Abstract

At the beginning of meiosis, chromosomes need to pair with their homologous partner and initiate recombination events that will ensure exchange of genetic information and proper segregation during anaphase I. Pairing events differ significantly between species. We and others recently uncovered in Drosophila that homologous chromosomes are not paired in primordial germ cells (PGCs), which are the first germ cells to be formed in the embryo; and that this absence of pairing is maintained for the autosomes until PGCs become germline stem cells (GSCs) in the adult. Surprisingly, of all cells in the adult fly, the only cells with unpaired chromosomes are the ones bound for meiosis. We were surprised to find that pairing takes place in premeiotic cells originating from GSCs and which undergo four rounds of mitosis before entering meiosis. In Drosophila females, we showed that homologous chromosomes start to pair when reaching the nuclear envelope in the mitotic zone. Once at the membrane, centromere movements are highly dynamic and we demonstrated that these movements are caused by rotations of the entire nucleus driven by microtubules and the SUN /KASH (LINC) complex in females. Meiosis in Drosophila males is known to be very different from females: males do not form synaptonemal complexes, males do not recombine and have no crossing-over; yet, males segregate their chromosomes perfectly normally. We thus wanted to investigate how males are pairing their homologues. Our results show that the initial events are strikingly similar to females: male GSCs have unpaired chromosomes and homologues start to pair during the four mitotic divisions. Even more surprisingly, we found that males do express SC proteins, which only localize at centromeres and do not elongate along chromosome arms. However, we found that chromosomes are much less dynamic than in females. I will present our latest results and model showing that males and females flies used similar molecular mechanisms but different kinetics to pair their homologous chromosomes.