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

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.