Oocyte polarity is established independently of the Balbiani body

In most animals, oocyte polarity establishes the embryonic body plan by asymmetrically localizing axis-determining transcripts. These transcripts first localize in Xenopus and zebrafish oocytes to the Balbiani body (Bb), a large membrane-less organelle conserved from insects to humans. The Bb is transient, disassembling and anchoring at one pole the axis-determining transcripts that establish the vegetal pole of the oocyte. Aggregation of the Bb depends on the Bucky ball (Buc) protein, an intrinsically disordered protein with a prion-like self-aggregation domain. In zebrafish buc null mutants, the Bb fails to form and oocytes lack polarity. Here, we established buc hypomorphic mutants that fail to form the Bb, but remarkably Buc protein and vegetal mRNAs localized normally at the vegetal cortex of the oocyte. Thus, these buc hypomorphic mutants displayed normal oocyte polarity, demonstrating that the Bb is not required to establish oocyte polarity. We found that both a reduced Buc protein level and truncation of the N-terminal 10 amino acids contribute to Bb failure in the hypomorphic mutants.