CHARACTERIZATION OF A HYPOMORPHIC ALLELE OF SPC25, SPINDLE POLE BODY COMPONENT 25, DURING MITOSIS AND MALE MEIOSIS IN DROSOPHILA MELANOGASTER

Accuracy of chromosome segregation during division is vital for maintaining genomic integrity and essential for cellular function and viability. Failed chromosome segregation leads to aneuploidy, which has different consequences for mitotic and meiotic cells. Aneuploid products of mitosis often have unregulated gene expression which can cause cellular over-proliferation or apoptosis. Chromosome nondisjunction (NDJ) events in meiosis generate aneuploid sperm or eggs. If incorporated during fertilization, their presence can lead to a range of consequences from spontaneous abortion to genetic conditions such as Down Syndrome or other aneuploid-associated syndromes. In this study, we identify a unique allele of spc25, spindle pole body component 25, which generates two mutant alternatively spliced spc25 transcripts, encoding two mutant Spc25 proteins. One protein contains an amino acid substitution, D71E (aspartate 71 glutamic acid), and a four amino-acid motif insertion, which we refer to as Spc25^ER. The other protein lacks its carboxy-terminal (CT) domain. Spc25, Spc24, Nuf2, and Ndc80 proteins form the Ndc80 complex, which establishes end-on kinetochore-microtubule (KT-MT) attachments. Stabilized end-on KT-MT attachments are required for proper biorientation and segregation of chromosomes. We show that the D71E substitution in Spc25^ER causes chromosome NDJ events in both meiosis I and II. Analysis of Nuf2 localization in mutants that express Spc25^ER shows premature loss of Nuf2 during anaphase. We propose that Spc25^ER incorporation into the Ndc80 complex leads to its instability when spindle forces are exerted onto kinetochores for the poleward movement of chromosomes. Our results also support previous work that shows that the CT region of Spc25 is critical for its localization to the kinetochore region.