Costly circRNAs, Effective Population Size, and the Origins of Molecular Complexity

While much excitement has attended the discovery and study of circular RNAs, a new study in Cell Reports suggests that most mammalian circRNAs are not only functionless, but in fact costly. Comparison across three species is also consistent with the influential but rarely tested Drift-Barrier Hypothesis of molecular complexity. According to this hypothesis, nonessential genomic elements are slightly deleterious elements that fix by genetic drift and, thus, are generally more abundant in species with small effective population sizes. I discuss the implications of these new results for the Drift-Barrier hypothesis. In particular, I note the distinction between two classes of genomic elements, based on whether they are created by 'standard' small-scale mutations (basepair substitutions, indels, etc.) or larger, more idiosyncratic mutations (segmental duplications, transposable element propagation, etc.) I suggest that the Drift-Barrier Hypothesis is likely to apply to the former class, but perhaps not the latter class.

Automated summary

The study suggests that most mammalian circRNAs may be functionless and costly, aligning with the Drift-Barrier Hypothesis where nonessential genomic elements are more abundant in species with small population sizes. The distinction between genomic elements created by small-scale mutations and larger, more idiosyncratic mutations is key to understanding the implications of the study for the Drift-Barrier Hypothesis.