RNA-based gene duplication: mechanistic and evolutionary insights

Burst of young retrogenes and independent retrogene formation in mammals

Retroposition and retrogenes gain increasing attention as recent studies show that they play an important role in human new gene formation. Here we examined the patterns of retrogene distribution in 8 mammalian genomes using 4 non-mammalian genomes as a contrast. There has been a burst of young retrogenes not only in primate lineages as suggested in a recent study, but also in other mammalian lineages. In mammals, most of the retrofamilies (the gene families that have retrogenes) are shared between species. In these shared retrofamilies, 14%–18% of functional retrogenes may have originated independently in multiple mammalian species. Notably, in the independently originated retrogenes, there is an enrichment of ribosome related gene function. In sharp contrast, none of these patterns hold in non-mammals. Our results suggest that the recruitment of the specific L1 retrotransposons in mammals might have been an important evolutionary event for the split of mammals and non-mammals and retroposition continues to be an important active process in shaping the dynamics of mammalian genomes, as compared to being rather inert in non-mammals.

Repeat mediated gene duplication in the Drosophila pseudoobscura genome

Genetic mutations can occur on a wide variety of scales, including those that change single nucleotides, those that add or remove content to/from a genome, and those that change the organization of a genome. Gene duplications are a specific class of mutations that add content to a genome, and they can arise via a wide variety of mechanisms. I examined the mechanisms responsible for recently duplicated genes in the D. pseudoobscura genome, and I observed both retroposed and DNA duplications. Many duplicated genes lack signatures of either retroposition or DNA-based mechanisms, but other features of these ambiguously duplicated genes suggest that most were generated via retroposition. Furthermore, close examination of sequences flanking DNA duplications and those found at the breakpoints of chromosomal inversions suggests a connection between these two events. In Drosophila, duplicated genes near inversion breakpoints can arise via unequal genetic exchange during the non-allelic crossing over event giving rise to the inversion. I observed one duplicated gene in the D. pseudoobscura genome that appears to have been generated by this mechanism. Additionally, many DNA duplications in the D. pseudoobscura genome are flanked by a repetitive sequence also found at the breakpoints of chromosomal inversions. This suggests that the molecular mechanisms responsible for chromosomal rearrangements and some duplicated genes have overlapping processes.

Multiple duplication of the bucentaur gene family, which recruits the APE-like domain of retrotransposon: Identification of a novel homolog and distinct cellular expression

The p97bcnt/cfdp2 is a ruminant-specific gene created by a combination of gene duplication of ancestral bcnt (bucentaur) or cfdp1 (craniofacial developmental protein 1), bcnt/cfdp1, and the insertion of a retrotransposable element-1 (RTE). As a result, p97Bcnt recruits the whole apurinic/apyrimidinic endonuclease (APE)-like domain of RTE in the middle of the molecule (RTE domain) as a region encoded by an exon. In addition, p97Bcnt contains two intramolecular repeats (IRs) of 40 amino acids each in the C-terminal region, whereas Bcnt/Cfdp1 contains one IR. We have identified an additional bovine homolog with a structure highly similar to p97Bcnt, designated p97Bcnt2, which contains three IRs. p97bcnt2 is located in tandem with bcnt/cfdp1 and p97bcnt within a 177-kb range on bovine chromosome 18, a syntenic region of human chromosome 16. The gene product is expressed as a protein with an apparent molecular mass of 102 kDa. The phylogenetic tree strongly suggests that p97bcnt-2 forms a third clade of the bcnt family and that the first duplication of the IR unit occurred prior to the divergence of p97bcnt and p97bcnt-2. To address the question of whether these bcnt members have distinct functions, we first examined the expression and localization of the p97Bcnt family members. p97Bcnt is substantially expressed in many tissues involved in responses to external and internal stress. In the testis, p97Bcnt localizes preferentially in the nuclei of spermatozoa, while Bcnt/Cfdp1 localizes predominantly in the cytosol of Leydig cells and some spermatogenic cells, implying that at least these two molecules of the Bcnt family play different functional roles. These results provide evidence for the direct contribution of RTE to gene diversity to form gene families that may support cellular function.