Fig. 1.

Coevolution of SH2 domains and tyrosine kinases. (A) A tree of the major divisions within the Eukaryote that are represented by the 21 organisms in this study. The numbers of organisms studied in the different kingdoms, supergroups, and phyla are indicated in yellow circles. (B) SH2 domain proteins and protein kinases were identified in five organisms that fall within the Bikonta, representing three supergroups and five phyla. (C) pTyr-binding SH2 domains appear in the Unikont branch of Eukaryotes. The total complement of kinases, PTKs, and SH2 domain–containing proteins is noted for each species analyzed. An approximate time line for the separation of the major branches is shown in fig. S1. The total number of kinases and PTKs were derived from the literature, as was the number of SH2 domain proteins for human. The total number of SH2 domain–containing proteins from the organisms listed was identified with SMART and Pfam as described in Materials and Methods. (D) The co-expansion of PTKs and SH2 genes is apparent as the numbers of each track one another closely from M. brevicollis to H. sapiens. (E) The percentage of the genome devoted to encoding PTKs and SH2 domains correlates across the different metazoan genomes (see table S1 for the list of two-letter representations of genus species) with a coefficient of determination (R²) of 0.95.