Inversion in Volvox tertius: the effects of con A

During the development of Volvox tertius spheroids, a single-celled gonidium enlarges and undergoes multiple incomplete cleavages to give an embryo which is 'inside-out' with respect to the adult organism. A morphogenetic movement, termed 'inversion', turns this hollow ball of cells 'inside-out' through a hole, the phialopore. In V. tertius this phialopore possesses 4 inwardly directed lips. Normal inversion was studied in vitro in slide chambers and involved cell-shape changes accompanied by the production of pseudopodia and the bending backwards of the phialopore lips. 100 micrograms/ml Con A specifically and reversibly blocked inversion. Despite the inhibitory effect on cell division, the blocking of inversion was not due to the blocking of the last cell division some 50-100 min prior to inversion. Neither did the first cell-shape change from pear- to spindle-shape appear blocked. A feature of inhibition by Con A was the enhanced production of pseudopodia by embryos blocked at inversion, and the abnormal production of pseudopodia by embryos blocked at earlier stages. Non-inverting embryos showed internal flagella. We suggest that the Con A block to inversion, which may be reversed by alpha-methyl mannoside, arises from the prevention of backwards-bending of the phialopore lips. Fluorescein-isothiocyanate-Con A bound to embryo and cell coat, ane more strongly to the embryo at pre-inversion. SDS-polyacrylamide gel analysis of proteins isolated from embryos showed 4 glycoprotein bands, but Con A binding to these bands could not be demonstrated.