ligand-blotting technique. Sodium salt of suramin (5 mM) inhibited binding of Vt to the 86 kDa protein. However, this protein was also detected in ovaries from small partially fed ticks (50-100 mg), suggesting that the inability of 20E to stimulate Vg-uptake in partially fed ticks may not have been due to the absence of a Vg-receptor.

Two feeding-induced proteins from the male gonad trigger engorgement of the female tick Amblyomma hebraeum

Most female ixodid ticks, once mated, feed to repletion within 6-10 days. Previous studies indicate that an engorgement factor (EF), passed to the female during copulation, may be the stimulus for engorgement. Here, we show that extracts of the testis/vas deferens of fed (but not unfed) male Amblyomma hebraeum contain EF bioactivity when injected into the hemocoel of feeding virgins. We have produced recombinant proteins (recproteins) from 28 feeding-induced genes in the male gonad and have identified a recombinant A. hebraeum engorgement factor (recAhEF) among these recproteins. recAhEF is a combination of two peptides, recAhEFalpha (16.1 kDa) and recAhEFbeta (11.6 kDa), neither of which has bioactivity on its own. recAhEF also stimulates salivary gland degeneration and partial development of the ovary, suggesting that it may be the same material as another male gonadal protein from this tick, male factor. We propose the name "voraxin" for the natural EF of ticks. When normal mated females were put on a rabbit immunized against recAhEF, 74% failed to feed beyond one-tenth the normal engorged weight within 14 days whereas all mated ticks put on a control rabbit engorged normally (mean duration of 8.8 +/- 0.8 days). This result constitutes preliminary evidence that an anti-tick vaccine might be developed from voraxin.

Rhipicephalus appendiculatus (Acari: Ixodidae): dynamics of Thogoto virus infection in female ticks during feeding on guinea pigs

Engorged nymphs (Rhipicephalus appendiculatus) were inoculated parenterally with Thogoto (THO) virus (approximately 1 microl per nymph; 10(6)-10(7) PFU/ml). The adult females which resulted were used as the source of infected ticks for this study. Hemolymph, salivary glands, synganglion, gut, ovary, and Malpighian tubules were collected on each day of the blood meal and titrated for THO virus by plaque assay. The percent of tissues infected with virus was 16% or less on the day of attachment. Percent infection rose for all tissues throughout 6-7 days of feeding, reaching 40-100% infection during the rapid phase of engorgement. For the first 4 days of feeding, virus titer in the synganglion was higher than in salivary glands (means of 6.4-34.7 PFU/synganglion and 1.6-8.8 PFU/salivary gland pair). From days 5-7, virus titer was generally higher in the salivary gland than the synganglion (means of 422, 408, and 817 PFU/gland pair and means of 62, 811, and 9 PFU/synganglion). However, because a salivary gland pair is much heavier than a synganglion, the virus concentration in the synganglion was much higher than in the salivary gland during the slow phase of feeding. During the rapid phase of feeding, the difference in virus titer between the synganglion and salivary gland reduced. This difference between the early and late stages of feeding may explain why a previous study [J. Gen. Virol. 70 (1989) 1093], using immunofluorescence and immuno-gold labelling, failed to detect virus in the salivary gland early in feeding. These data provide evidence to explain that R. appendiculatus can transmit THO virus within 24h of attachment, an important epidemiological finding.

Quantification of vitellogenesis and its control by 20-hydroxyecdysone in the ixodid tick, Amblyomma hebraeum

Ovaries of the ixodid tick, Amblyomma hebraeum Koch, grew rapidly after engorgment as a result of yolk uptake. At 26 degrees C, oviposition began by day 10 post-engorgement, plateaued on days 16-18, and ended by day 38. Vitellin (Vt) was partially purified from ovaries of day 10 engorged ticks by gel filtration and ion exchange chromatography. This Vt comprises seven major and several minor polypeptides. Two polypeptides (211 and 148 kD) from haemolymph of engorged female ticks corresponded to minor polypeptides of similar molecular weight in the ovary. The haemolymph titre of the 211 and 148 kD polypeptides increased up to the onset of oviposition. These polypeptides were absent in males and non-vitellogenic females (day 0 engorged or day 10 partially-fed females), and were thus designated as vitellogenin (Vg). Antibodies raised against haemolymph Vg211 and 148 recognized these polypeptides in partially purified Vt, as well as six of the seven major polypeptides. Using these antibodies we developed an indirect, competitive ELISA to quantify Vg. Rise in haemolymph Vg-concentration lagged slightly behind the rise in haemolymph ecdysteroid (ES)-concentration, and Vg-synthesis was stimulated by injections of 20E into non-vitellogenic females. These observations indicate that an ES is the vitellogenic hormone in A. hebraeum.

