Figure 5: Highly complex combinations of sDscam 5′ variable exons.
From: A large family of Dscam genes with tandemly arrayed 5′ cassettes in Chelicerata
(a) Schematic diagram for splicing patterns of the 5′ variable exons. Symbols used are the same as in Fig. 1. Canonical splicing isoforms were joined in neighbouring junctions in variable cassettes, according to the previous ‘cap-proximal splicing’ model19. Non-canonical splicing isoforms included: (I) splicing isoforms from the same cassette with either exon 2, 3 or 4 skipped; (II) splicing isoforms that contained variable exons from tandem cassettes; as well as (III) the isoforms that contained within-cassette introns. (b) Quantification of the canonical and non-canonical splicing isoforms. (c) Schematic diagram of the splicing patterns of the 5′ variable exons in M. martensii sDscamβ1. Splice isoforms within a single tandem cassette are shown as a black line above the gene structure diagram, while splice isoforms from different tandem cassettes are represented below by coloured lines. (d) Alternative splicing junctions from different cassettes were validated using reverse transcription–PCR (RT–PCR). Due to the low expression of sDscam variable exons, nested PCR was necessary to amplify the products; only the primers used in the second PCR are depicted and same in panels below. The RT–PCR products were confirmed by cloning and sequencing. These experiments revealed the splicing of multiple cassette variants from different tandem cassettes. (e) Splicing patterns of the 5′ variable exons in sDscamβ3. (f) RT–PCR was used to detect alternative splice isoforms in sDscamβ3. (g) A summary of several types of isoforms with distinct Ig numbers generated by alternative splicing.
