Polycomb group (PcG) proteins prevent the assembly of abnormal synaptonemal complex structures during meiosis

The synaptonemal complex (SC) is a proteinaceous scaffold that is assembled between paired homologous chromosomes during the onset of meiosis. Timely expression of SC coding genes is essential for SC assembly and successful meiosis. However, SC components have an intrinsic tendency to self-organize into abnormal repetitive structures, which are not assembled between the paired homologs and whose formation is potentially deleterious for meiosis and gametogenesis. This creates an interesting conundrum, where SC genes need to be robustly expressed during meiosis, but their expression must be carefully regulated to prevent the formation of anomalous SC structures. In this manuscript, we show that the Polycomb group protein Sfmbt, the Drosophila ortholog of human MBTD1 and L3MBTL2, is required to avoid excessive expression of SC genes during prophase I. Although SC assembly is normal after Sfmbt depletion, SC disassembly is abnormal with the formation of multiple synaptonemal complexes (polycomplexes) within the oocyte. Overexpression of the SC gene corona and depletion of other Polycomb group proteins are similarly associated with polycomplex formation during SC disassembly. These polycomplexes are highly dynamic and have a well-defined periodic structure. Further confirming the importance of Sfmbt, germ line depletion of this protein is associated with significant metaphase I defects and a reduction in female fertility. Since transcription of SC genes mostly occurs during early prophase I, our results suggest a role of Sfmbt and other Polycomb group proteins in downregulating the expression of these and other early prophase I genes during later stages of meiosis.

Differentiating Drosophila female germ cells initiate Polycomb silencing by regulating PRC2-interacting proteins

Polycomb silencing represses gene expression and provides a molecular memory of chromatin state that is essential for animal development. We show that Drosophila female germline stem cells (GSCs) provide a powerful system for studying Polycomb silencing. GSCs have a non-canonical distribution of PRC2 activity and lack silenced chromatin like embryonic progenitors. As GSC daughters differentiate into nurse cells and oocytes, nurse cells, like embryonic somatic cells, silence genes in traditional Polycomb domains and in generally inactive chromatin. Developmentally controlled expression of two Polycomb repressive complex 2 (PRC2)-interacting proteins, Pcl and Scm, initiate silencing during differentiation. In GSCs, abundant Pcl inhibits PRC2-dependent silencing globally, while in nurse cells Pcl declines and newly induced Scm concentrates PRC2 activity on traditional Polycomb domains. Our results suggest that PRC2-dependent silencing is developmentally regulated by accessory proteins that either increase the concentration of PRC2 at target sites or inhibit the rate that PRC2 samples chromatin.

The Mitochondrial DNA Polymerase Promotes Elimination of Paternal Mitochondrial Genomes

Mitochondrial DNA (mtDNA) is typically inherited from only one parent [1-3]. In animals, this is usually the mother. Maternal inheritance is often presented as the passive outcome of the difference in cytoplasmic content of egg and sperm; however, active programs enforce uniparental inheritance at two levels, eliminating paternal mitochondrial genomes or destroying mitochondria delivered to the zygote by the sperm [4-13]. Both levels operate in Drosophila [8, 12, 13]. As sperm formation begins, hundreds of doomed mitochondrial genomes are visualized within the two huge mitochondria of each spermatid. These genomes abruptly disappear during spermatogenesis. Genome elimination, which is not in the interests of the restricted genomes, is directed by nuclear genes. Mutation of EndoG, which encodes a mitochondria-targeted endonuclease, retarded elimination [8]. Here, we show that knockdown of the nuclear-encoded mtDNA polymerase (Pol γ-α), Tamas, produces a more complete block of mtDNA elimination. Tamas is found in large particles that localize to mtDNA during genome elimination. We discount a simple possible mechanism by showing that the 3'-exonuclease function of the polymerase is not needed. While DNA elimination is a surprising function for DNA polymerase, it could provide a robust nexus for nuclear control of mitochondrial genome copy number, since use of common interactions for elimination and replication might limit options for the mitochondrial genome to escape restriction. We suggest that the DNA polymerase may play this role more widely and that inappropriate activation of its elimination ability might underlie association of DNA loss syndromes with mutations of the human mtDNA polymerase [14-16].

A genetic toolkit for tagging intronic MiMIC containing genes

Previously, we described a large collection of Minos-Mediated Integration Cassettes (MiMICs) that contain two phiC31 recombinase target sites and allow the generation of a new exon that encodes a protein tag when the MiMIC is inserted in a codon intron (Nagarkar-Jaiswal et al., 2015). These modified genes permit numerous applications including assessment of protein expression pattern, identification of protein interaction partners by immunoprecipitation followed by mass spec, and reversible removal of the tagged protein in any tissue. At present, these conversions remain time and labor-intensive as they require embryos to be injected with plasmid DNA containing the exon tag. In this study, we describe a simple and reliable genetic strategy to tag genes/proteins that contain MiMIC insertions using an integrated exon encoding GFP flanked by FRT sequences. We document the efficiency and tag 60 mostly uncharacterized genes.

Barriers to male transmission of mitochondrial DNA in sperm development

Across the eukaryotic phylogeny, offspring usually inherit their mitochondrial genome from only one of two parents: in animals, the female. Although mechanisms that eliminate paternally derived mitochondria from the zygote have been sought, the developmental stage at which paternal transmission of mitochondrial DNA is restricted is unknown in most animals. Here, we show that the mitochondria of mature Drosophila sperm lack DNA, and we uncover two processes that eliminate mitochondrial DNA during spermatogenesis. Visualization of mitochondrial DNA nucleoids revealed their abrupt disappearance from developing spermatids in a process requiring the mitochondrial nuclease, Endonuclease G. In Endonuclease G mutants, persisting nucleoids are swept out of spermatids by a cellular remodeling process that trims and shapes spermatid tails. Our results show that mitochondrial DNA is eliminated during spermatogenesis, thereby removing the capacity of sperm to transmit the mitochondrial genome to the next generation.

Manipulating the metazoan mitochondrial genome with targeted restriction enzymes

High copy number and random segregation confound genetic analysis of the mitochondrial genome. We developed an efficient selection for heritable mitochondrial genome (mtDNA) mutations in Drosophila, thereby enhancing a metazoan model for study of mitochondrial genetics and mutations causing human mitochondrial disease. Targeting a restriction enzyme to mitochondria in the germline compromised fertility, but escaper progeny carried homoplasmic mtDNA mutations lacking the cleavage site. Among mutations eliminating a site in the cytochrome c oxidase gene, mt:CoI(A302T) was healthy, mt:CoI(R301L) was male sterile but otherwise healthy, and mt:CoI(R301S) exhibited a wide range of defects, including growth retardation, neurodegeneration, muscular atrophy, male sterility, and reduced life span. Thus, germline expression of mitochondrial restriction enzymes creates a powerful selection and has allowed direct isolation of mitochondrial mutants in a metazoan.