| Chromosome segregation during anaphase involves multiple mechanisms of motion that might interact and/or cooperate. The experiments in this thesis to test this hypothesis are grouped in four main projects: myosin-microtubule interactions, effect of hyperactivated myosin, myosin-actin interactions, and role of elastic spindle matrix in anaphase movements.;To further study the role of myosin in anaphase, we treated crane-fly spermatocytes with an enhancer of myosin activity, calyculinA, which caused anaphase chromosomes to accelerate poleward. Based on other studies that showed that calyculinA caused hyperactivation of myosin, we conclude that hyperactive myosin caused chromosome acceleration in crane-fly spindles, thereby confirming the role of myosin in anaphase motion.;To test whether the mechanisms of anaphase are similar in different insect cells, we studied chromosome movement in spermatocytes from another insect (locust), using the same battery of inhibitors used for crane-fly spermatocytes. The results showed that actin and myosin are involved in chromosome movements in locusts, similar to one of the anaphase mechanisms described in crane-fly spermatocytes.;To test whether a spindle matrix is present in insect spindles, we studied the distribution of the elastic protein titin in locust and crane-fly spermatocytes. We showed that titin is present in insect spindles and it associates with myosin, actin and with matrix proteins. Myosin, actin and titin normally function together in myofibrils and their arrangement in the spindle and their association with matrix proteins suggest that an elastic spindle matrix might be involved in spindle rigidity, force production and tension generation during anaphase.;Previous studies, using actin inhibitors, showed that actin filaments are required for chromosome movement in normal spindles from crane-fly spermatocytes. Our experiments, using in vivo actin-filament-free spindles, showed that spindles can nonetheless function normally when they are depleted of actin filaments. We showed, for the first time, that the mechanism that moves the chromosomes in these actin-filament-free spindles involves force-producing interactions between spindle myosin and kinetochore microtubules.;Overall, we showed that various mechanisms of anaphase might be active in insect spermatocytes, depending on what molecular components are present and functional in the division spindle in these cells. |
