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Review
. 2019 Sep;20(9):650-660.
doi: 10.1111/tra.12674. Epub 2019 Jul 31.

Germ granules in Drosophila

Tatjana Trcek  1 , Ruth Lehmann  1
Affiliations
Review

Germ granules in Drosophila

Tatjana Trcek et al. Traffic. 2019 Sep.

Abstract

Germ granules are hallmarks of all germ cells. Early ultrastructural studies in Drosophila first described these membraneless granules in the oocyte and early embryo as filled with amorphous to fibrillar material mixed with RNA. Genetic studies identified key protein components and specific mRNAs that regulate germ cell-specific functions. More recently these ultrastructural studies have been complemented by biophysical analysis describing germ granules as phase-transitioned condensates. In this review, we provide an overview that connects the composition of germ granules with their function in controlling germ cell specification, formation and migration, and illuminate these mysterious condensates as the gatekeepers of the next generation.

Keywords: Oskar; RNA granules; RNA localization; germ granules; localized translation; mRNA clusters; phase separation; vasa, nanos.

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Conflict of interest statement

The authors declare that there is no conflict of interest.

Figures

Figure 1
Figure 1
Formation of germ granules in Drosophila embryo (figure adapted from Reference 4. A, Germ granules form in the specialized cytoplasm called germ plasm at the embryo's posterior pole. B, An EM image showing that germ granules (labeled by immunogold particles staining Vasa protein; marked with green “GG”) are more electron dense than surrounding germ plasm and are closely associated with mitochondria (marked with orange “M”). C, Germ granules accumulate nos (green) and pgc (magenta) homotypic mRNA clusters,5, 6, 7 that are often colocalized within the same granule but that do not mix with each other.7 White arrows point at granules that are populated by only nos or pgc demonstrating that germ granules are heterogeneous in mRNA composition. D, oskar mRNA translates into Long and Short Osk isoforms that regulate distinct aspects of germ plasm and germ granules formation. R‐me indicates a methylated arginine. Scale bar in B is 500 nm and in C it is 1000 nm
Figure 2
Figure 2
Spatial organization of germ granules and of granule‐associated posttranscriptional regulation through early embryo development. (A‐C) Initially, germ granules are uniformly distributed within germ plasm (0‐1.5 hours old fertilized eggs). Once the pole buds form at the posterior pole, germ granules then transport via dynein motors coupled to astral microtubules toward the centrosomes associated with pole bud nuclei.41 As such they become organized into crescents surrounding dividing nuclei in pole buds and pole cells (1.5‐3 hours old fertilized eggs) (A).41, 42 During this time, nuclear germ granules form that are also often hollow (B, C).38 Image in B shows a nucleus of a pole cell stained with DAPI (blue) that accumulates hollow nuclear germ granules stained with Vasa:GFP protein (green). EM image in B shows nuclear germ granules formed by Short Osk in Drosophila‐cultured cells lines. (D, E) nos, pgc and CycB mRNAs persistently localize in germ granules throughout early embryogenesis (D) but they nevertheless display distinct onsets of translation (E) to allow the body patterning of the early embryo (Nanos), cellularization of pole buds into primordial germ cells (Gcl) and transcriptional silencing of newly formed primordial germ cells (Pgc)42, 43, 44, 45 (figure adapted from Reference 42). Scale bar in B is 5 μm (fluorescent image) and 1 μm (EM image)
Figure 3
Figure 3
Mechanisms of mRNA enrichment to germ granules. mRNAs such as nos (pink) are transcribed in nurse cells during oogenesis and afterward dumped into a transcriptionally silent oocyte. Cytoplasmic streaming swirls these mRNA through the oocyte, which brush along the germ granules (green) formed at the posterior pole. mRNAs enrich in germ granules passively in a diffusion/entrapment‐dependent mechanism. Figure adapted from Reference 36
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References

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