Molecular individuality and adaptation of the tick Rhipicephalus appendiculatus in changed feeding environments

The tick Rhipicephalus appendiculatus Neumann (Acari: Ixodidae) naturally infests many host species. However, the mechanisms that enable it to feed on such a wide range of hosts are unclear. One possibility is that a tick population maintains molecular (genotypic and/or phenotypic) diversity among individuals such that individuals vary in their competency in taking bloodmeals under different feeding conditions. As a first step in testing this hypothesis, we showed that the polymorphism of salivary gland proteins, previously demonstrated in unfed ticks, was maintained during feeding on guinea-pigs. We then compared feeding performance under standard laboratory rearing conditions: one instar (adults or nymphs) feeding on guinea-pigs, with three changed conditions: (1) two instars (adults and nymphs) feeding together on guinea-pigs; (2) one instar (adults or nymphs) feeding on hamsters; and (3) two instars (adults and nymphs) feeding together on hamsters. The mean engorged weight of adult females was significantly reduced under all changed conditions, indicating that most of the adult individuals were significantly challenged by the changed conditions. However, some individuals achieved successful engorgement, indicating competence to the changed condition, and demonstrating variation in adaptive ability among individuals. Engorged females produced egg masses positively correlated to the engorged weights. More interestingly, the correlation coefficient (R) increased when feeding condition was changed. This may lead to more efficient selection for population adaptation under the changed conditions. As the feeding success of ixodid ticks depends on the efficiency of the cocktail of immunomodulatory saliva, the relevance of the polymorphism of salivary gland proteins and host adaptation is discussed.

The relationship between 'critical weight' and 20-hydroxyecdysone in the female ixodid tick, Amblyomma hebraeum

THE FEEDING CYCLE OF ADULT FEMALE TICKS (ACARI: Ixodidae) is divided into preparatory, slow and rapid feeding phases. At the transition from slow to rapid feeding, Amblyomma hebraeum (Koch) females reach a 'critical weight' (CW; approx. 10x the unfed weight) that is characterized by several behavioural and physiological changes. Five of these changes were used as criteria to establish a more precise estimate of CW than we have to date. The CW as defined by re-attachment to the host was 9x the unfed weight, while for haemolymph ecdysteroid titre, salivary gland degeneration, ovary weight, oocyte length and oocyte vitellin content the CW was 10x, 10x, 12x, 12x and 13x, respectively. CW thus varies depending on the parameter measured. Although previous studies have established the influence of ecdysteroids on salivary gland degeneration and vitellogenesis, here we demonstrate a further effect of ecdysteroids: inhibiting re-attachment to the host.

Profile of the ecdysteroid hormone and its receptor in the salivary gland of the adult female tick, Amblyomma hebraeum

We examined the hemolymph ecdysteroid titer (by radioimmunoassay) and profile of the ecdysteroid receptor (EcR/USP; by [3H]ponasterone A binding, gel mobility shift assay, Western blot) in the salivary gland of the ixodid tick, Amblyomma hebraeum Koch (Acari: Ixodidae) throughout the tick feeding period and first 6 days post-engorgement. Throughout the slow phase of feeding, the hemolymph ecdysteroid titer was approximately 18 pg/microliter. The titer peaked at approximately 52 pg/microliter during the rapid phase of feeding, falling back to approximately 22 pg/microliter on the day of engorgement. Ecdysteroid titer rose again to approximately 750 pg/microliter by day 6 post-engorgement. EcR was undetectable by any of the three assays in unfed ticks. Following the onset of feeding, there appeared both specific ponasterone A binding and two major EcR bands detected by Western blot analysis. Both measurements were sustained throughout the feeding period, but declined after detachment when the salivary glands were degenerating. After ticks reached about 100 mg (by which time most females are mated), a discrete DNA-binding band was shown by gel mobility shift assay using Drosophila hsp27 EcRE as a probe. Moreover, the band intensified when hemolymph ecdysteroid titer reached its peak during the rapid phase of feeding; it declined along with decreasing EcR/USP levels, and with specific ligand binding activity following engorgement. This study suggests a role for the small hemolymph ecdysteroid peak during the rapid phase of feeding in initiating salivary gland degeneration.

Ecdysteroid regulation of salivary gland degeneration in the ixodid tick, Amblyomma hebraeum: a reconciliation of in vivo and in vitro observations

Salivary gland degeneration in the female tick Amblyomma hebraeum Koch is triggered by an ecdysteroid (ES) hormone. Under both in vivo and in vitro conditions, degeneration requires 4 days for completion. In partially fed females that have fed beyond a "critical weight," the commitment period for salivary gland degeneration occurs between 24 and 48 h after removal from the host. Although tissue degeneration begins within 24 h postengorgement, ES titer as measured by radioimmunoassay (RIA) does not rise to threshold levels until 48 h postengorgement. To explain this anomaly we examined two hypotheses: (1) there is an early hormonal signal (e.g., 3-dehydroecdysone; 3DE) that is an ES not detectable by the antibody used in our RIA; and (2) the low hemolymph titer during the first 2 days postengorgement is not an accurate reflection of the ES concentration within the tissue itself. 3-Oxoecdysteroid 3 beta-reductase (ketoreductase) was present in salivary glands, but neither ketoreductase nor 3DE was detected in hemolymph. The ES concentration of salivary gland homogenates was similar to that of hemolymph, while that of saliva was undetectable. Together, these results support our second hypothesis that the metabolically active tissue of the salivary gland experiences a suprathreshold concentration of hormone even though the concentration in hemolymph is below threshold levels.