Fig. 7

Comparisons of organisation and origin of clustered Dscam and Pcdh genes. a Schematic diagram for the sDscamα gene in Chelicerata. Symbols used are the same as in Fig. 1. Each variable cassette was transcribed by an alternative promoter followed by alternative splicing. The variable cassette encoded the N-terminal Ig1 domain (blue). Tertiary structure model of Ig1 of I. scapularis sDscam is shown on the left. b Schematic diagram for the Pcdhα genes in vertebrates. The Pcdh gene cluster contains exons that encode 14 extracellular and TM domains [18]. Each repeat is preceded by a promoter, and encodes extracellular and TM domains. Tertiary structure model of the EC1 domain of Pcdhα is shown on the left, which is similar to that of the Ig1 domain of sDscam. c, d Comparisons between the evolutionary origin and expansion of clustered Dscam (c) and Pcdh (d) genes. The 5′ clustered organisation of both sDscam and Pcdh genes may originate from the shortening and expansion of the ectodomains of canonical Dscam via sequential duplication and mutation