Germ Granule Evolution Provides Mechanistic Insight into Drosophila Germline Development

The copackaging of mRNAs into biomolecular condensates called germ granules is a conserved strategy to posttranscriptionally regulate germline mRNAs. In Drosophila melanogaster, mRNAs accumulate in germ granules by forming homotypic clusters, aggregates containing multiple transcripts from the same gene. Nucleated by Oskar (Osk), homotypic clusters are generated through a stochastic seeding and self-recruitment process that requires the 3 & PRIME; untranslated region (UTR) of germ granule mRNAs. Interestingly, the 3 & PRIME; UTR belonging to germ granule mRNAs, such as nanos (nos), have considerable sequence variations among Drosophila species and we hypothesized that this diversity influences homotypic clustering. To test our hypothesis, we investigated the homotypic clustering of nos and polar granule component (pgc) in four Drosophila species and concluded that clustering is a conserved process used to enrich germ granule mRNAs. However, we discovered germ granule phenotypes that included significant changes in the abundance of transcripts present in species' homotypic clusters, which also reflected diversity in the number of coalesced primordial germ cells within their embryonic gonads. By integrating biological data with computational modeling, we found that multiple mechanisms underlie naturally occurring germ granule diversity, including changes in nos, pgc, osk levels and/or homotypic clustering efficacy. Furthermore, we demonstrated how the nos 3 & PRIME; UTR from different species influences nos clustering, causing granules to have & SIM;70% less nos and increasing the presence of defective primordial germ cells. Our results highlight the impact that evolution has on germ granules, which should provide broader insight into processes that modify compositions and activities of other classes of biomolecular condensate.

Automated summary

mRNA packaging into germ granules is a conserved strategy for posttranscriptional regulation of germline mRNAs. We studied the homotypic clustering of nos and pgc in four Drosophila species and found that clustering is a conserved process that enriches germ granule mRNAs. Natural variations in nos 3ʹ UTR sequence influence clustering, resulting in decreased nos levels and increased defective germ cells. Our findings highlight the impact of evolution on germ granules and provide insight into the diversity and regulation of biomolecular condensates.