regA , a Volvox gene that plays a central role in germ - soma differentiation , encodes a novel regulatory protein

Volvox carteri, a simple multicellular green alga, is an attractive model for studying the genetic control of germ-soma differentiation, because each spheroid (or ‘individual’) contains only two cell types: mortal somatic cells and immortal germ cells (Kirk, 1998; Schmitt et al., 1992). A typical asexual spheroid contains 16 large asexual reproductive cells, or ‘gonidia’, just internal to a surface monolayer of small, biflagellate, Chlamydomonas-like somatic cells (Fig. 1A). Gonidia are immotile and specialized for reproduction; each will divide to produce a juvenile containing a new cohort of gonidia and somatic cells (Starr, 1969, 1970). Somatic cells, in contrast, are specialized for motility but have no reproductive potential; once differentiated, they never divide, and they eventually undergo programmed death (Pommerville and Kochert, 1981, 1982). The regA locus plays a central role in establishing this germsoma division of labor by acting as a negative regulator of reproductive functions in somatic cells. Whereas regA+ somatic cells can not be forced to divide, regA− somatic cells exhibit all of the reproductive potentials of the species (Huskey and Griffin, 1979; Kirk et al., 1987; Starr, 1970). In the asexual phase of the life cycle, regA− mutants exhibit the ‘somatic regeneration’ (or ‘Reg’) phenotype, in which somatic cells appear to differentiate normally, but then redifferentiate as gonidia and divide to produce juveniles of like phenotype; in the sexual phase regA− somatic cells produce offspring containing eggs or sperm (Starr, 1970). All of the >100 Reg mutants that have been analyzed to date (Huskey and Griffin, 1979; Kirk et al., 1987; D. Kirk, M. Kirk and K. A. Stamer, unpublished observations) have lesions that map to regA, indicating that this is the only locus that can mutate to yield a Reg phenotype. No effect of regA mutations on development of ‘true’ gonidia can be detected, indicating that the gene is not expressed in gonidia. Molecular studies reinforce the view that regA acts as a negative regulator of reproductive functions in somatic cells, because regA− somatic cells accumulate transcripts that are normally accumulated only by developing gonidia (Tam and Kirk, 1991b). We conclude that regA is a master regulatory gene that plays a central role in germ-soma differentiation by acting in somatic cells to prevent expression of functions required for germ cell development. Prior efforts to clone the regA locus by performing a chromosome walk from two different linked polymorphisms (Adams et al., 1990; Harper et al., 1987) did not succeed (D. L. Kirk and J. K. Miller, unpublished). Here we report success in using the cold-inducible Volvox transposon, Jordan, to tag and recover the regA locus, and we show that regA encodes a nuclear protein with a combination of features similar to ones found in various transcription factors.