Mitotic chromosomes scale to nuclear-cytoplasmic ratio and cell size in Xenopus

During the rapid and reductive cleavage divisions of early embryogenesis, subcellular structures such as the nucleus and mitotic spindle scale to decreasing cell size. Mitotic chromosomes also decrease in size during development, presumably to scale coordinately with mitotic spindles, but the underlying mechanisms are unclear. Here we combine in vivo and in vitro approaches using eggs and embryos from the frog Xenopus laevis to show that mitotic chromosome scaling is mechanistically distinct from other forms of subcellular scaling. We found that mitotic chromosomes scale continuously with cell, spindle, and nuclear size in vivo. However, unlike for spindles and nuclei, mitotic chromosome size cannot be reset by cytoplasmic factors from earlier developmental stages. In vitro, increasing nuclear-cytoplasmic (N/C) ratio is sufficient to recapitulate mitotic chromosome scaling, but not nuclear or spindle scaling, through differential loading of maternal factors during interphase. An additional pathway involving importin alpha scales mitotic chromosomes to cell surface area/volume ratio (SA/V) during metaphase. Finally, single-chromosome immunofluorescence and Hi-C data suggest that mitotic chromosomes shrink during embryogenesis through decreased recruitment of condensin I, resulting in major rearrangements of DNA loop architecture to accommodate the same amount of DNA on a shorter chromosome axis. Together, our findings demonstrate how mitotic chromosome size is set by spatially and temporally distinct developmental cues in the early embryo.

Automated summary

This study reveals that the size of mitotic chromosomes is controlled by spatially and temporally distinct developmental cues. The scaling of mitotic chromosomes involves mechanisms different from other subcellular structures such as spindles and nuclei. It is regulated by factors such as cell, spindle and nuclear size, as well as the nuclear-cytoplasmic ratio and importin alpha. Additionally, the shrinking of mitotic chromosomes during embryogenesis is attributed to decreased recruitment of condensin I and rearrangement of DNA loop architecture.