Gene expression profiles associated with beef cattle resistance to Rhipicephalus ticks

[摘要] ENGLISH ABSTRACT: Tick resistance is a complex trait influenced by numerous environmental, physiological and genetic factors. The length of the association between cattle breeds and tick species may play a vital role in the potency of the immune responses generated by the host post-infestation. The genetically determined components of host resistance, which may have evolved due to long periods of evolution of breeds in the presence of specific tick species, are regarded the most important factors of host resistance to ticks. The isolation and characterisation of genes associated with natural host resistance may provide a low-cost, environmentally sound and sustainable chemical-free alternative for tick control through gene introgression and improved accuracy of selection in breeding programs. This study examined the tick burdens and associated gene expression profiles in two ancient (Nguni – R. decoloratus and Brahman – R. microplus) and four modern (Nguni – R. microplus, Brahman – R. decoloratus, Angus – R. decoloratus and Angus – R. microplus) host-tick associations following artificial infestation. Approximately 100 unfed tick larvae of a single species were used to infest each animal, thereafter tick counts were enumerated 18-days post-infestation. Skin biopsies, from which RNA was extracted for use in the gene expression analyses, were collected pre-infestation from non-parasitized sites and 12-hours post-infestation at visible tick-bite sites. The panel of genes analysed comprised of cytokines (TLR5, TLR7, TLR9, TRAF6, CD14), chemokines and their receptors (CCR1, CCL2, CCL6), toll-like receptors (IL-1β, CXCL8, IL-10, TNF) and other candidate genes (BDA20, OGN, TBP, LUM, B2M) whose expression was normalized against RN18S1 (or β-actin-like). Custom 96-well RT2 Profiler PCR arrays, fitted with primers designed and optimised by Qiagen, were used for real-time PCR analyses using RT2 SYBR® Green dye and an ABI 7500 Standard real-time PCR cycler. The effects of breed, tick species and breed by tick species interaction on tick count were analysed using XLSTAT (2016) and SAS Enterprise Guide (2016). The fold regulation/change values were generated via the online RT2 Profiler PCR Array Data Analysis Web-portal (SABioscience - Qiagen), using the ΔΔCT method. The effects of breed, tick species and breed by tick species interaction on the differential gene expression of each gene were analysed using XLSTAT and SAS (2016). The expression levels of LUM, B2M, TRAF6 and TPB showed significant breed variations. The Nguni and Angus differed for TBP and TRAF6, while the Brahman and Angus differed for LUM and B2M. LUM and B2M displayed significantly higher expression levels in the Brahman and Nguni cattle. Significant breed, tick species and breed by tick species interaction effects were detected from the tick count data, with the Brahman carrying less ticks than both the Angus and Nguni cattle, while the R. microplus resulted in heavier tick burdens than the R. decoloratus ticks. In both experiments, there was a lack of evidence of any breed by tick species interaction which would implicate the effect of length of association between breeds and tick species in the host response to tick challenge in respect with gene expression and tick burden.

[发布机构] Stellenbosch University