A maternally inherited Chromosomal Passenger Complex regulates germ plasm ribonucleoparticle aggregation in Zebrafish

Abstract

In zebrafish, the formation of primordial germ cells depends on the inheritance of a compartmentalized membrane-less subcellular structure containing a pool of maternally expressed germ plasm ribonucleoparticles (gpRNPs) and proteins. Interactions between cytoskeletal components and gpRNPs are crucial for the movement and collection of gpRNPs into the furrows during the first few cellular division of the early embryo. Previous work has identified motley/ birc5b, a maternally-expressed homolog of a known Chromosomal Passenger Complex (CPC) component, Survivin, as a linker between gpRNPs and microtubules during gpRNP aggregation. However, Survivin can also function independent of the CPC in other cellular contexts. Here we investigated whether a maternally inherited CPC is necessary for gpRNP aggregation. We identified cdca9 as a maternally-expressed duplicated homolog of Borealin, another member of the CPC. Similar to motley, embryos from homozygous cdca9 mutant females exhibit defects in chromosome segregation and cytokinesis during meiosis and mitosis, phenotypes associated with mutations in CPC members. Additionally, embryos lacking Cdca9 displayed decreased gpRNP aggregation prior to furrow formation in the early embryo, a phenotype indistinguishable from that observed in motley mutants. As previously shown for Birc5b, Cdca9 and other CPC components INCENP and Aurora B kinase colocalize at the tips of astral microtubules as gpRNPs are transported to the forming furrow. Unexpectedly, Birc5b, but not other CPC components, accumulates within the growing gpRNP aggregate prior to and during furrow formation. The association of Birc5b with germ plasm masses continues during their asymmetric segregation in the cleavage stages, ceasing only when gpRNPs undergo cytoplasmic dispersal during gastrulation. Our studies reveal a role for a non-conventional, maternally-inherited CPC for spindle and furrow formation, and, unexpectedly, gpRNP aggregation during early development. Additionally, we find that Birc5b, but not other CPC proteins, remains a component of zebrafish germ plasm during and after its aggregation. Author SummaryMaternal products are necessary for early development across species, and the removal of these products from the embryo can cause developmental defects or death. The zebrafish has been widely used to discover the role of maternal products during early development. Using zebrafish, we discovered that a mutation in a maternal-specific duplicated borealin gene not only affects early development but also the aggregation of germ cell determinants. We also find that this duplicated Borealin interacts within a specialized Chromosomal Passenger Complex, a complex that traditionally regulates multiple steps of cellular division. This specialized Chromosomal Passenger Complex acts as a linker between germ cell determinants and the cytoskeleton during early development. These results highlight a unique role for the Chromosomal Passenger Complex outside of cellular division during early development. Further, these findings underscore the intricate mechanisms by which gene duplications contribute to the regulation of early developmental processes, providing valuable insight into the molecular events of embryogenesis.