The Deadbeat Paternal Effect of Uncapped Sperm Telomeres on Cell Cycle Progression and Chromosome Behavior in Drosophila melanogaster

Abstract

Telomere-capping complexes (TCCs) protect the ends of linear chromosomes from illegitimate repair and end-to-end fusions and are required for genome stability. The identity and assembly of TCC components have been extensively studied, but whether TCCs require active maintenance in nondividing cells remains an open question. Here we show that Drosophila melanogaster requires Deadbeat (Ddbt), a sperm nuclear basic protein (SNBP) that is recruited to the telomere by the TCC and is required for TCC maintenance during genome-wide chromatin remodeling, which transforms spermatids to mature sperm. Ddbt-deficient males produce sperm lacking TCCs. Their offspring delay the initiation of anaphase as early as cycle 1 but progress through the first two cycles. Persistence of uncapped paternal chromosomes induces arrest at or around cycle 3. This early arrest can be rescued by selective elimination of paternal chromosomes and production of gynogenetic haploid or haploid mosaics. Progression past cycle 3 can also occur if embryos have reduced levels of the maternally provided checkpoint kinase Chk2. The findings provide insights into how telomere integrity affects the regulation of the earliest embryonic cell cycles. They also suggest that other SNBPs, including those in humans, may have analogous roles and manifest as paternal effects on embryo quality.KEYWORDS telomere maintenance; telomere capping; cell cycle checkpoint; paternal effect; sperm nuclear basic protein T ELOMERES are the natural ends of linear chromosomes and have distinct properties from broken ends. Multiple proteins are enriched predominantly, if not exclusively, at telomeres and form a capping complex that protects telomeric DNA from engaging in aberrant DNA repair activities. Protein components of the telomere-capping complex (TCC) vary among organisms, in part because species differ in telomeric sequences and whether telomeres are maintained by telomerase or alternative mechanisms (Raffa et al. 2011;Mason et al. 2015). Nonetheless, the TCC's essential functions are well conserved. Failure to assemble TCCs results in telomeric DNA degradation, telomere fusions, and genomic instability.Mutations in at least 12 loci of Drosophila melanogaster lead to telomere fusions in neuroblasts. Their analysis has led to the identification of telomere-enriched and telomereexclusive proteins required for telomere elongation or TCC assembly, maintenance, or function (Cenci et al. 2005;Pimpinelli 2006). Absence of any one component results in telomere fusions but components have distinct activities (Pimpinelli 2006). For example, heterochromatin protein 1a (HP1a) binds modified histone H3-MeK9 and represses transcription of telomeric retrotransposons and telomere elongation. HP1a also binds DNA and this activity is required for its capping function. The TCC protein HOAP binds DNA and HP1a. Although HOAP is required for capping, it does not affect retrotransposon transcription or telomere elongation. A third protein, HipHop, binds both HP1a and HOAP. HOAP and HipHo...

Versager is expressed at the histone-to-protamine transition during spermiogenesis and is required for embryonic chromosome transmission in Drosophila melanogaster

“…We noted a phenotype consistent with a mitotic arrest in a few percent of the embryos at 0-30 min AED, and this phenotype increased to 25% in embryos observed at 1-1.5hr AED. This phenotype appears similar to a "mitotic catastrophe" observed in embryos of ddbt fathers, in which telomere-fusion induced DNA damage triggers a Chk2-mediated cell cycle arrest [13]. However, a more detailed analysis of embryos of vrs fathers is required to better understand the nature of the arrest.…”

“…A number of other genes have been identified that are specifically required for paternal chromosome behavior in Drosophila embryos: paternal loss (pal) [16], loser (lsr) [1], deadbeat(ddbt) [13], ms(3)K81 [29] and Horka (a dominant allele of lodestar) [15]. Of these, only ms(3)K81, ddbt and Horka have been characterized at the molecular level.…”

“…The common thread between these genes is that they all affect some aspect of sperm chromatin formation that impacts chromosome behavior in the embryo. The ms(3)K81 and deadbeat proteins are involved in the normal maintenance of telomeric function [11][12][13]. Horka is a member of the Snf2 family of helicase-related genes and it remains unclear how the dominant mutation affects normal structure of paternal chromosomes [14].…”

“…Horka is a member of the Snf2 family of helicase-related genes and it remains unclear how the dominant mutation affects normal structure of paternal chromosomes [14]. In embryos from ms(3)K81 and deadbeat, the entire set of paternal chromosomes are affected as a result of telomere fusions and/or associations [11][12][13]. Horka, on the other hand, shows a chromosome-specificity in which all chromosomes are affected except the Y chromosome [15].…”

“…In ms(3)K81 and deadbeat(dbt) mutants, defects in assembly of the sperm telomere capping complex lead to post-fertilization fusion and loss of paternal chromosomes [11][12][13]. A dominant mutation in the lodestar helicase, Horka, [14] causes chromosome instability and subsequent chromosome loss in the embryo, leading to the formation of diplo// haplo mosaics [15].…”