Fig. 2

Minimal model for the genetic program of cellular differentiation in V. carteri. Four master regulatory genes are involved in programming differentiation, namely glsA, hsp70A, lag, and regA [8, 27, 30, 32, 33, 35,36,37, 39, 40, 42]. At the 32-cell stage, expression of glsA and hsp70A genes is required to promote the asymmetric divisions that produce large-small sister-cell pairs. Then, the lag gene acts only in the large cells to repress the development of somatic characteristics, while the regA gene acts only in the small cells to repress reproductive development. In contrast to the glsA, hsp70A, and regA genes, which have been cloned and sequenced [38, 40, 46], the lag gene is actually unknown. The role for lag in the model is based on previously existing phenotypic mutants [8, 27], but the phenotype-gene relationship is missing and, therefore, the lag gene itself is out of reach. However, if the counterpart of regA in large cells evolved from an ancient regA gene, then rlsM could be a candidate for the missing lag gene. As shown here, the regA-related rlsM gene is only expressed in large, reproductive cells