Since ticks are classified as hematophagous ectoparasites, the primary feeding event involves a bloodmeal on a vertebrate host. Such activities facilitate the ingestion of microorganisms which may be detrimental to the survival of a tick. It is observed, however, that ticks are able to survive such invasion by microorganisms and in several cases, facilitate the transmission of pathogens, while themselves remaining unaffected. This phenomenon is attributed to the innate immune system of ticks. The focus of this project is on stimulus-induced immunoreactive peptides known as antimicrobial peptides. In chapter 2, an attempt was made to identify a homolog of the anti Gram-positive and bacteriostatic peptide microplusin, in the salivary glands of the argasid tick Ornithodoros savignyi. It was reported previously that tissue and life stage specific expression of this transcript occurs in the fat body of adult, fully fed, female Rhipicephalus (Boophilus) microplus ticks. The positive control used for this study was unsuccessful due to the incorrect tissue and life stage of R. (B.) microplus ticks. No significant homolog was identified due to the possible existence of stringent regulation of expression as well as differences in the induction stimuli between argasid and ixodid ticks. Lysozyme catalyzes the cleavage of the β-1,4 glycosidic bond between N-acetyl muramic acid and N-acetyl glucosamine of the peptidoglycan layer of bacterial cell walls affording the molecule antibacterial activity. In argasid ticks, lysozyme was observed to be induced by feeding. In chapter 3, an attempt was made to elucidate the O. savignyi homolog of the O. moubata lysozyme molecule. The partial sequence obtained revealed the presence of a lysozyme homolog in O. savignyi. The tissue expression profile revealed constitutive expression in the midgut and ovaries and induction of transcription in the hemolymph upon feeding. In salivary glands, upregulation was observed following ingestion of Gram-positive bacteria. In chapter 4, the tissue expression profile of O. savignyi defensin was investigated. It was found that transcription is induced following the ingestion of Gram-positive bacteria, while in the hemolymph upregulation was observed upon feeding. Furthermore, chapter 4 saw the attempts made at the RNAi mediated silencing of the lysozyme and defensin transcripts. Silencing, analysed by real time PCR, was not efficient as no statistically significant silencing was observed. Observation of the phenotype revealed mortality. However, statistical analysis of silencing revealed that the mortality observed was not due to silencing, but non-specific and possibly the result of injury during injection. Overall, the abovementioned experiments revealed the tissue specificity of expression of ixodid microplusin and that a more strategic approach is required for the elucidation of the argasid homolog. The partial O. savignyi lysozyme sequence was elucidated together with the tissue expression profile of this molecule and O. savignyi defensin. The RNAi experiments require optimization for future studies. / Dissertation (MSc)--University of Pretoria, 2010. / Biochemistry / unrestricted