A Cross-Species Study of X-Y Chromosome Pairing in Drosophila Male Meiosis

Meiosis is a specialized cell division responsible for the production of sperm and eggs. The goal of meiosis is to reduce the chromosome number by half, a crucial step in ensuring genetic balance across generations. When this reductional division fails to segregate the chromosomes properly, aneuploidies may arise which can lead to a variety of genetic diseases. In order to ensure accurate chromosome segregation, homologous chromosomes must pair at the onset of meiosis. Canonically, chromosome pairing is stabilized by the synaptonemal complex (SC) and the repair of double strand DNA breaks through recombination. However, Drosophila males undergo a non-canonical meiosis that proceeds without an SC or recombination, presenting a unique model to study chromosome pairing mechanisms. Our research explores how meiotic cells in Drosophila males achieve stable pairing by focusing on the X and Y chromosomes, which pair by using a 240bp repeat in the intergenic spacer (IGS) of the rDNA locus–the only known pairing site for any chromosomes in D. melanogaster. Previous research has shown that IGS, in combination with the pairing proteins SNM (stromalin in meiosis) and MNM (modifier of mdg4), is both necessary and sufficient for X-Y chromosome pairing. Interestingly, some of the D. melanogaster sister species (D. simulans, D. sechellia, and D. mauritiana) have lost the rDNA locus from their Y chromosome, yet retain IGS, implying that IGS may also be utilized for X-Y pairing in sister species. Using DNA fluorescence in situ hybridization (FISH) and confocal microscopy, we visualized chromosome pairing dynamics in D. melanogaster, its sister species, and hybrids between these species. Whereas wild type flies (parental species) were highly competent in X-Y pairing as expected, hybrid species exhibited a considerable increase in X-Y nondisjunction. These data suggest that X-Y pairing mechanisms between species are not compatible, despite the fact that their IGS sequences are highly conserved. Interestingly, a comparative analysis of SNM protein structure across sister species revealed a truncated form of SNM in D. mauritiana, raising a possibility that SNM, which is known to mediate X-Y pairing, may be incompatible between D. mauritiana and other sister species. Our finding may hint at how IGS and its binding proteins are utilized to facilitate X-Y pairing in non-canonical pairing mechanisms. Understanding pairing mechanisms may help future therapeutic use of artificial chromosomes that can be efficiently transmitted through the germline.