Rising from the ashes; an unanticipated failure with the engorgement factor voraxin leads to advances in three areas of tick biology: Developmental abnormalities and parthenogenesis, identification of a Coxiella-like symbiont and the molecular characterization of vitellogenesis in the southern African bont tick, Amblyomma hebraeum Koch (1844).

摘要

Weiss and Kaufman (2004, PNAS 101: 5874-5879) demonstrated that injections of fed male gonad homogenates stimulate engorgement in virgin female Amblyomma hebraeum ticks, due to the presence of the engorgement factor voraxin. They also showed voraxin's potential as part of an anti-tick vaccine: 74% of mated females fed on a rabbit immunized against voraxin failed to engorge. The original aim of this thesis was to further characterize and better evaluate the potential of voraxin as a component of a novel anti-tick vaccine. However, I was unable to confirm Weiss and Kaufman's results. Injections of male gonad homogenates or recombinant voraxin produced in two different bacterial expression systems into virgin females, both on and off the host, failed to stimulate engorgement. Immunization of rabbits against the recombinant voraxin proteins also failed to inhibit tick feeding. Silencing of voraxin via RNAi was unsuccessful, but I was able to silence subolesin, an important modulator of tick feeding and reproduction.;I also observed a large number of morphological abnormalities in our ticks, as well as several virgin females that engorged and laid viable eggs. Although low levels of parthenogenesis have been reported in many other normally bisexual tick species, it has not been previously reported in A. hebraeum. The causes of these abnormalities are unknown, but could be due, in part, to the high degree of inbreeding in our lab colony.;In addition to the numerous bacterial pathogens transmitted by ticks, many tick species also harbour a number of potentially mutualistic endosymbiotic bacteria. Here I have determined that out tick colony possess both Coxiella-like and Rickettsia-like symbionts. Their impact on the biology of the tick is unknown.;Although much is known about vitellogenesis and its regulation in insects, our knowledge in this area is much more limited in ticks. I have sequenced the full-length cDNAs encoding two vitellogenins and the vitellogenin receptor from A. hebraeum, which are similar to those of other arthropods. RT-PCR analysis of gene expression showed that the vitellogenin receptor is expressed only in the ovary of fed females, whereas vitellogenin is produced in the fat body and midgut.