Control of differentiation in volvox carteri: a model explaining pattern formation during embryogenesis.

The green flagellate J’olvox is a simple muhicellular organism having only two kinds of cells, somatic and reproductive. The cells are arranged on the periphery of a spheroid. Asexual individu~s of I’olvox carten’ are composed of 2000-4000 somatic cells and about 16 large reproductive cells (gonidia), located in the posterior region of the spheroid. Mature asexual spheroids show a very regular spatial arrangement of the gonidia. The development of all types of spheroids asexual, male and female is through successive divisions of the asexual reproductive cells, the gonidia [l-3]. In the developing asexual embryo differentiation into somatic and reproductive cells is usually seen at the division from 32-64 cells. At this division 16 cells of the embryo undergo unequal cleavage, forming a small somatic and a large gonidial initial. While cell division ceases in the gonidial initials, the remaining cells (somatic initials) continue their synchronous divisions. At the termination of cell divisions, the many somatic cells form a hollow sphere with the gonidia conspicuously placed on the outer surface. After cleavage is complete, the embryo enters the process of inversion, turning itself inside-out. The gonidia remain in the same relative positions where they were formed during early embryogenesis. The young spheroid now enters a period of expansion. Formation of a gelatinous matrix of the colony begins and the cells become separated from one another. Finally, the mature daughter colonies are released from the parent colony. Its simple life cycle makes Volvox a promising material for the study of the mechanism controlling the orderly ~tegration of cells during embryogenesis. In this paper I would like to propose a model to explain the following problems: