Structural and functional organization of germ plasm condensates

Abstract

Reproductive success of metazoans relies on germ cells. These cells develop early during embryogenesis, divide and undergo meiosis in the adult to make sperm and oocytes. Unlike somatic cells, germ cells are immortal and transfer their genetic material to new generations. They are also totipotent, as they differentiate into different somatic cell types. The maintenance of immortality and totipotency of germ cells depends on extensive post-transcriptional and post-translational regulation coupled with epigenetic remodeling, processes that begin with the onset of embryogenesis [1, 2]. At the heart of this regulation lie germ granules, membraneless ribonucleoprotein condensates that are specific to the germline cytoplasm called the germ plasm. They are a hallmark of all germ cells and contain several proteins and RNAs that are conserved across species. Interestingly, germ granules are often structured and tend to change through development. In this review, we describe how the structure of germ granules becomes established and discuss possible functional outcomes these structures have during development.

Evolutionary changes in germ granule mRNA content are driven by multiple mechanisms inDrosophila

“…In D. mel , a sub-granule organization for homotypic clusters has been described in that larger clusters tend to be located centrally within the granule. Specifically, nos clusters tend to be located more centrally than pgc clusters in D. mel , given the average size differential between the two cluster types [6, 18]. Although no function has yet been identified for this sub-granule organization [18], our results where a species’ nos and pgc clusters tend to be of similar size (Supp Fig.…”

“…The mRNA from an estimated 61 genes localize to the germ plasm in D. mel [11], including four that have been evaluated for homotypic clustering, nos , pgc, gcl and cycB (Fig. 1) [6]. Here, we focused on only two, nos and pgc , since clustering factor have been previously described for both mRNAs types [20, 22].…”

“…Throughout the animal kingdom, germline function and maintenance require the formation of ribonucleoprotein (RNP) granules called germ granules [1][2][3][4][5][6][7]. Specifically, germ granules are biomolecular condensates that function in the post-transcriptional regulation of mRNAs that have critical roles in germline differentiation, proliferation, and function [1][2][3][4][5][6][7][8][9][10].…”

“…Throughout the animal kingdom, germline function and maintenance require the formation of ribonucleoprotein (RNP) granules called germ granules [1][2][3][4][5][6][7]. Specifically, germ granules are biomolecular condensates that function in the post-transcriptional regulation of mRNAs that have critical roles in germline differentiation, proliferation, and function [1][2][3][4][5][6][7][8][9][10]. In Drosophila, germ granules develop within a highly specialized cytoplasm called the germ plasm that develops at the posterior oocyte and persists in the early embryo for ~1-2 hours post fertilization [11,12].…”

“…Recent studies have employed a combination of techniques such as single molecule fluorescent in situ hybridization (smFISH), super-resolution microscopy, quantitative image analysis, and computational modeling to investigate germ granule development by quantifying the mRNA content of germ granules in Drosophila melanogaster ( D. mel ) [5, 6, 17, 19, 20, 22, 23]. These interdisciplinary methods have enabled the quantification of the number of transcripts found within a homotypic cluster (referred to as cluster size), the characterization of homotypic cluster size distributions for thousands of clusters found throughout a germ plasm (referred to as germ plasm landscape), the measuring of the frequency at which different types of homotypic clusters populate the same granule (referred to as co-localization), and the description of the relationship between the sizes of different homotypic clusters that are detected within the same granule [17, 19, 20].…”