Regulative recovery in the sea urchin embryo and the stabilizing role of fail-safe gene network wiring

“As discussed elsewhere, the diverse modules of a GRNs have separate evolutionary histories; they change at different rates and are apparently loaded into the GRN at different stages in the evolution of the crown group lineages (30).”

“This capacity has been described as a ‘failsafe’ mechanism, at least in micromeres, in which one set of experiments shows reactivation of the micromere network after blocking upstream initiation of specification (Smith and Davidson, 2009).”

“mouse and sea urchins), the timing of determination and segregation of fate potential for precursor cells can be altered in response to environmental influences, both internal and external (Smith and Davidson 2009; Sharma and Ettensohn 2011; Martinez Arias et al. 2013).”

“There is a great deal of experimental evidence as to the details of operation of this gate in the skeletogenic domain of the sea urchin embryo, and for its cis-regulatory basis [11,16,26], and using entirely different genes it is also deployed in another domain of this embryo, the oral ectoderm (E.”

“purpuratus embryos, in which the regulatory gene directly responsive to Tcf, pmar1, operates at the top of the GRN hierarchy (1, 9, 10).”

“In Sp, this combinatorial input is used to trigger pmar1 transcription in micromeres (4, 34), whereas in Et this combination is not functional in skeletogenic micromere specification by direct test, and Otxα is not used at all in skeletogenic specification.”

“Wnt pathway members rescue missing vegetal cortex Previous experiments showed several Wnt pathway molecules to be distributed asymmetrically by 4th cleavage, and each is involved in the asymmetrical activation of endomesoderm (Chuang et al., 1996; Kenny et al., 1999; Logan et al., 1999; Wikramanayake et al., 2004; Min…”

“…and Davidson, 2008; Peter and Davidson, 2011), the endoderm– mesoderm cell fate decision (Ben-Tabou de Leon and Davidson, 2010; Peter and Davidson, 2011), and the activation of sets of differentiation and structural genes in a specific cell lineage (Oliveri et al., 2008; Smith and Davidson, 2009).”

“Smith and Davidson (Smith and Davidson, 2009) have recently proposed the hypothesis that the DNG, consisting of hesC and blimp1, is an ancestral system in echinoids and that the micro1 initiation system might have developed more recently in the modern euechinoid lineage.”

“Not only that, GRNs have been successfully used to predict system-level properties and they have proven to be extremely useful “tools” helping researchers identifying critical aspects of their models (e.g. Bolouri and Davidson, 2009; Smith and Davidson, 2009).”

“Representatives from families (or orders in the case of the Stomopneustoida) and nonechinoid echinoderm classes for which there is experimentally derived evidence in favor or in disfavor of the presence of the double-negative repression of hesc by Pmar1 and of alx1, tbr, or ets1 by Hesc were used to score extant taxa…”

“This allows specific transcription-factor target sequences, often just 4 – 8 bp, to be tested for network function within what approximates a genomic context (e.g., Smith and Davidson, 2009).”

“We have shown that these additional linkages not only hinder analysis further, they also play critical roles in governing outcomes (Smith et al. 2007; Smith and Davidson 2008, 2009).”

“In addition, HesC represses pmar1 transcription (Smith and Davidson, 2009).”

“…of a gene of interest (GOI) that can be engineered to drive the expression of reporter genes under the regulatory control of that gene (Smith 2008; Smith and Davidson 2008a, 2008b, 2009; Wahl et al. 2009; de-Leon and Davidson 2010; Damle and Davidson 2011; Ohguro et al. 2011; Materna and Davidson…”