In with the Old, in with the New: The Promiscuity of the Duplication Process Engenders Diverse Pathways for Novel Gene Creation
Figure 2
Small-scale DNA-mediated duplication events can yield daughter loci with varying degrees of structural resemblance to the ancestral copy depending on the extent of the duplication span and the location of the duplication breakpoints. Rectangles represent exons and solid horizontal lines through exons denote introns and flanking region sequences. Corresponding colours between the ancestral (top) and the derived (bottom) locus denote sequence homology. (a) Complete gene duplication wherein the duplication event spans, at a minimum, the entire ORF of the ancestral locus from the initiation codon to the termination codon. The duplication event may or may not encompass upstream and downstream flanking region sequences. In the schematic, complete duplication of an ancestral locus yields a derived copy (bottom) comprising three exons and intervening introns as well as some 5′ and 3′ flanking region sequences. (b) Partial gene duplication wherein only a portion of the ancestral ORF is duplicated. In the schematic, the downstream duplication breakpoint occurred within intron 2 of the ancestral copy (top), yielding a truncated derived copy (bottom) comprising some of the ancestral 5′ flanking region sequence, exons 1 and 2 and part of intron 2. (c) Chimeric gene duplication or partial gene duplication with recruitment. In instances wherein the derived locus (bottom) has unique exon(s) to the exclusion of the ancestral copy, the type of duplication event depends on the genomic source(s) of the unique coding region(s). In the schematic, the derived copy (bottom) has a unique exon 3 (yellow) to the exclusion of the ancestral locus (top). If a BLAST query of the unique exonic sequence yields no hits in the genome, it is suggestive of a partial gene duplication event and subsequent recruitment of intergenic sequence by the duplicate copy from its new genomic neighbourhood to yield an intact ORF. To qualify as a chimeric duplicate, this unique coding region of the derived locus must also exhibit evidence of duplication from another genomic source, be it intergenic, genic or a combination of genic and intergenic regions. The creation of such fusion genes could have occurred as a single evolutionary event (a single duplication event encompassing portions of two adjacent genes) or may represent independent duplication events and subsequent fusion of these fragments via shuffling.
