CHROMOSOMES INITIATE SPINDLE ASSEMBLY UPON EXPERIMENTAL DISSOLUTION OF THE NUCLEAR-ENVELOPE IN GRASSHOPPER SPERMATOCYTES

Chromosomes are known to enhance spindle microtubule assembly in grass hopper spermatocytes, which suggested to us that chromosomes might pla y an essential role in the initiation of spindle formation. Chromosome s might, for example, activate other spindle components such as centro somes and tubulin subunits upon the breakdown of the nuclear envelope. We tested this possibility in living grasshopper spermatocytes. We ru ptured the nuclear envelope during prophase, which prematurely exposed the centrosomes to chromosomes and nuclear sap. Spindle assembly was promptly initiated. In contrast, assembly of the spindle was completel y inhibited if the nucleus was mechanically removed from a late propha se cell. Other experiments showed that the trigger for spindle assembl y is associated with the chromosomes; other constituents of the nucleu s cannot initiate spindle assembly in the absence of the chromosomes. The initiation of spindle assembly required centrosomes as well as chr omosomes. Extracting centrosomes from late prophase cells completely i nhibited spindle assembly after dissolution of the nuclear envelope. W e conclude that the normal formation of a bipolar spindle in grasshopp er spermatocytes is regulated by chromosomes. A possible explanation i s an activator, perhaps a chromosomal protein (Yeo, J.-P., F. Alderucc io, and B.-H. Toh. 1994a. Nature (Lend.). 367: 288-291), that promotes and stabilizes the assembly of astral microtubules and thus promotes assembly of the spindle.