Cysteine protease cathepsin B promotes high population density-induced mutagenesis, driving genome evolution and competitive growth in response to the crowding stress

In a wide variety of species from invertebrates to mammals, overpopulation has been shown to induce low fertility and high mortality. Although density-dependent population regulation is widespread in the animal kingdom, the underlying molecular mechanisms remain poorly understood. We show here that <i>C. elegans</i> animals respond to the crowding stress by secreting CPR-4, a homologue of human cathepsin B cysteine protease, to promote chromosomal DNA damage in germ cells, leading to high density-induced deficiencies that include increased embryonic lethality and larval arrest and decreased brood size. CPR-4 mediates these crowding responses through the insulin-like growth factor receptor DAF-2, multiple components in the insulin signaling pathway, and the SKN-1/Nrf transcription factor. Whole genome sequencing analyses of animals from 10 generations of continual growth in the crowded condition reveal that CPR-4-induced DNA damage produces an average of 2.9 more de novo genome mutations per animal per generation and a 75% increase in mutation rate compared with animals grown in the uncrowded condition. CPR-4-induced mutagenesis also facilitates evolution of the genomes through multi-generational crowding selection, leading to biased mutation distributions towards the intergenic regions over the gene bodies and crowd-inducible growth advantage. Our findings suggest that CPR-4 acts as a crucial crowd-responding factor to induce chromosomal DNA damage, leading to density-dependent deficiencies, increased genome mutation rates, and genome evolution and competitive growth of animals in response to the crowding stress.