De novo ORFs are more likely to shrink than to elongate during neutral evolution.

For protein coding genes to emerge de novo from a non-genic DNA, the DNA sequence must gain an open reading frame (ORF) and the ability to be transcribed. The newborn de novo gene can further evolve to accumulate changes in its sequence. Consequently, it can also elongate or shrink with time. Existing literature shows that older de novo genes have longer ORF, but it is not clear if they elongated with time or remained of the same length since their inception. To address this question we developed mathematical model of ORF elongation as a Markov-jump process, and show that ORFs tend to keep their length in short evolutionary timescales. We also show that if change occurs it is likely to be a truncation. Our genomics and transcriptomics data analyses of seven Drosophila melanogaster populations is also in agreement with the models prediction. We conclude that selection could facilitate ORF length extension that may explain why longer ORFs were observed in old de novo genes in studies analysing longer evolutionary time scales. SignificanceNew protein coding genes can emerge from non-genic DNA through a process called de novo gene emergence. Genes thus emerged usually have a small open reading frame (ORF). However, studies show that de novo genes with an older evolutionary origin have longer ORF than younger genes. To understand how ORF length evolves, we use a combination of mathematical modeling and population level genome data analysis. We find that in the absence of evolutionary selection, ORFs tend to become shorter than becoming longer. Therefore, long ORFs are probably selected by evolution to be retained in the genome.