Relaxation of natural selection in the evolution of the giant lungfish genomes.

Nonadaptive hypotheses on the evolution of eukaryotic genome size predict an expansion when purifying selection is weaker. Accordingly, species with huge genomes, such as lungfish, are expected to show a genome-wide relaxation signature of selection compared with other organisms. However, few studies have empirically tested this prediction using genomic data in a comparative framework. Here we show i) that the newly assembled transcriptome of the Australian lungfish, Neoceratodus forsteri, is characterized by an excess of pervasive transcription, or transcriptional leakage, possibly due to suboptimal transcriptional control, and ii) a significant relaxation signature in coding genes in lungfish species compared with other vertebrates. Based on these observations, we propose that the largest known animal genomes evolved in a nearly neutral scenario whereby genome expansion is less efficiently constrained.