Differentiation in Volvox carteri: Study of Pattern Variation of Reproductive Cells

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.