A REVIEW ON THE EVOLUTION OF DEVELOPMENT IN VOLVOX - MORPHOLOGICAL AND PHYSIOLOGICAL-ASPECTS

This paper presents a morphophysiological concept of the evolution of Volvox development. We use published data concerning differences in size of the mature gonidia, rates of their division and peculiarities in segregation of somatic and reproductive cell lines. Based on this, four programmes (types) of asexual development of Volvox are recognized, and the evolutionary relationships among these programmes (but not among any concrete species of Volvox) are elucidated. The first developmental programme (Volvox powersii - V. pocockiae) is clearly primitive for the genus. This programme is characterized by ancestral features: large gonidia, division is fast, and there is no unequal (asymmetric) division into two cellular types. The second developmental programme(V. carteri), the third programme (V. tertius) and the fourth programme (V. aureus) are all derived, but constitute different phylogenetic trends. They each have some derived features: the second programme involves asymmetric division, the third programm involves slow division, while the fourth programme involves small gonidia and slow division. The evolutionary concept is supplemented by data on sexual reproduction in various species of Volvox.