Abstract
Germ granules, specialized ribonucleoprotein particles, are a hallmark of all germ cells. In Drosophila, an estimated 200 mRNAs are enriched in the germ plasm, and some of these have important, often conserved roles in germ cell formation, specification, survival and migration. How mRNAs are spatially distributed within a germ granule and whether their position defines functional properties is unclear. Here we show, using single-molecule FISH and structured illumination microscopy, a super-resolution approach, that mRNAs are spatially organized within the granule whereas core germ plasm proteins are distributed evenly throughout the granule. Multiple copies of single mRNAs organize into ‘homotypic clusters' that occupy defined positions within the center or periphery of the granule. This organization, which is maintained during embryogenesis and independent of the translational or degradation activity of mRNAs, reveals new regulatory mechanisms for germ plasm mRNAs that may be applicable to other mRNA granules.
Highlights
Germ granules, specialized ribonucleoprotein particles, are a hallmark of all germ cells
Given their size of up to 500 nm and round shape (Fig. 1c, Supplementary Fig. 2h–m, Supplementary Table 1) we concluded that the particles we detected by light microscopy corresponded to the electron-dense granules previously observed by electron microscopy (EM)[9,10]
The Pearson correlation coefficient (PCC) analysis demonstrated that these proteins highly co-localized with each other (PCC40.86; Fig. 1d), which was expected given that Vasa and Osk physically interact in the germ plasm granule[11] while Aub physically interacts with both Vasa and Tud[12]
Summary
Germ granules, specialized ribonucleoprotein particles, are a hallmark of all germ cells. In Drosophila, an estimated 200 mRNAs are enriched in the germ plasm, and some of these have important, often conserved roles in germ cell formation, specification, survival and migration. Genetic analysis in Drosophila melanogaster identified highly conserved proteins that are common to all germ granules and critical for their assembly in vivo[3] How these specific factors interact with each other and other protein and mRNA components of the germ granules and how they exert their function on germ cell biology is subject of intense study. Osk becomes translationally competent, produces Oskar protein, which later recruits Vasa protein along with other germ plasm proteins as well an estimated 200 maternally provided mRNAs, such as cyclinB (cycB), nanos (nos), polar granule component (pgc) and germ cell less (gcl)[4]. It is believed that other maternally provided mRNAs enriched at the posterior are localized by this process
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