Rattini (Rodentia, Murinae) species relationships and involvement as reservoirs for Scrub Typhus : a comparative molecular cytogenetic gene expression approach

[摘要] ENGLISH ABSTRACT: The Rattus sensu lato complex, which harbours the important biomedical model speciesRattus norvegicus, represents a group of rodents that are of clinical, agricultural andepidemiological importance. This study presents a comprehensive comparative molecularcytogenetic investigation of this complex. Karyotypes of 11 Rattus s. l. species andHapalomys delacouri were analysed using conventional cytogenetic techniques. These datasuggest that, contrary to previous claims, the usefulness of karyotypes for identifying theseAsian murid species is limited, as few species-specific chromosomal characters could beidentified. In order to understand the events that shaped the morphology and composition ofthe extant karyotypes, nine out of the 11 Rattus s. l. species were analysed by chromosomepainting. This allowed the detection of a rare convergent sex-autosome translocation presentin H. delacouri and B. savilei, whose establishment is hypothesised to have been favoured bythe presence of interstitial heterochromatic blocks (IHBs) at the boundaries of thetranslocated segments. These results indicate that Rattus s. l. is characterised by slow tomoderate rates of chromosome evolution in contrast to the extensive chromosomerestructuring identified in most other murid rodents. Based on these data the firstcomprehensive putative Rattini ancestral karyotype was constructed. Their integration withpublished comparative maps enabled a revision of the previously postulated ancestral muridkaryotype. BAC-mapping unequivocally demonstrated that the widely reportedpolymorphisms affecting chromosome pairs 1, 12 and 13 in the Rattus karyotypes are due topericentric inversions. The analysis of genomic features, such as telomeres, Ag-NORs andsatellite DNA suggest a constrained pattern of chromosome evolution. The investigated ratsatellite I DNA family appears to be taxon (Rattus) specific, and of recent origin (consistentwith 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 geneexpression 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 observedbetween Rattini and Murini with carrier status, a link was identified between underexpressionof syndecan-4 in Murini and seropositive Rattini rodents, compared to those that wereseronegative. This suggests that the reduced levels of syndecan-4 transcription in Murini andRattini is linked to the poor carrier status of Murini, and to increased longevity ofseropositive Rattini (i.e., predominant host of Orientia), highlighting aspects that need furtherinvestigation.

[发布机构] Stellenbosch University