By imaging homologous chromosome movements in living nematodes, Wynne et al. describe how they pair up during meiosis to facilitate recombination.
In C. elegans, homologue pairing relies on DNA sequences near one end of each chromosome called pairing centers. These regions bind zinc finger proteins that link chromosomes to a pair of nuclear membrane proteins called SUN-1 and ZYG-12 that span the nuclear envelope and connect to microtubule-based dynein motors in the cytoplasm. The motors are thought to drag chromosomes around the inner surface of the nucleus until they meet their homologous partner.
Wynne et al. followed the movements of individual pairing centers and ZYG-12 molecules in living worm oocytes. Chromosomes were relatively immobile before meiosis, but, soon after meiotic entry, pairing centers and their nuclear envelope linker proteins showed two different modes of movement: rapid translocations in a single direction interspersed with periods of slower, more meandering motions on the nuclear surface. The faster, directional movements were inhibited if microtubules were depolymerized or dynein was depleted, but the slower motions continued in the absence of dynein, which may explain why dynein knockdown delays, but doesn't prevent, homologue pairing.
Dynein is required, however, for synapsis—the stable association of homologous chromosomes after their initial coupling. Senior author Abby Dernburg now wants to expand the live imaging approach to investigate how the motor protein contributes to this process.