Euchromatic homology is sufficient for pairing of rDNA-deficient X chromosomes in male meiosis. John E. Tomkiel, Andrew Bourgeios, Christina Morgan, Katie Hansen, Kayla Hill, Aboubakar Doura. Dept Biol, Univ North Carolina, Greensboro, NC.

   Meiotic sex chromosome pairing in male Drosophila occurs at the rDNA, located in both the X and Y heterochromatin, and is required for both normal chromosome segregation and spermatogenesis. Deletion of rDNA pairing sites results in XY nondisjunction, meiotic drive, and sterility when in combination with certain T(1;Y) translocations. It is unknown if these phenomenon are related to interactions between the X and Y specifically at the rDNA, or if it is pairing itself that is important. It is also unknown if other homology between the X and Y might substitute as a pairing site. We examined these questions by monitoring the ability of a collection of T(1;Y) chromosomes to pair and segregate from an rDNA-deficient X homolog. We found that euchromatic X homology was sufficient for pairing and segregation and for suppression of meiotic drive, but not all X segments behaved the same. This differential segregational ability of X segments suggests that there may be discrete pairing and/or conjunction sites distributed through X euchromatin. These may differ in nature from autosomal conjunction sites, as segregation ability did not correlate with the presence or absence of binding sites for Teflon, a protein required for autosomal conjunction. Because euchromatic X pairing sites would not normally function in hemizygous males, their conservation may indicate that the underlying mechanism of pairing in males and females is the same. We suggest that the location of sex chromosome pairing sites in heterochromatin may not be functionally important, but rather may merely reflect the location of remaining homology after evolution of heteromorphic sex chromosomes.