Approximately 10% of BubR1^Rev1 homozygotes reach adult stages. Homozygous mutant females are sterile and lay fewer eggs than wild-type flies.

Homozygous BubR1^Rev1 mutants and BubR1^k06109/BubR1^Rev1 mutant neuroblasts exhibit a significant decrease in prophase-metaphase mitotic figures and a significant increase in sister chromatid separation. BubR1^Rev1 homozygous neuroblasts fail to exhibit a mitotic arrest in response to spindle damage.

Approximately 25% of embryos derived from BubR1^Rev1 homozygous females fertilised by wild-type males arrest development before cycle 10 (when nuclei reach the cortex); 62% develop up to cycle 10-13, although these embryos do not cellularise and nuclei are not found evenly distributed throughout the cortex; 10% initiate cellularisation, and only 3% gastrulate, although these exhibit pyknotic nuclei. BubR1^Rev1 embryos accumulate defective nuclei throughout the syncytial stages and after cycle 10 most exhibit regions without cortical nuclei, with extensive areas of micronuclei, abnormally larger-sized nuclei or paired nuclei with extensive DNA bridges.

BubR1^Rev1 embryos show abnormal mitotic progression, including precocious sister chromatid separation, lagging chromosomes, DNA bridges, irregular chromosome separation and aneuploidy. A similar range of abnormalities are observed in nuclei within mit

Almost 50% of BubR1^Rev1 display asynchronous nuclear behaviour following cycle 5. Nuclei in anaphase, prophase and nuclei initiating sister chromatid separation can be found at the same time. At later times, some nuclei show unequal chromosome segregation with lagging chromatids and chromatin bridges. In addition, during mitotic progression, cytoplasmic movements are strongly affected in mutant embryos. Among the synchronous embryos, there is a significant decrease in interphase nuclei and proper metaphase alignment is not seen.

More than 90% of BubR1^Rev1 mitotic cycle 10-13 embryos exhibit cortical areas devoid of nuclei with abundant free centrosomes. During the syncytial stage many nuclei appear to lose their attachment to centrosomes as the centrosomes migrate to the cortex in the absence of DNA.

At the time of cellularisation, the centrosome number in BubR1^Rev1 embryos is 30-40% higher relative to the control, indicating that the centrosomes initiate an extra round of replication, which is never fully completed. A small number of embryos (<5%) exhibit very few nuclei and centrosomes, suggesting that development has arrested completely in these embryos.

BubR1^Rev1 embryos exhibited aberrant spindle formation, with spindles abnormally distributed within the syncytium and some spindle being shorter in length. In other cases, fused spindles were seen, while neighbouring nuclei display an almost normal an

BubR1^Rev1 embryos display a decrease over time in the proportion of nuclei showing mitotic arrest after colchicine incubation. Although 63% of embryos arrest at cycle 3-6, only 40% arrest at cycle 7-9 and 23% arrest at cycle 10-13. The remaining embry

BubR1^Rev1 embryos display cycles of chromatin condensation and decondensation during early syncytial development.