Early programming of the oocyte epigenome temporally controls late prophase I transcription and chromatin remodelling

Results

Drosophila oocytes reactivate transcription during meiosis

Consistent with previous reports11, we observed using a ethynyl uridine (EU) incorporation assay that the Drosophila oocyte is transcriptionally inactive throughout most of the prophase I arrest (from oogenesis stage 5 until the end of stage 8; Fig. 1A, Supplementary Fig. 1). This transcriptional quiescence starts from the onset of the prophase I arrest, lasts for ∼25 h, and is associated with the reorganization of oocyte chromatin into a highly compact cluster of meiotic chromosomes referred to as the karyosome. Despite the prolonged transcriptional inactivity, we observed that Drosophila oocytes reactivate gene expression ∼13 h before meiotic resumption (at oogenesis stage 9). This precisely-timed oocyte transcriptional reactivation is intriguing, as the polyploid nurse cells ensure essentially all transcriptional activity in the Drosophila female germ line. Such observation raises the possibility that successful meiotic progression requires oocyte-specific transcription during the prophase I arrest.

Figure 1: Drosophila oocytes have a unique, dynamic and diversified epigenome.

(A) Schematic of Drosophila oogenesis. The functional unit of the Drosophila ovary is the ovarian follicle or egg chamber, which consists of the cyst defined by the oocyte and its supporting nurse cells surrounded by a monolayer of somatic cells (follicle cells). The morphological features of the ovarian follicles define 14 distinct developmental stages (S). The oocyte progresses through the initial phases of prophase I until it arrests at diplotene at oogenesis stage 5. The onset of this arrest is preceded by the clustering of oocyte chromosomes into a highly compact and transcriptionally quiescent structure—the karyosome. As oogenesis progresses the oocyte grows in size mainly through cytoplasmic transfer from the nurse cells. At stage 9 the oocyte reactivates transcription, which lasts until the start of stage 11. The prophase I arrest is lifted at stage 13 and results in the formation of a metaphase I-arrested gamete. (B) Distinct patterns of histone post-translation modifications (PTMs) in the Drosophila ovarian follicle. Three main patterns were identified: (i) oocyte-specific expression (histone H2B lysine 16 acetylation—H2BK16ac; a–a′′), (ii) oocyte+somatic (follicle) cell expression (histone H4 lysine 12 acetylation—H4K12ac; b–b′′), and generalized distribution (oocyte (albeit sometimes at lower levels)+nurse cells+somatic cells; histone H3 lysine 4 trimethylation—H3K4me3; c–c′′). Arrowheads point to the chromatin of different cell types, insets depict oocyte chromatin. Development time in relation to the start of oogenesis is expressed in hours post-germ line stem cell division (h.p.d.). Scale bars, 10 μm for ovarian follicles and 1 μm for oocyte insets. (C) Heatmap representing the expression of 21 different histone PTMs across the three different cell types of the Drosophila ovarian follicle. (D). Temporal analysis of the highly dynamic levels of oocyte H2BK16ac, histone H3 lysine 36 dimethylation (H3K36me2) and histone H4 lysine 5 acetylation (H4K5ac) throughout oogenesis. Relative levels are expressed in fluorescence arbitrary units (a.u.). Error bars represent s.d. See Supplementary Fig. 2 for illustrative micrographs and complete temporal analysis of the tested PTMs.