Co-orientation stability by physical tension: a demonstration with experimentally interlocked bivalents

Using living spermatocyte cultures of the grasshopper Melanoplus differentialis three experiments were successfully carried out in which bivalents were interlocked at metaphase I, using a micromanipulator. Two rod-shaped bivalents, with terminal kinetochores, were each made unipolar to opposite poles and placed together in such a way that each put tension upon the other when pulled by its sets of spindle fibers. The experiments unambiguously demonstrated and supported the importance of physical tension in maintaining metaphase I coorientation stability. In two of these experiments the interlocked structures remained stable for 168 and 100 minutes prior to anaphase I separation. In the third experiment, following 25 minutes stability under tension the two bivalents pulled apart. No longer under tension, each bivalent reoriented within 15–30 min, became bipolar, and congressed.