PLK1- and PLK4-mediated asymmetric mitotic centrosome size and positioning in the early zebrafish embryo.

Factors that regulate mitotic spindle positioning have been elucidated in vitro, however it remains unclear how a spindle is placed within the confines of extremely large cells. Our studies identified a uniquely large centrosome structure in the early zebrafish embryo (246.44{+/-}11.93m2 mitotic centrosome in a 126.86{+/-}0.35m diameter cell), whereas C. elegans centrosomes are notably smaller (6.75{+/-}0.28m2 mitotic centrosome in a 55.83{+/-}1.04m diameter cell). During early embryonic cell divisions, cell size changes rapidly in C. elegans and zebrafish embryos. Notably, mitotic centrosome area scales closely with changing cell size compared to changes in spindle length for both organisms. One interesting difference between the two is that mitotic centrosomes are asymmetric in size across embryonic zebrafish spindles, with the larger mitotic centrosome being 2.14{+/-}0.13-fold larger in size than the smaller. The largest mitotic centrosome is placed towards the embryo center in a Polo-Like Kinase (PLK) 1 and PLK4 dependent manner 87.14{+/-}4.16% of the time. We propose a model in which uniquely large centrosomes direct spindle placement within the disproportionately large zebrafish embryo cells to orchestrate cell divisions during early embryogenesis.