Fig. 3. Mechanistic models for bivalent positioning and congression in meiotic spindles of grasshopper spermatocytes, using the force from flux and plus-end kMT assembly. (A) Bivalent positioning. A bivalent maintains an equilibrium position at the spindle equator owing to an equivalent amount of flux (blue arrows) on homologous kinetochores attached to an equal number of kMTs. Moderate tension on the kinetochores induces kMT plus-end assembly (red arrows). Laser irradiation (green line) partially destroys a kinetochore, which reduces the number of kMTs to which it can bind. This sudden imbalance in kMT number increases the stress on the remaining kMTs of the irradiated kinetochore, inducing a greater rate of kMT plus-end assembly (red arrow). Compressive force on the opposing kMTs inhibits or decreases the rate of plus-end assembly, and, consequently, the bivalent moves (orange arrow) towards the pole that has the greater number of kMT attachments and the larger flux-generated force (larger blue arrow). As chromosome arms bind to an increasing density of spindle MTs that resist their poleward movement (yellow arrows), the increased tension on the unirradiated kinetochore induces plus-end polymerization. A new equilibrium position is reached when tension-induced plus-end assembly on the unirradiated kinetochore equals the rate of disassembly at the pole. (B) Congression. Initially, a bivalent close to one spindle pole becomes mono-oriented and is pulled poleward by flux (blue arrow). Chromosome arms bind to an increasing density of spindle MTs and the resulting resistance (yellow arrows) increases kinetochore tension to induce plus-end polymerization (red arrows). Poleward movement stops, facilitating the capture of the homologous kinetochore by the opposite pole (blue arrows in the spindle). Capture of the unattached kinetochore produces an even greater amount of tension and polymerization (red arrow) at the opposite kinetochore, allowing the bi-oriented bivalent to move to the spindle equator (orange arrow), even though kMT number and flux-generated force (blue arrows) are greater at the lagging homologous kinetochore. Finally, an equilibrium position is established at the spindle equator when the leading kinetochore is captured by an equal number of kMTs. Poleward force from flux (blue arrows) is equivalent in each half-spindle and is balanced by an equal rate of kMT plus-end assembly (red arrows) induced by moderate tension.