Intron evolution and small non-coding RNAs

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As we already knew, many spliceosomal introns exist in eukaryotic genes. These introns are extra bits of DNA sequences that have to be spliced out after the genes were transcribed into RNA (and hence the name spliceosomal introns). Two natural questions that should be raised are when did these introns originate and what is their role? Solving these questions is vital for understanding the evolution of the genome. We have carried out the following works : (1)Construction of a database of the ribosomal protein (RP) genes ; (2)Construction of a database of small nucleolar RNAs (snoRNAs) ; (3)Inference of the process of intron gain and loss during eukaryotic evolution ; (4)Comparison of mitochondrial and cytoplasmic RP genes ; and (5)Analysis of introns using zebrafish.The results show that overall intron gains are 〜25% greater than intron losses and the mitochondrial RP genes were intronless at the time of endosymbiosis. We propose that eukaryotic introns function as carriers for non-coding RNAs and, through the process of intron gain and loss, may affect the level and variation of expression of non-coding RNAs. Consequently, the speciation of eukaryotes may have occurred through the process of intron gain and loss during evolution, giving birth to various present-day biological systems.