Bivalent orientation and behavior in crane-fly spermatocytes recovering from cold exposure.

At metaphase in crane-fly primary spermatocytes, the two sister kinetochores at the centromere of each homologue in a bivalent normally are adjacent and face the same pole; one homologue has all its kinetochore microtubules (kMTs) extending toward one pole and its partner has all its kMTs extending toward the opposite pole. In contrast, during recovery from exposure to 2 degrees C, one or both homologues in many metaphase bivalents had bipolar malorientations: all kMTs of one kinetochore extended toward one pole and some or all those of its sister extended toward the other. Metaphase sister kinetochores that had most of their kMTs extending toward the same pole were adjacent, and those with most extending toward opposite poles were separated from each other. Distances between homologous centromeres were similar to those in properly oriented bivalents. Maloriented bivalents were tilted relative to the spindle axis, and analysis of living cells showed that tilted configurations were rare during prometaphase in untreated cells but frequently arose in cold-recovering cells as initial configurations, then persisted through metaphase. This was in contrast to unipolar configurations of bivalents (configurations suggesting orientation of both homologous centromeres toward the same pole), which always reoriented shortly after the configuration arose. We conclude that in cold-recovering cells, bipolar malorientations are more stable than unipolar malorientations, and the orientation process is affected such that bipolar malorientations arise in bivalents upon initial interaction with the spindle and persist through metaphase.