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Fig. 2 BMP homodimers fail to rescue embryos lacking BMP antagonists and heterodimers: In bubble plots, circle area reflects the percent embryos of a given phenotype (Left), fill color reflects the MO condition, and line color reflects the RNA injection condition. Ns are in brackets above each column. Injection conditions and genotypes are labeled below each column. Raw phenotype scores are shown in Dataset S1. MO concentrations and combinations are listed in SI Appendix, Table S4. Hypothesis 1: BMP heterodimers prevail because antagonists preferentially bind homodimers (A) and, if true, then BMP homodimers should signal in the absence of antagonists and heterodimers (B). (C) Bmp2 homodimers cannot signal at endogenous expression levels, even without CNF. Column 1: 1/4 of the offspring of a bmp2b+/− incross are C5 bmp2b−/−. Column 2: 15 pg FLAG-bmp2b RNA injection rescues most C5 mutant embryos and ventralizes nonmutants. Other columns: bmp2b homodimers fail to rescue bmp7a mutant embryos with or without CNF and additional bmp2b RNA. (D–H) Column numbers below images refer to the experimental condition of columns in C. The otx2b expression can be seen in the following: WT embryos (D), bmp7a mutant embryos (E), bmp7a mutants with additional bmp2b RNA (F), CNF MO-injected bmp7a mutants (G), and bmp7a mutants injected with CNF MO and bmp2b RNA (H). (I) Bmp7 homodimers cannot signal at endogenous concentrations, even without CNF. Column 1: bmp7a−/− embryos are C5 dorsalized. Column 2: 40 pg bmp7a RNA rescues DV patterning in bmp7a mutants. Other columns: Bmp7 homodimers fail to rescue bmp2b mutants with or without CNF and additional bmp7a RNA.