UNDERSTANDING THE MEIOTIC ROLES OF SISTERS UNBOUND IN DROSOPHILA MELANOGASTER

During meiosis, cohesin is required for sister chromatid cohesion and for formation of chromosome cores. Multiple processes including chromosome segregation, recombination and synaptonemal complex (SC) are dependent on cohesin. Cohesin complex consists of two SMC subunits- SMC1, SMC3 and two non-SMC subunits RAD21/REC8 in meiosis and SA. But in Drosophila, non-SMC subunits have not been shown to be required for cohesion. We have identified a gene sisters unbound, which along with previously identified ord and solo, form a group of three genes (sos) which do not have any sequence similarity to cohesins but performs functions demonstrated by cohesins.

Proper chromosome segregation requires that homologs are connected by chiasmata during meiosis I and that sister centromeres are mono-oriented at meiosis I and bi-oriented at anaphase II. For both these functions cohesion is necessary. Cohesins are also required for proper assembly axial elements/lateral elements (AE/LE), SC, inhibiting sister chromatid exchange (SCE) and recombination. SUNN is required for all these functions and it localizes to chromosomes in a pattern similar to cohesion proteins ORD, SOLO and cohesin subunit SMC1 and is mutually interdependent on SOLO, SMC1 for localization. Bioinformatics analysis suggests that SUNN is a structural homolog of SA. Based on functional and structural similarity to cohesin complex components we predict that SUNN is a part of the Drosophila meiotic cohesin complex.