Control of spindle form and function in grasshopper spermatocytes

Abstract

The influence of the mitotic organizing centers, the kinetochores and the polar organizers, in controlling the dynamic spindle form and function has been investigated in the primary spermatocytes of two grasshoppers, Arphia xanthoptera and Melanoplus differentialis. A new measure of the total birefringent material in the spindle is introduced—volume-birefringence. This measure avoids many of the problems associated with the traditional retardation measurements of spindle organization.—The number of chromosomes (and their kinetochores) in a spindle can be altered with a piezoelectric micromanipulator in three ways: 1) chromosomes can be removed permanently from the cell, 2) chromosomes can be detached from the spindle and allowed to reenter the spindle at a later time, and 3) chromosomes can be transferred from one spindle to another in cells containing two spindles. Such operations show the volume-birefringence of the spindle is proportional to the number of chromosomes in the spindle. A residual volume-birefringence is seen and attributed to the contribution of the polar organizers to spindle structure. The relative polar contribution differs in the two species. Chromosome motion and spindle elongation in anaphase are unaffected by the number of chromosomes in the spindle. The proportion of volume-birefringence associated with a kinetochore is used to estimate the number of microtubules one might expect to see if the birefringence of the spindle is of microtubular origin. These calculations predict about twice the number of microtubules per kinetochore than seen with the electron microscope. Reasons are suggested to explain this discrepancy.— It is argued that chromosome detachment releases spindle component subunits into the total subunit pool, but that these excess subunits do not influence the metaphase form nor the anaphase function of the spindle; therefore, spindle dynamics are under the direct control of the kinetochores and the polar organizing centers.