Activation of pmar1 controls specification of micromeres in the sea urchin embryo

pmarl is a transcription factor in the paired class homeodomain family that was identified and found to be transcribed in micromeres beginning at the fourth cleavage of sea urchin development [Dev. Biol. 246 (2002), 209]. Based on in situ data, molecular perturbation studies, and QPCR data, the recently published gene regulatory network (GRN) model for endomesoderm specification [Science 295 (2002) 1669; Dev. Biol. 246 (2002), 162] places pmarl early in the micromere specification pathway, and upstream of two important micromere induction signals. The goal of this study was to test these three predictions of the network model. A series of embryo chimeras were produced in which pmarl activity was perturbed in one cell that was transplanted to control hosts. At the fourth cleavage, micromeres bearing altered pmarl activity were combined with a normal micromereless host embryo. If [beta]-catenin signaling is blocked, the micromeres remain unspecified and are unable to signal to the host cells. When such [beta]-catenin-blocked micromeres also express Pmarl, all observed micromere functions are rescued. The rescue includes expression of the primary mesenchyme cell (PMC) differentiation program, expression and execution of the Delta signal to induce secondary mesoderm cell (SMC) specification in macromere progeny, and expression of the early endomesoderm induction signal necessary for full specification of the endoderm. Additionally, Pmarl expressed mosaically from inserted DNA constructs causes induction of ectopic Endo 16 in adjacent cells, demonstrating further that Pmarl controls expression of the early endomesoderm induction signal. Based on these experiments, Pmarl is an important transcription factor necessary for initiating the micromere specification program and for the expression of two inductive signals produced by micromeres. Each of the tests we describe supports the placement and function of Pmarl in the endomesoderm GRN model. Keywords: Gene regulatory network; Sox; Delta; [beta]-Catenin; Paired homeodomain