Temperature-induced orientation instability during meiosis: An experimental analysis

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Abstract
Low-temperature (10 °C) treatments alternated with periods at room temperature (25 °C) were found to induce grossly abnormal kinetochore orientation behaviour during metaphase I in spermatocytes of the grasshopper Melanoplus differentialis. These orientation abnormalities involved multiple reorientations before a stable bipolar co-orientation was finally achieved. Similar orientation abnormalities have been induced in the past by heat-treatments in the locust Schistocerca. The unstable orientation behaviour of bivalents in eight cold-treated living spermatocytes was analysed with the aid of time-lapse cinematographic records. Analyses made included the relationship between kinetochore position within the spindle and reorientation, the frequency of reorientation and the velocity of chromosome motion after reorientation. It had been shown previously, in untreated cells, that physical tension applied by micromanipulation could stabilize experimentally induced unipolar bivalents and inhibit their reonentation. Two experiments of this type with cold-treated spermatocytes were successful in demonstrating the importance of tension in the maintenance of stability, even in unstable, cold-treated material. In discussing the interpretation of these results it is proposed that bivalents have an inherent tendency to reorient periodically. In untreated cells the time between reorientations is long enough to enable interkinetochoric tensions, and resulting stable bipolar orientation, to be achieved, but in cold-treated cells this time is reduced, thereby hindering the establishment of tension and stability.