R. Robilotto

Comparative analysis of processed ribosomal protein pseudogenes in four mammalian genomes

BackgroundThe availability of genome sequences of numerous organisms allows comparative study of pseudogenes in syntenic regions. Conservation of pseudogenes suggests that they might have a functional role in some instances.ResultsWe report the first large-scale comparative analysis of ribosomal protein pseudogenes in four mammalian genomes (human, chimpanzee, mouse and rat). To this end, we have assigned these pseudogenes in the four organisms using an automated pipeline and make the results available online. Each organism has a large number of ribosomal protein pseudogenes (approximately 1,400 to 2,800). The majority of them are processed (generated by retrotransposition). However, we do not see a correlation between the number of pseudogenes associated with a ribosomal protein gene and its mRNA abundance. Analysis of pseudogenes in syntenic regions between species shows that most are conserved between human and chimpanzee, but very few are conserved between primates and rodents. Interestingly, syntenic pseudogenes have a lower rate of nucleotide substitution than their surrounding intergenic DNA. Moreover, evidence from expressed sequence tags indicates that two pseudogenes conserved between human and mouse are transcribed. Detailed analysis shows that one of them, the pseudogene of RPS27, is likely to be a protein-coding gene. This is significant as previous reports indicated there are exactly 80 ribosomal protein genes encoded by the human genome.ConclusionsOur analysis indicates that processed ribosomal protein pseudogenes abound in mammalian genomes, but few of these are conserved between primates and rodents. This highlights the large amount of recent retrotranspositional activity in mammals and a relatively larger amount of it in the rodent lineage.

Comprehensive analysis of the pseudogenes of glycolytic enzymes in vertebrates: the anomalously high number of GAPDH pseudogenes highlights a recent burst of retrotrans-positional activity

BackgroundPseudogenes provide a record of the molecular evolution of genes. As glycolysis is such a highly conserved and fundamental metabolic pathway, the pseudogenes of glycolytic enzymes comprise a standardized genomic measuring stick and an ideal platform for studying molecular evolution. One of the glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has already been noted to have one of the largest numbers of associated pseudogenes, among all proteins.ResultsWe assembled the first comprehensive catalog of the processed and duplicated pseudogenes of glycolytic enzymes in many vertebrate model-organism genomes, including human, chimpanzee, mouse, rat, chicken, zebrafish, pufferfish, fruitfly, and worm (available at http://pseudogene.org/glycolysis/). We found that glycolytic pseudogenes are predominantly processed, i.e. retrotransposed from the mRNA of their parent genes. Although each glycolytic enzyme plays a unique role, GAPDH has by far the most pseudogenes, perhaps reflecting its large number of non-glycolytic functions or its possession of a particularly retrotranspositionally active sub-sequence. Furthermore, the number of GAPDH pseudogenes varies significantly among the genomes we studied: none in zebrafish, pufferfish, fruitfly, and worm, 1 in chicken, 50 in chimpanzee, 62 in human, 331 in mouse, and 364 in rat. Next, we developed a simple method of identifying conserved syntenic blocks (consistently applicable to the wide range of organisms in the study) by using orthologous genes as anchors delimiting a conserved block between a pair of genomes. This approach showed that few glycolytic pseudogenes are shared between primate and rodent lineages. Finally, by estimating pseudogene ages using Kimuras two-parameter model of nucleotide substitution, we found evidence for bursts of retrotranspositional activity approximately 42, 36, and 26 million years ago in the human, mouse, and rat lineages, respectively.ConclusionOverall, we performed a consistent analysis of one group of pseudogenes across multiple genomes, finding evidence that most of them were created within the last 50 million years, subsequent to the divergence of rodent and primate lineages.