Fig. 4

Evolution of capa/pk genes. Transcript sequence data indicate rapid evolution of the capa/pk gene(s) with the divergence of Remipedia and the hexapods. This rapid evolution includes the appearance of novel ligands, gene duplications, and subsequent sorting of the three putative ligands in the resulting capa and pk genes. The most derived pattern (pattern C3, Zygentoma) appears to be typical of many insect taxa. For D. melanogaster, it has been verified that PVK (yellow), PK (blue) and trypto-PK (green) each exhibit specific receptors (see [19]). a Sequences of CAPA/PK precursors assigned to different types with respect to the evolution of capa/pk genes. Predicted signal peptides (highlighted in grey), amidation signals (bold), cleavage signals (italics, bold), splice variants (a, b), and supposed bioactive mature peptides (underscored) are indicated. Incomplete sequences are indicated with “…”. Peptidomic studies have not been performed for any of these species. b Overview of the putative evolution of capa/pk genes, as indicated by analyses of transcript sequences. According to the phylogenetic relationships of the non-pterygote hexapods (Fig. 1), gene duplication and subsequent evolutionary development of discrete capa and pk genes must have occurred at least twice, in Diplura and in Zygentoma. The evolution of the type C3 pattern (Thermobia domestica, Zygentoma), which likely also represents the basic pattern in winged insects (Pterygota), took place either via the type C2 pattern or directly from the type B pattern