MBE Advance Access originally published online on August 30, 2006
Molecular Biology and Evolution 2006 23(12):2259-2262; doi:10.1093/molbev/msl098
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Letters |
Smoke Without Fire: Most Reported Cases of Intron Gain in Nematodes Instead Reflect Intron Losses
Allan Wilson Centre for Molecular Ecology and Evolution, Massey University, Palmerston North, New Zealand
E-mail: scottwroy{at}gmail.com.
Identification of recently gained spliceosomal introns would provide crucial evidence in the continuing debate concerning the age and evolutionary significance of introns. A previously published genomic analysis reported to have identified 122 introns that had been gained since the divergence of the nematodes Caenorhabidits elegans and Caenorhabditis briggsae 
100 MYA. However, using newly available genomic sequence from additional Caenorhabditis species, we show that 74% (60/81) of the reported gains in C. elegans are present in a C. briggsae relative. This pattern indicates that these introns represent losses in C. briggsae, not gains in C. elegans. In addition, 61% (25/41) of the reported gains in C. briggsae are present in the more distant C. briggsae relative, in a pattern suggesting that additional reported gains in C. elegans and/or C. briggsae may in fact represent unrecognized losses. These results underscore the dominance of intron loss over intron gain in recent eukaryotic evolution, the pitfalls associated with parsimony in inferring intron gains, and the importance of genomic sequencing of clusters of closely related species for drawing accurate inferences about genome evolution.
Key Words: intron gain • genome complexity • genome annotation • genome sequencing • genome evolution • parsimony
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