Molecular Signals During Sexual Induction of Volvox Carteri F. Nagariensis

Abstract

Sexual reproduction is often induced by environmental stress. Thus, in many unicellular green algae nitrogen starvation triggers the production of thick-walled zygotes which endure long adverse periods. In the multicellular green alga Volvox carteri, however, sexual reproduction occurs only under optimal growth conditions and is initiated by a milieu-independent, intrinsic mechanism (Jaenicke and Gilles, 1985). One reason for this divergence might be that - in contrast to other algae in which vegetative cells are directly converted into gametes - Volvox spheroids have to undergo transformation into new sexual organisms bearing eggs or sperm before mating can occur (Starr, 1969). This is too long to afford survival in a quickly deteriorating environment. It may be a better strategy to reproduce sexually in time and to store the dormant zygotes before resources become limiting. For this purpose Volvox has developed an elaborate sexual induction system consisting of an intrinsic triggering mechanism, a chemical intra-species messenger, and a signal cascade within the spheroids.