Epigenetic regulation of oogenesis and germ stem cell maintenance by the Drosophila histone methyltransferase Eggless/dSetDB1

Abstract

The Drosophila melanogaster histone lysine methyltransferase (HKMT) Eggless (Egg/dSETDB1) catalyzes methylation of Histone H3 lysine 9 (H3K9), a signature of repressive heterochromatin. Our previous studies showed that H3K9 methylation by Egg is required for oogenesis. Here we analyze a set of EMS-induced mutations in the egg gene, identify the molecular lesions of these mutations, and compare the effects on oogenesis of both strong loss-of-function and weak hypomorphic alleles. These studies show that H3K9 methylation by Egg is required for multiple stages of oogenesis. Mosaic expression experiments show that the egg gene is not required intrinsically in the germ cells for their early differentiation, but is required in the germ cells for their survival past stage 5 of oogenesis. egg is also required in germ stem cells for their maintenance, since egg− germ stem cells initially survive but are not maintained as females age. Mosaic analysis also reveals that the early egg chamber budding defects in egg− ovaries are due to an intrinsic requirement for egg in follicle stem cells and their descendents, and that egg plays a non-autonomous role in somatic cells in the germarium to influence the differentiation of early germ cells.

Essential roles of Windei and nuclear monoubiquitination of Eggless/ SETDB 1 in transposon silencing

“…The domain structures and predicted Egg post‐translational modification sites are summarized in Fig C. We first immunoisolated Egg from OSCs and probed with anti‐ubiquitin (Ub) antibodies.…”

“…In Drosophila, H3K9 methylation is governed by three histone methyltransferases (HMTs), Su(var)3-9, G9a, and Eggless/SETDB1 (Egg) [4][5][6][7][8][9][10]. Of those, Egg is the only essential H3K9 methyltransferase in Drosophila and is required for H3K9 trimethylation in the ovaries [4,8,11].…”

Transcription factors, chromatin proteins and the diversification of Hemiptera

“…Mutations in eggless (including a complete deletion of the SET domain) result in meiosis arrest and defective oogenesis. In addition, these mutations were also linked to wing and locomotion defects in D. melanogaster (Clough et al, 2007; Clough et al, 2014). Given the role of TAM/MDB domains in binding sequences with the CpG dinucleotide with cytosine methylation, the expansion of this family of SET domains in A. pisum might correlate with the evolution of a more prominent role for DNA methylation in controlling gene expression and development in the pea aphid.…”

A nuclear licence to silence transposons

“…SETDB1 catalyses histone 3 lysine 9 trimethylation (H3K9me3), a repressive histone mark, which leads to heterochromatin formation and silencing of endogenous retroviruses (ERVs) and genes and is thus essential for early development . Eggless (Egg) is the fly homolog of SETDB1 and is indispensable for H3K9me3 deposition in the ovaries and PIWI‐interacting RNA (piRNA)‐mediated transposon silencing . Previously, SETDB1/Egg has been proposed to be regulated by post‐translational modifications and proteasomal degradation and pivotal recent work has highlighted that the catalytic activity of SETDB1 depends on its licencing by ubiquitination .…”