Cytosystematics, sex chromosome translocations and speciation in African mole-rats (Bathyergidae: Rodentia)

Thesis (PhD (Botany and Zoology))--University of Stellenbosch, 2008.The Bathyergidae are subterranean rodents endemic to Africa south of the Sahara. They are characterised by divergent diploid numbers that range from 2n=40 in Fukomys mechowi to 2n=78 in F. damarensis. In spite of this variation there is limited understanding of the events that shaped the extant karyotypes and in an attempt to address this, and to shed light on the mode and tempo of chromosomal evolution in the African mole-rats, a detailed analysis of both the autosomal and sex chromosome components of the genome was undertaken. In addition to G- and Cbanding, Heterocephalus glaber (2n=60) flow-sorted painting probes were used to conduct cross-species chromosome painting among bathyergids. This allowed the detection of a balanced sex chromosome-autosome translocation in F. mechowi that involved a complex series of rearrangements requiring fractionation of four H. glaber autosomes and the subsequent translocation of segments to sex chromosomes and to the autosomal partners. The fixation of this rare rearrangement has probably been favoured by the presence of an intercalary heterochromatic block (IHB) that was detected at the boundary with the translocated autosomal segment. Male meiosis in Cryptomys, the Fukomys sister clade, was investigated by immunostaining of the SCP1 and SCP3 proteins involved in the formation of the synaptonemal complex. This allowed confirmation of a Y-autosome translocation that is shared by C. hottentotus subspecies. We discuss reduced recombination between Y and X2 that seems to be heterochromatin dependent in the C hottentotus lineage, and the implications this holds for the evolution of a meiotic sex chromosome chain such as has been observed in platypus. By extending cross-species chromosome painting to Bathyergus janetta, F. damarensis, F. darlingi and Heliophobius argenteocinereus, homologous chromosomal regions across a total of 11 species/subspecies and an outgroup were examined using cladistic and bioinformatics approaches. The results show that Bathyergus, Georychus and Cryptomys are karyotypically highly conserved in comparison to Heterocephalus, Heliophobius and Fukomys. Fukomys in particular is characterised by a large number of rearrangements that contrast sharply with the conservative Cryptomys. The occurrence and fixation of rearrangements in these species has probably been facilitated by vicariance in combination with life history traits that are particular to these mammals

Antagonist Xist and Tsix co-transcription during mouse oogenesis and maternal Xist expression during pre-implantation development calls into question the nature of the maternal imprint on the X chromosome

International audienceDuring the first divisions of the female mouse embryo, the paternal X-chromosome is coated by Xist non-coding RNA and gradually silenced. This imprinted X-inactivation principally results from the apposition, during oocyte growth, of an imprint on the X-inactivation master control region: the X-inactivation center (Xic). This maternal imprint of yet unknown nature is thought to prevent Xist upregulation from the maternal X (XM) during early female development. In order to provide further insight into the XM imprinting mechanism, we applied single-cell approaches to oocytes and pre-implantation embryos at different stages of development to analyze the expression of candidate genes within the Xic. We show that, unlike the situation pertaining in most other cellular contexts, in early-growing oocytes, Xist and Tsix sense and antisense transcription occur simultaneously from the same chromosome. Additionally, during early development, Xist appears to be transiently transcribed from the XM in some blastomeres of late 2-cell embryos concomitant with the general activation of the genome indicating that XM imprinting does not completely suppress maternal Xist transcription during embryo cleavage stages. These unexpected transcriptional regulations of the Xist locus call for a re-evaluation of the early functioning of the maternal imprint on the X-chromosome and suggest that Xist/Tsix antagonist transcriptional activities may participate in imprinting the maternal locus as described at other loci subject to parental imprinting

Antagonist <i>Xist</i> and <i>Tsix</i> co-transcription during mouse oogenesis and maternal <i>Xist</i> expression during pre-implantation development calls into question the nature of the maternal imprint on the X chromosome

