Pairing between homologous sequences on the X and chromosome 3 in Drosophila male meiosis

Pairing between sex chromosomes in male Drosophila normally occurs at intergenic spacer (IGS) sequences within the tandemly repeated rDNA genes that are located proximally in the heterochromatin on both the X and Y. Pairing is not limited to these sequences, however, and can also occur with high fidelity between the X and segments of X euchromatin that have been translocated to the Y. Such euchromatic pairings can lead to segregation of the X and Y, even when the X is rDNA-deficient, suggesting X-Y conjunction remains at these euchromatic sequences until anaphase I. From these previous observations, however, it was unclear if conjunction occurred directly at euchromatic sequences, or if conjunction occurred due to residual IGS repeats remaining on the rDNA-deleted X. Here, to ask if pairing and conjunction of X euchromatin could occur completely independent of the rDNA, we used fluorescent in situ hybridization to examine pairing between the X chromosome and Dp(1;3) chromosomes that contain a transposed segment of the X. We found that as little as 120 kb of euchromatic homology was sufficient to ensure nearly complete pairing and could contribute to directing segregation. The ability to direct segregation was independent of the conjunction complex proteins Mod(mdg4)-in-meiosis and Teflon. We conclude that pairing can occur at X euchromatin homologies, and these interactions may persist even in the absence of the conjunction complex and contribute to segregation of the paired elements to opposite spindle poles at meiosis I.