Biology subjects

Nhim, S.

Publications and source records attributed to Nhim, S..

4 recordsLinked to original sources

bioRxiv2024

Heterochromatin dynamics during the initial stages of sexual development in Plasmodium falciparum

Asexual replication of Plasmodium falciparum in the human blood results in exponential parasite growth and causes all clinical symptoms of malaria. However, at each round of the replicative cycle, some parasites convert into sexual precursors called gametocytes, which are essential for transmission to mosquito vectors. After sexual conversion, parasites develop through the sexual ring stage and then gametocyte stages I to V before they are infective to mosquitoes. Heterochromatin, a type of chromatin generally refractory to gene expression, plays an important role in the regulation of sexual conversion by silencing the master regulator of the process, PfAP2-G, in asexual parasites. Additionally, previous reports have described changes in the genome-wide distribution of heterochromatin in stage II/III or older gametocytes, including expansion of heterochromatin at several subtelomeric regions and reduced occupancy at a few specific loci. However, it is not known if these changes occur concomitantly with sexual conversion or at a later time during gametocyte development. Using a transgenic line in which sexual conversion can be conditionally induced, here we show that the genome-wide distribution of heterochromatin at the initial stages of sexual development (i.e., sexual rings and stage I gametocytes) is almost identical to parasites at asexual blood stages, and major changes do not occur until stage II/III. We also show that transcriptional changes associated with sexual development typically precede, rather than follow, changes in heterochromatin occupancy at their loci, which raises the possibility that PfAP2-G operates as a pioneer factor. IMPORTANCEEpigenetic processes and chromatin structure play an important role in the regulation of gene expression in malaria parasites. In particular, a type of chromatin called heterochromatin is involved in the regulation (silencing) of many genes. Parasite sexual development is essential for transmission to mosquito vectors. Here we characterised the global distribution of heterochromatin at different stages of sexual development, and found that initially it is identical to asexual blood stages, but at later transmission stages it is altered. This informs about the putative roles of general heterochromatin redistribution in parasite life cycle progression. By integrating multi-omic datasets, we also found that changes in the expression of several genes precede changes in their heterochromatin occupancy. This indicates that during sexual development some genes can be activated in spite of having heterochromatin.

microbiology↗

bioRxiv2019

Macrophage plasticity is Rac signalling and MMP9 dependant

In vitro, depending on extracellular matrix (ECM) architecture, macrophages migrate either in amoeboid or mesenchymal mode; while the first is a general trait of leukocytes, the latter is associated with tissue remodelling via Matrix Metalloproteinases (MMPs). To assess whether these stereotyped migrations could be also observed in a physiological context, we used the zebrafish embryo and monitored macrophage morphology, behaviour and capacity to mobilisation haematopoietic stem/progenitor cells (HSPCs), as a final functional readout. Morphometric analysis identified 4 different cell shapes. Live imaging revealed that macrophages successively adopt all four shapes as they migrate through ECM. Treatment with inhibitors of MMPs or Rac GTPase to abolish mesenchymal migration, suppresses both ECM degradation and HSPC mobilisation while differently affecting macrophage behaviour. This study depicts real time macrophage behaviour in a physiological context and reveals extreme reactivity of these cells constantly adapting and switching migratory shapes to achieve HSPCs proper mobilisation.

cell biology↗

bioRxiv2018

Genomic differentiation is initiated without physical linkage among targets of divergent selection in Fall armyworms

The process of speciation involves whole genome differentiation by overcoming gene flow between diverging populations. We have ample knowledge which evolutionary forces may cause genomic differentiation, and several speciation models have been proposed to explain the transition from genetic to genomic differentiation. However, it is still unclear what are critical conditions enabling genomic differentiation in nature. The Fall armyworm, Spodoptera frugiperda, is observed as two sympatric strains that have different host-plant ranges, suggesting the possibility of ecological divergent selection. In our previous study, we observed that these two strains show genetic differentiation across the whole genome with an unprecedentedly low extent, suggesting the possibility that whole genome sequences started to be differentiated between the strains. In this study, we analyzed whole genome sequences from these two strains from Mississippi to identify critical evolutionary factors for genomic differentiation. The genomic Fst is low (0.017) while 91.3% of 10kb windows have Fst greater than 0, suggesting genome-wide differentiation with a low extent. We identified nearly 400 outliers of genetic differentiation between strains, and found that physical linkage among these outliers is not a primary cause of genomic differentiation. Fst is not significantly correlated with gene density, a proxy for the strength of selection, suggesting that a genomic reduction in migration rate dominates the extent of local genetic differentiation. Our analyses reveal that divergent selection alone is sufficient to generate genomic differentiation, and any following diversifying factors may increase the level of genetic differentiation between diverging strains in the process of speciation.

evolutionary biology↗

bioRxiv2018

Transcriptional plasticity evolution in two strains of Spodoptera frugiperda (Lepidoptera: Noctuidae) feeding on alternative host-plants

AO_SCPLOWBSTRACTC_SCPLOWSpodoptera frugiperda, the fall armyworm (FAW), is an important agricultural pest in the Americas and an emerging pest in sub-Saharan Africa, India, East-Asia and Australia, causing damage to major crops such as corn, sorghum and soybean. While FAW larvae are considered polyphagous, differences in diet preference have been described between two genetic variants: the corn strain (sf-C) and the rice strain (sf-R). These two strains are sometimes considered as distinct species, raising the hypothesis that ost plant specialization might have driven their divergence. To test this hypothesis, we irst performed controlled reciprocal transplant (RT) experiments to address the impact of plant diet on several traits linked to the fitness of the sf-C and sf-R strains. The phenotypical data suggest that sf-C is specialized to corn. We then used RNA-Se to identify constitutive transcriptional differences between strains, regardless of diet, in laboratory as well as in natural populations. We found that variations in mitochon rial transcription levels are among the most substantial and consistent differences between the two strains. Since mitochondrial genotypes also vary between the strains, we believe the mitochondria may have a significant role in driving strain divergence.

evolutionary biology↗