Identification of Two Proteins Required for Conjunction and Regular Segregation of Achiasmate Homologs in Drosophila Male Meiosis

In Drosophila males, homologous chromosomes segregate by an unusual process involving physical connections not dependent on recombination. We have identified two meiotic proteins specifically required for this process. Stromalin in Meiosis (SNM) is a divergent member of the SCC3/SA/STAG family of cohesin proteins, and Modifier of Mdg4 in Meiosis (MNM) is one of many BTB-domain proteins expressed from the mod(mdg4) locus. SNM and MNM colocalize along with a repetitive rDNA sequence known to function as an X-Y pairing site to nucleolar foci during meiotic prophase and to a compact structure associated with the X-Y bivalent during prometaphase I and metaphase I. Additionally, MNM localizes to autosomal foci throughout meiosis I. These proteins are mutually dependent for their colocalization, and at least MNM requires the function of teflon, another meiotic gene. SNM and MNM do not colocalize with SMC1, suggesting that the homolog conjunction mechanism is independent of cohesin.