Early and late gene expression programs in developing somatic cells of Volvox carteri.

In situ hybridization was used to resolve details of the temporal and spatial aspects of expression of a number of cell-type-specific genes of Volvox carteri. In confirmation of earlier results obtained by Northern-blot analysis, this study revealed that accumulation of transcripts of somatic genes was largely restricted to the somatic-cell lineage. In extension of the previous study, two distinct gene expression programs during somatic-cell development were more fully defined: "early" somatic genes were expressed in the somatic-cell lineage even before visible differentiation began, whereas "late" somatic genes were not expressed until after somatic cells had been visibly differentiated for more than a day and had already reached the presenescent-adult stage of development. We postulate that products of the early somatic genes may be involved in generalized somatic functions that are required throughout somatic-cell development, whereas the late somatic gene products may be involved in more specialized processes that characterize mature, parental somatic cells. The most significant finding of the present study is that transcripts of early somatic genes are significantly more abundant in presumptive somatic cells than in presumptive gonidia during embryonic stages, while the two cell types are linked by a network of cytoplasmic bridges. The fact that the two cell lineages can establish and maintain different transcript populations despite the extensive cytoplasmic continuity that exists between them is somewhat surprising and must be taken into account in future attempts to elucidate the way in which dichotomous differentiation is initiated in the organism.