Extended Data Fig. 10: Summary of PAR ultrastructure and molecular determinants of axis remodelling and DSB formation.

Schematic of the meiotic Y chromosome loop–axis structure before the pairing or synapsis of X and Y chromosomes at the transition between zygonema and pachynema. The chromosome axis comprises the meiosis-specific axial proteins SYCP2, SYCP3, HORMAD1 and HORMAD2; cohesin subunits (only REC8 is represented); and RMMAI proteins (REC114, MEI4, MEI1, ANKRD31 and IHO1). On the non-PAR portion of the Y chromosome axis (left), RMMAI protein loading and DSB formation are partly dependent on HORMAD1 and ANKRD31, and strictly dependent on MEI4, REC114¹⁹, IHO1²¹ and, presumably, MEI1¹⁸. The DNA is organized into large loops, with a low number of axis-associated RMMAI foci. By contrast, in the PAR (right), the hyperaccumulation of RMMAI proteins at mo-2 minisatellites (possibly spreading into the adjacent chromatin) promotes the elongation and subsequent splitting of the PAR sister chromatid axes. Short mo-2-containing chromatin loops stretch along this extended PAR axis, which increases the physical distance between the PAR boundary and the distal PAR sequences, including the telomere. The degree of RMMAI protein loading, PAR axis differentiation and DSB formation are proportional to the mo-2 FISH signal (which we interpret as reflecting mo-2 copy number), and depend on MEI4, ANKRD31 and, presumably, REC114.