The tumour suppressor brain tumour (Brat) regulates linker histone dBigH1 expression in the Drosophila female germline and the early embryo

Maternal histone mRNAs are uniquely processed through polyadenylation in a Stem-Loop Binding Protein (SLBP) dependent manner

During early embryogenesis the zygotic genome remains transcriptionally silent and expression relies on maternally deposited products. Maternal deposition of histones is crucial to preserve chromatin integrity during early embryo development, when the number of nuclei exponentially increases in the absence of zygotic expression. In the Drosophila embryo, histones are maternally deposited as both proteins and mRNAs. Histone transcripts are the only nonpolyadenylated cellular mRNAs. They contain a highly conserved 3'UTR stem-loop structure, which is recognized by the Stem-Loop Binding Protein (SLBP) that, in conjunction with U7 snRNP, regulates their unique 3'-end processing. Here we report that, unexpectedly, maternal histone mRNAs are polyadenylated and have a truncated 3' stem-loop. This noncanonical 3'-end processing of maternal histone mRNAs occurs at their synthesis during oogenesis and requires SLBP, but not U7 snRNP. We show that maternal histone transcripts are subjected to cytoplasmic poly(A) tail elongation by Wisp, which results in their stabilization and is a requisite for translation. We also show that maternal histone transcripts remain largely quiescent and that their translation is activated upon loss of the embryonic linker histone dBigH1, which impairs chromatin assembly and induces DNA damage. Here, we discuss possible models to integrate these observations.

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

A rapidly evolving single copy histone H1 variant is associated with male fertility in a parasitoid wasp

The linker histone H1 binds to the nucleosome core particle at the site where DNA enters and exits, and facilitates folding of the nucleosomes into a higher-order chromatin structure in eukaryotes. Additionally, some variant H1s promote specialized chromatin functions in cellular processes. Germline-specific H1 variants have been reported in some model species with diverse roles in chromatin structure changes during gametogenesis. In insects, the current understanding of germline-specific H1 variants comes mainly from the studies in Drosophila melanogaster, and the information on this set of genes in other non-model insects remains largely unknown. Here, we identify two H1 variants (PpH1V1 and PpH1V2) that are specifically predominantly expressed in the testis of the parasitoid wasp Pteromalus puparum. Evolutionary analyses suggest that these H1 variant genes evolve rapidly, and are generally maintained as a single copy in Hymenoptera. Disruption of PpH1V1 function in the late larval stage male by RNA interference experiments has no phenotype on spermatogenesis in the pupal testis, but results in abnormal chromatin structure and low sperm fertility in the adult seminal vesicle. In addition, PpH1V2 knockdown has no detectable effect on spermatogenesis or male fertility. Collectively, our discovery indicates distinct functions of male germline-enriched H1 variants between parasitoid wasp Pteromalus and Drosophila, providing new insights into the role of insect H1 variants in gametogenesis. This study also highlights the functional complexity of germline-specific H1s in animals.

Chromatin Organization and Function in Drosophila

Eukaryotic genomes are packaged into high-order chromatin structures organized in discrete territories inside the cell nucleus, which is surrounded by the nuclear envelope acting as a barrier. This chromatin organization is complex and dynamic and, thus, determining the spatial and temporal distribution and folding of chromosomes within the nucleus is critical for understanding the role of chromatin topology in genome function. Primarily focusing on the regulation of gene expression, we review here how the genome of Drosophila melanogaster is organized into the cell nucleus, from small scale histone–DNA interactions to chromosome and lamina interactions in the nuclear space.