Cytotype Regulation of P Transposable Elements in Drosophila melanogaster: Repressor Polypeptides or piRNAs?

Abstract

The telomeric P elements TP5 and TP6 are associated with the P cytotype, a maternally inherited condition that represses P-element-induced hybrid dysgenesis in the Drosophila germ line. To see if cytotype repression by TP5 and TP6 might be mediated by the polypeptides they could encode, hobo transgenes carrying these elements were tested for expression of mRNA in the female germ line and for repression of hybrid dysgenesis. The TP5 and TP6 transgenes expressed more germ-line mRNA than the native telomeric P elements, but they were decidedly inferior to the native elements in their ability to repress hybrid dysgenesis. These paradoxical results are inconsistent with the repressor polypeptide model of cytotype. An alternative model based on the destruction of P transposase mRNA by Piwi-interacting (pi) RNAs was supported by finding reduced P mRNA levels in flies that carried the native telomeric P elements, which are inserted in a known major piRNA locus.T RANSPOSABLE elements are significant components of the genomes of many organisms. These elements can cause gene mutations and chromosome breakage, and over evolutionary time, they can alter the composition and structure of genomes. There is, therefore, considerable interest in elucidating the mechanisms that foster or repress their activity. For example, many researchers have studied the regulation of P transposable elements in Drosophila melanogaster-a model family of elements in a model genetic organism (Engels 1989;Rio 1990).P elements are cut-and-paste transposons whose activity is catalyzed by an 87-kDa polypeptide, the P transposase, which is encoded by complete members of the P-element family (Karess and Rubin 1984;Rio et al. 1986). This polypeptide is restricted to the germ line because the last of the element's three introns is removed from P RNA only in that tissue . In somatic cells, retention of this intron results in the production of a 66-kDa polypeptide instead of the transposase. Tissue-specific splicing is therefore an important mechanism for controlling P-element activity. However even within the germ line, where the P transposase is made, P-element activity is regulated.Genetic evidence for this regulation was obtained from early studies that defined a maternally transmitted state called cytotype (Engels 1979;Kidwell 1981). The M cytotype permits P-element activity whereas the P cytotype represses it. Thus, when P elements are combined with the M cytotype by crossing P-bearing males to M-cytotype females, the P elements are mobilized in the offspring, where they cause a syndrome of germ-line abnormalities called hybrid dysgenesis (Kidwell et al. 1977;Engels 1989). This syndrome includes gonadal dysgenesis (GD) in both of the sexes, chromosome breakage, and elevated mutation rates. By contrast, crosses between P-bearing males and P-cytotype females produce offspring that seldom show dysgenic traits. These early studies also demonstrated that the P cytotype depends on the presence of P elements themselves (Engels 1979;Sved 1987); thus, t...

Reexamining the P-Element Invasion of Drosophila melanogaster Through the Lens of piRNA Silencing

“…Transgenes encoding both type I (Robertson and Engels 1989;Misra and Rio 1990;Rasmusson et al 1993) and type II repressors (Gloor et al 1993;Simmons et al 2002;Jensen et al 2008) reduce the excision rate of P elements in vivo. Although the precise mechanism of repressor protein action remains unclear, they are posited to act as competitive inhibitors of P transcription.…”

“…However, recent research has revealed that the piRNA pathway is the molecular basis of cytotype regulation (Reiss et al 2004;Brennecke et al 2008;Jensen et al 2008;Belinco et al 2009;Khurana et al 2011). Indeed, P-cytotype genetics and piRNA-mediated silencing are increasingly synergistic, with classic observations motivating modern molecular experiments and new mechanisms shedding light on old discoveries.…”

“…In new the piRNA model, P-cytotype regulation is transmitted and enforced by P element-derived piRNAs, which are maternally deposited into oocytes ( Figure 4B) (Brennecke et al 2008;Jensen et al 2008). Maternally deposited piRNAs from P-cytotype females are proposed both to target the transcriptional and post-transcriptional silencing of P elements in offspring germlines, and feed forward the production of additional P-derived piRNAs through the ping-pong cycle (Figure 4B).…”

“…However, P cytotypeconferring elements were observed to be either full length (Ronsseray et al 1991(Ronsseray et al , 1996 or internally deleted (Nitasaka et al 1987;Marin et al 2000;Stuart et al 2002), suggesting that there is no particular mutation in the P element itself that determines repression. Indeed, for two internally-deleted elements, which at their endogenous locus confer the P cytotype, the transgenic introduction of the element to novel genomic sites did not confer repression (Jensen et al 2008). Thus, it is the genomic position of an insertion that determines its regulatory capacity, and not the polypeptide that it encodes.…”

“…Until recently, the molecular mechanism(s) that underlie P-element repression baffled Drosophila geneticists. Although early studies demonstrated that P elements are partially regulated by self-encoded repressor proteins, this autoregulation does not explain the very strong germline repression exhibited by many wildderived strains (Robertson and Engels 1989;Misra and Rio 1990;Gloor et al 1993;Misra et al 1993;Jensen et al 2008). The recent discovery of the Piwi-interacting RNA (piRNA) pathway, a conserved silencing mechanism that controls germline TE activity in both metazoans and ciliates (reviewed in Aravin et al 2007;Mani and Juliano 2013), provided the critical missing piece to the puzzle: the activity of P elements in D. melanogaster is also regulated by piRNAs (Brennecke et al 2008;Jensen et al 2008;Khurana et al 2011).…”