Fig. 8.

Evolution of complex biological systems. (A) The components of the vascular, muscular, nervous, and immune systems evolve in the respective organisms indicated by the colored arrows. SH2-encoding genes disrupted by mouse knockout that results in defects in specialized tissues (3) or genes found exclusively in specific tissues with gene expression data are included within the circles (fig. S10). (B) The evolution of pTyr signaling within the adaptive immune system suggests that such complex systems make extensive use of existing SH2 components to develop new and robust signaling networks. The TCR and BCR signaling networks are indicated along with SH2 domain-containing proteins and the general signaling circuitry that they connect. Specific SH2 domains are represented as ovals colored on the basis of their functional categories (3). The evolutionary origin of each SH2 domain protein is indicated with a colored bar code displaying whether that gene and its ortholog were present in the indicated organisms studied.