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Sexual differentiation of the reproductive cells (gonidia) of the green alga Volvox carteri f. nagariensis Iyengar is triggered by a glycoprotein inducer which is released into the medium by sexual male spheroids. If added, it induces the next generation cycle to become sexual. About 3000 pheromone molecules per spheroid will give full induction. Specific binding sites for the pheromone have been identified in the extracellular matrix in which characteristic events take place. Inducer binding and action are reversible for a certain time period until the start of cell division; thereafter, determination of the gonidia is final. Extent of sexual induction is proportional to the duration of inducer pulses. Inhibition of sexual induction reveals as essential extracellular components of the induction system high molecular acidic glycoproteins, cyclic nucleotides, Ca2+/calmodulin, and pheromone-controlled gene products. Low matrix levels of cyclic AMP and cyclic GMP seem to stimulate induction; high levels to inhibit.

Pheromone-initiated sexual reproduction of Volvox carteri f. nagariensis is started by a “spontaneous” generation of sexual male spheroids at a rate of 10-4. As previously suggested, these males are mutants (sexc) which become independent of pheromone-controlled sex-induction. Unequivocal evidence for a mutative event now comes from the observation that the sexctrait is transmitted through cell lineages during embryogenesis, thus confirming earlier morphological analysis, Mutation semms to be restricted to cell division, The high frequency of mutation points to a special enchacing mechanism for whick the possibility of a genome rearrangement is discussed.

Asexual spheroids of the multicellular green alga Volvox are composed of two types o f cells: non-flagellated reproductive gonidia and Chlamydomonas-like flagellated somatic cells. They are committed by a differentiating cleavage during embryogenesis. The gonidia of the adult spheroids form a symmetrical pattern consisting of four layers of four gonidia each; their position is established already in the embryos by the gonidial initials. Whereas, generally, the 16-gonidia pattern is assumed to be the basic one, most o f the spheroids have fewer gonidia (down to 8). The nine possible gonidial patterns (8 to 16 gonidia) are described and correlated to the gonidial stem cells which have been differentiated. Defects in gonidial pattern are of particular interest, since any model of differentiation has to explain not only the basic pattern formed, but also its systematic variations. Our study shows that the pattern reduction is by no means random, but governed by an intrinsic mechanism which shifts the first unequal cleavage from the 32-celled stage to the 16-celled stage. All the patterns formed can be deduced from cleavage pathways involving non-synchronous differentiation of the stem cells. Thus, pattern formation can be correlated to timing and spacing signals regulating events during embryogenesis.