Browsing by Author "Gilbert, Clement"

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    Chromosomal evolution and phylogeny of golden moles and tenrecs (Mammalia : Afrosoricida)
    (Stellenbosch : University of Stellenbosch, 2008-03) Gilbert, Clement; Robinson, T. J.; Hassanin, A.; University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology.
    Afrosoricida is a 65 million years old (my) eutherian order that together with the Tubulidentata (aardvark) and Macroscelidea (elephant shrews) form the Afroinsectiphillia, a subclade of Afrotheria. It includes two families – Chrysochloridae (nine genera of golden moles) and Tenrecidae (11 genera of tenrecs) – that collectively represent ~59% of the afrotherian generic diversity. This study presents the first comprehensive cytogenetic comparison between members of these two families (seven genera and 11 species/subspecies of golden moles, and two genera and 11 species of tenrecs) using G- and C-banding and chromosome painting. All detected rearrangements are interpreted in a strict cladistic framework. In the case of Chrysochloridae, this provides evidence for a sister relationship between Chrysochloris and Cryptochloris, the monophyly of the Amblysomus genus, and for the elevation of A. hottentotus meesteri to specific rank. The detection of telomeric-like repeats in the centromeres of all chromosomes of the Amblysomus species/subspecies but not in those of A. h. meesteri further strengthens its recognition as a distinct species. Parsimony analysis of chromosomal rearrangements within Tenrecidae, the second Afrotheria assemblage studied, showed that rearrangements which could be interpreted as Whole Arm Reciprocal Translocations (WARTs) were more likely to be the result of Robertsonian translocations. Four interspecific associations are recovered within Microgale that are consistent with morphological and molecular characters. It was also possible to infer ancestral karyotypes for the Chrysochloridae, Oryzorictinae and the two tenrecid genera, Oryzorictes and Microgale. Given the relatively high karyotypic diversity observed among some Microgale species and the prevailing debates on chromosomal evolution and regional palaeoenvironmental fluctuations, it is suggested that Microgale be added to the list of taxa where structural rearrangements are likely to have played a role in speciation. Using Genbank sequences and a relaxed Bayesian clock method, we estimate the age of the family Chrysochloridae at ~28.5 my and that of the genus Microgale at ~9.9 my. Based on these dates, it can be shown that most of the evolutionary branches are characterized by a slow rate of chromosomal change, but that markedly high rates are observed in some Microgale species and to a lesser extent in the lineage leading to A. robustus. The rates of chromosomal evolution and other cytogenetic features highlighted in this study are discussed in light of recent advances in understanding the molecular mechanims that underpin changes to genomic architecture.
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    Evolution from XIST-independent to XIST-controlled X-chromosome inactivation : epigenetic modifications in distantly related mammals
    (Public Library of Science, 2011-04-25) Chaumeil, Julie; Waters, Paul D.; Koina, Edda; Gilbert, Clement; Robinson, Terence J.; Graves, Jennifer A. Marshall
    X chromosome inactivation (XCI) is the transcriptional silencing of one X in female mammals, balancing expression of X genes between females (XX) and males (XY). In placental mammals non-coding XIST RNA triggers silencing of one X (Xi) and recruits a characteristic suite of epigenetic modifications, including the histone mark H3K27me3. In marsupials, where XIST is missing, H3K27me3 association seems to have different degrees of stability, depending on cell-types and species. However, the complete suite of histone marks associated with the Xi and their stability throughout cell cycle remain a mystery, as does the evolution of an ancient mammal XCI system. Our extensive immunofluorescence analysis (using antibodies against specific histone modifications) in nuclei of mammals distantly related to human and mouse, revealed a general absence from the mammalian Xi territory of transcription machinery and histone modifications associated with active chromatin. Specific repressive modifications associated with XCI in human and mouse were also observed in elephant (a distantly related placental mammal), as was accumulation of XIST RNA. However, in two marsupial species the Xi either lacked these modifications (H4K20me1), or they were restricted to specific windows of the cell cycle (H3K27me3, H3K9me2). Surprisingly, the marsupial Xi was stably enriched for modifications associated with constitutive heterochromatin in all eukaryotes (H4K20me3, H3K9me3). We propose that marsupial XCI is comparable to a system that evolved in the common therian (marsupial and placental) ancestor. Silent chromatin of the early inactive X was exapted from neighbouring constitutive heterochromatin and, in early placental evolution, was augmented by the rise of XIST and the stable recruitment of specific histone modifications now classically associated with XCI. © 2011 Chaumeil et al.