STUDIES ON THE DEVELOPMENT OF THE SEA URCHIN STRONGYLOCENTROTUS DROEBACHIENSIS. II. REGULATION OF MITOTIC SPINDLE EQUILIBRIUM BY ENVIRONMENTAL TEMPERATURE

¹ Marine Biological Laboratory, Woods Hole, Massachusetts 02543, and the Department of Biology, Brandeis University, Waltham, Massachusetts 02154

1. The assembly and function of the mitotic apparatus in first division eggs of the sea urchin Strongylocentrotus droebachiensis were studied at 0° C and at 8° C by polarization microscopy in vivo and after isolation in hexylene glycol at controlled pH.

2. No differences in the amounts of total tubulin synthesized over comparable periods of the cell cycle were observed.

3. Mitotic apparatuses from a cell grown at 0° C are anastral, while those grown at 8° C are amphiastral.

4. At a 0° C growth temperature only about one-half of the spindle fiber monomer is available for polymerization as at 8° C, indicating a natural variation in pool size.

5. The amount of spindle monomer made available to the usable pool is specified only by the temperature (during early prophase, with temperature prehistory having no effect.

6. The temperature coefficient for this apparent activation differs markedly from that of the mitotic process as a whole.

7. The amount of tubulin obtained from an isolated mitotic apparatus, as determined by electrophoretic separation of tubulin on SDS-acrylamide gels, is directly proportional to its measured retardation, implying that birefringence is a true measure of oriented microtubules.

8. These results are compatible with the mitotic dynamic equilibrium theory of Inoué and complement this theory by providing evidence for natural variation in pool size (and thus for the potential local mobilization of monomer during anaphase movement) and by providing a direct correlation between birefringence and tubulin content of spindles.