Untitled

Abstract

Origin and subsequent accumulation of spliceosomal introns are prominent events in the evolution of eukaryotic gene structure. Recently gained introns would be especially useful for the study of the mechanisms of intron gain because randomly accumulated mutations might erase the evolutionary traces. The mechanisms of intron gain remain unclear due to the presence of very few solid cases. A widely cited model of intron gain is tandem genomic duplication, in which the duplication of an AGGT-containing exonic segment provides the GT and AG splicing sites for the new intron. We found that the second intron of the potato RNA-dependent RNA polymerase gene PGSC0003DMG402000361 originated mainly from a direct duplication of the 3 side of the upstream intron. The 5 splicing site of this new intron was recruited from the upstream exonic sequence. In addition to the new intron, a downstream exonic segment of 178 bp also arose from duplication. Most of the splicing signals were inherited directly from the parental intron/exon structure, including a putative branch site, the polypyrimidine tract, the 3 splicing site, two putative exonic splicing enhancers and the GC contents differentiated between the intron and exon. We propose a new version of the tandem genomic duplication model, termed as the partial duplication of the preexisting intron/exon structure. This new version and the widely cited version are not mutually exclusive.3