FIGURE 3 | Models and examples of intron loss.

A | The two main models of intron loss. The classical model of intron loss is shown in panel Aa^{101, 102, 103, 104, 105, 106}. A gene is transcribed and the intron spliced out. This spliced transcript is then reverse transcribed and the resultant cDNA undergoes recombination with the genomic copy, leading to intron loss. Panel Ab shows the genomic deletion model of intron loss^{72, 107, 108}, which leads to exact or inexact deletion of the DNA sequence that encodes the intron sequence. B | An apparent example of intron loss by genomic deletion. A genomic deletion of most of an intron sequence from the jingwei gene in Drosophila teissieri left a 12-bp residue¹⁰. This residual 12 bp is an effective insertion of 4 codons, encoding the amino-acids serine, proline, phenylalanine and proline in the new allele. This case represents the only known case of an intron presence/absence polymorphism within a species. The new intron-loss allele segregates at 77% in the species and is associated with a decrease in expression levels. Population studies show that the new intron-loss allele is evolving under positive selection, although it is not known whether this is due to the coding sequence insertion, the change in mRNA level, or the absence of the intron itself. Data are from Ref. 10.

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