<div><p>During the first divisions of the female mouse embryo, the paternal X-chromosome is coated by <i>Xist</i> non-coding RNA and gradually silenced. This imprinted X-inactivation principally results from the apposition, during oocyte growth, of an imprint on the X-inactivation master control region: the X-inactivation center (<i>Xic</i>). This maternal imprint of yet unknown nature is thought to prevent <i>Xist</i> upregulation from the maternal X (X^M) during early female development. In order to provide further insight into the X^M imprinting mechanism, we applied single-cell approaches to oocytes and pre-implantation embryos at different stages of development to analyze the expression of candidate genes within the <i>Xic</i>. We show that, unlike the situation pertaining in most other cellular contexts, in early-growing oocytes, <i>Xist</i> and <i>Tsix</i> sense and antisense transcription occur simultaneously from the same chromosome. Additionally, during early development, <i>Xist</i> appears to be transiently transcribed from the X^M in some blastomeres of late 2-cell embryos concomitant with the general activation of the genome indicating that X^M imprinting does not completely suppress maternal <i>Xist</i> transcription during embryo cleavage stages. These unexpected transcriptional regulations of the <i>Xist</i> locus call for a re-evaluation of the early functioning of the maternal imprint on the X-chromosome and suggest that <i>Xist</i>/<i>Tsix</i> antagonist transcriptional activities may participate in imprinting the maternal locus as described at other loci subject to parental imprinting.</p></div

Sperm-inherited organelle clearance in C-elegans relies on LC3-dependent autophagosome targeting to the pericentrosomal area

International audienceMacroautophagic degradation of sperm-inherited organelles prevents paternal mitochondrial DNA transmission in C. elegans. The recruitment of autophagy markers around sperm mitochondria has also been observed in mouse and fly embryos but their role in degradation is debated. Both worm Atg8 ubiquitin-like proteins, LGG1/GABARAP and LGG-2/LC3, are recruited around sperm organelles after fertilization. Whereas LGG-1 depletion affects autophagosome function, stabilizes the substrates and is lethal, we demonstrate that LGG-2 is dispensable for autophagosome formation but participates in their microtubule-dependent transport toward the pericentrosomal area prior to acidification. In the absence of LGG-2, autophagosomes and their substrates remain clustered at the cell cortex, away from the centrosomes and their associated lysosomes. Thus, the clearance of sperm organelles is delayed and their segregation between blastomeres prevented. This allowed us to reveal a role of the RAB5/RAB-7 GTPases in autophagosome formation. In conclusion, the major contribution of LGG-2 in sperm-inherited organelle clearance resides in its capacity to mediate the retrograde transport of autophagosomes rather than their fusion with acidic compartments: a potential key function of LC3 in controlling the fate of sperm mitochondria in other species

Spontaneous Reactivation of Clusters of X-Linked Genes Is Associated with the Plasticity of X-Inactivation in Mouse Trophoblast Stem Cells

International audienceRandom epigenetic silencing of the X-chromosome in somatic tissues of female mammals equalizes the dosage of X-linked genes between the sexes. Unlike this form of X-inactivation that is essentially irreversible, the imprinted inactivation of the paternal X, which characterizes mouse extra-embryonic tissues, appears highly unstable in the trophoblast giant cells of the placenta. Here, we wished to determine whether such instability is already present in placental progenitor cells prior to differentiation toward lineage-specific cell types. To this end, we analyzed the behavior of a GFP transgene on the paternal X both in vivo and in trophoblast stem (TS) cells derived from the trophectoderm of XX(GFP) blastocysts. Using single-cell studies, we show that not only the GFP transgene but also a large number of endogenous genes on the paternal X are subject to orchestrated cycles of reactivation/de novo inactivation in placental progenitor cells. This reversal of silencing is associated with local losses of histone H3 lysine 27 trimethylation extending over several adjacent genes and with the topological relocation of the hypomethylated loci outside of the nuclear compartment of the inactive X. The "reactivated" state is maintained through several cell divisions. Our study suggests that this type of "metastable epigenetic" states may underlie the plasticity of TS cells and predispose specific genes to relaxed regulation in specific subtypes of placental cells