Mitotic chromosome size scaling in Xenopus

Abstract

As rapid divisions without growth generate progressively smaller cells within an embryo, mitotic chromosomes must also decrease in size to permit their proper segregation, but this scaling phenomenon is poorly understood. We demonstrated previously that nuclear and spindle size scale between egg extracts of the related frog species Xenopus tropicalis and Xenopus laevis, but show here that dimensions of isolated mitotic sperm chromosomes do not differ. This is consistent with the hypothesis that chromosome scaling does not occur in early embryonic development when cell and spindles sizes are large and anaphase B segregates chromosomes long distances. To recapitulate chromosome scaling during development, we combined nuclei isolated from different stage Xenopus laevis embryos with metaphase-arrested egg extracts. Mitotic chromosomes derived from nuclei of cleaving embryos through the blastula stage were similar in size to replicated sperm chromosomes, but decreased in area approximately 50% by the neurula stage, reproducing the trend in size changes observed in fixed embryos. Allowing G2 nuclei to swell in interphase prior to mitotic condensation did not increase mitotic chromosome size, but progression through a full cell cycle in egg extract did, suggesting that epigenetic mechanisms determining chromosome size can be altered during DNA replication. Comparison of different sized mitotic chromosomes assembled in vitro provides a tractable system to elucidate underlying molecular mechanisms.

Condensin and the Spindle Midzone Prevent Cytokinesis Failure Induced by Chromatin Bridges in C. elegans Embryos

“…To test whether condensin I and II are necessary to prevent cleavage furrow regression, we induced chromosome segregation defects by top-2 RNAi in hypomorphic mutants of condensin I, dpy-28(s939) and dpy-26(n199) , and of condensin II, hcp-6(mr17) [40–42]. We used milder RNAi conditions against top-2 that did not induce significant cytokinesis failure (1/80) in the 2-cell embryos, to measure any enhancement of the cytokinesis defect by the hypomorphic condensin mutants.…”

Biological Scaling Problems and Solutions in Amphibians

“…Studies of chromosome scaling during Xenopus development show that mitotic chromosome sizes do not scale with cell size in the early embryo, but become progressively smaller through the blastula and neurula stages (Micheli et al 1993; Kieserman and Heald 2011). Compared with scaling of the nucleus and spindle by cytoplasmic factors, developmental mitotic chromosome scaling seems to be intrinsically determined by chromosome architecture and regulated expression of chromatin proteins, although the relevant scaling factors remain to be identified.…”

“…Compared with scaling of the nucleus and spindle by cytoplasmic factors, developmental mitotic chromosome scaling seems to be intrinsically determined by chromosome architecture and regulated expression of chromatin proteins, although the relevant scaling factors remain to be identified. Interestingly, whereas increasing nuclear size in egg extracts before mitosis did not affect mitotic chromosome size unless the DNA replicated (Kieserman and Heald 2011), inhibiting nuclear growth resulted in shorter, thicker mitotic chromosomes, suggesting that mitotic chromosome size may be modulated by increasing but not decreasing nuclear DNA density (Hara et al 2013). Overall, Xenopus developmental scaling of mitotic chromosome size mirrors spindle scaling as cells become small enough that the distance that chromosomes can separate is constrained (Wühr et al 2008; Field and Lenart 2011; Kieserman and Heald 2011; Levy and Heald 2012).…”

Sizing and shaping the nucleus: mechanisms and significance

“…During C. elegans embryogenesis, reductions in nuclear size were shown to correlate with increased mitotic chromosome condensation [79] and interphase genome reorganization resulting in activated genes shifting towards the nuclear lumen and silenced genes localizing to the NE [80]. Interestingly, increasing the size of Xenopus embryonic nuclei did not result in increased mitotic chromosome length or width [81]. During T-cell activation, both actin-mediated nuclear elongation and activation of signaling intermediates were required to alter gene expression [82].…”