De novo emergence, existence, and demise of a protein-coding gene in murids.

Background: Genes, principal units of genetic information, vary in complexity and evolutionary history. Less-complex genes (e.g., long non-coding RNA (lncRNA) expressing genes) readily emerge de novo from non-genic sequences and have high evolutionary turnover. Genesis of a gene may be facilitated by adoption of functional genic sequences from retrotransposon insertions. However, protein-coding sequences in extant genomes rarely lack any connection to an ancestral protein-coding sequence.

Results: We describe remarkable evolution of the murine gene D6Ertd527e and its orthologs in the rodent Muroidea superfamily. The D6Ertd527e emerged in a common ancestor of mice and hamsters most likely as a lncRNA-expressing gene. A major contributing factor was a long terminal repeat (LTR) retrotransposon insertion carrying an oocyte-specific promoter and a 5' terminal exon of the gene. The gene survived as an oocyte-specific lncRNA in several extant rodents while in some others the gene or its expression were lost. In the ancestral lineage of Mus musculus, the gene acquired protein-coding capacity where the bulk of the coding sequence formed through CAG (AGC) trinucleotide repeat expansion and duplications. These events generated a cytoplasmic serine-rich maternal protein. Knock-out of D6Ertd527e in mice has a small but detectable effect on fertility and the maternal transcriptome.

Conclusions: While this evolving gene is not showing a clear function in laboratory mice, its documented evolutionary history in Muroidea during the last ~ 40 million years provides a textbook example of how a several common mutation events can support de novo gene formation, evolution of protein-coding capacity, as well as gene's demise.