Doctoral Degrees (Botany and Zoology)

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Browsing Doctoral Degrees (Botany and Zoology) by Author "Badenhorst, Daleen"

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    Rattini (Rodentia, Murinae) species relationships and involvement as reservoirs for Scrub Typhus : a comparative molecular cytogenetic gene expression approach
    (Stellenbosch : Stellenbosch University, 2011-12) Badenhorst, Daleen; Robinson, Terence J.; Dobigny, Gauthier; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: The Rattus sensu lato complex, which harbours the important biomedical model species Rattus norvegicus, represents a group of rodents that are of clinical, agricultural and epidemiological importance. This study presents a comprehensive comparative molecular cytogenetic investigation of this complex. Karyotypes of 11 Rattus s. l. species and Hapalomys delacouri were analysed using conventional cytogenetic techniques. These data suggest that, contrary to previous claims, the usefulness of karyotypes for identifying these Asian murid species is limited, as few species-specific chromosomal characters could be identified. In order to understand the events that shaped the morphology and composition of the extant karyotypes, nine out of the 11 Rattus s. l. species were analysed by chromosome painting. This allowed the detection of a rare convergent sex-autosome translocation present in H. delacouri and B. savilei, whose establishment is hypothesised to have been favoured by the presence of interstitial heterochromatic blocks (IHBs) at the boundaries of the translocated segments. These results indicate that Rattus s. l. is characterised by slow to moderate rates of chromosome evolution in contrast to the extensive chromosome restructuring identified in most other murid rodents. Based on these data the first comprehensive putative Rattini ancestral karyotype was constructed. Their integration with published comparative maps enabled a revision of the previously postulated ancestral murid karyotype. BAC-mapping unequivocally demonstrated that the widely reported polymorphisms affecting chromosome pairs 1, 12 and 13 in the Rattus karyotypes are due to pericentric inversions. The analysis of genomic features, such as telomeres, Ag-NORs and satellite DNA suggest a constrained pattern of chromosome evolution. The investigated rat satellite I DNA family appears to be taxon (Rattus) specific, and of recent origin (consistent with a feedback model of satellite evolution). The comparative nature of the study led to the further analysis of the species within Rattus s. l. complex and their possible involvement as reservoirs of scrub typhus using a qPCR gene expression approach based on real-time PCR. The structure and transcription of syndecan-4, which had previously been linked to Orientia tsutsugamushi (causative agent of scrub typhus) infection, was compared among Rattini (typhus-positive) and Murini (typhus-negative) rodents. Although, it was not possible to conclusively link the structural variation observed between Rattini and Murini with carrier status, a link was identified between underexpression of syndecan-4 in Murini and seropositive Rattini rodents, compared to those that were seronegative. This suggests that the reduced levels of syndecan-4 transcription in Murini and Rattini is linked to the poor carrier status of Murini, and to increased longevity of seropositive Rattini (i.e., predominant host of Orientia), highlighting aspects that need further investigation.