The excess of 5′ introns in eukaryotic genomes

In this work, 21 completely sequenced eukaryotic genomes were analyzed using an intragene comparison approach. We found that all of these genomes show a significant 5'-biased distribution of introns of protein-coding genes. Our findings are different from previous studies based on the intergene method, where introns are biased towards the 5' end of genes only in intron-poor genomes, but are evenly distributed in intron-rich genomes. In addition, by analyzing the patterns of intron distribution of a set of well-compiled housekeeping genes from human and their respective orthologs identified by a bidirectional best BLAST hit method from the other genomes, we found that the trend of 5'-biased intron positions of the set of housekeeping genes for each genome is much more skewed than that of all genes of the same genome, and rarely if any of the housekeeping genes examined have an extremely 3'-biased position distribution in which all introns of a gene are located only at the 3' portion of the gene. The most parsimonious explanation for our findings may be the model in which intron loss is caused by homologous recombination between the genomic copy of a gene and a reverse transcriptase product of a spliced mRNA.