CELLULAR MECHANISMS OF THE EVOLUTION OF ONTOGENY IN VOLVOX

The green flagellates of the genus Volvox can be conditionally subdivided into two groups according to the size of gonidia (asexual reproductive cells) at the onset of cleavage. In Volvox carteri and several other species the gonidium undergoes an extended period of hypertrophied growth, after which a series of rapid fissions occurs. Embryonic cells do not grow during the intervals between consecutive divisions. In representatives of the second group (e.g., V. aureus) the period of gonidial enlargement is comparatively nondurable; thus the cleavage begins when the gonidium is relatively small, and each division is followed by a period of cellular growth. In this paper the evolutionary relationships between two types of asexual life cycle in Volvox are analysed on the basis of literary and our own data. It is supposed that the V. aureus type of development with slow divisions of small gonidia is more advanced in the evolutionary respect than the V. carteri type of development with rapid divisions of large gonidia. Experimental analysis of the role of light and dark for embryonic cleavage progression in V. aureus, V. carteri f. nagariensis and V. tertius as well as the experiments with several metabolic inhibitors (aminopterin, actinomycin D, cycloheximide and streptomycin) have enabled us to elucidate cellular mechanisms of the evolution of ontogenesis in Volvox.