Mutation, selection, and the prevalence of the C. elegans mortal germline phenotype

Abstract

C. elegans strains with the mortal germline (Mrt) phenotype become progressively sterile over the course of a few tens of generations. Mrt is proximately controlled epigenetically, and is typically temperature-dependent, being penetrant at temperatures near the upper range of C. elegans’ tolerance. Previous studies have suggested that Mrt presents a relatively large mutational target, and that Mrt is not uncommon in natural populations of C. elegans. The Mrt phenotype is not monolithic. Some strains exhibit a strong Mrt phenotype, in which individuals invariably become sterile over a few generations, whereas other strains show a weaker (less penetrant) phenotype in which the onset of sterility is slower and more stochastic. We present results in which we (1) quantify the rate of mutation to the Mrt phenotype, and (2) quantify the frequency of Mrt in a collection of 95 wild isolates. Over the course of ~16,000 meioses, we detected one mutation to a strong Mrt phenotype, resulting in a point estimate of the mutation rate ⋃_Mrt≈ 6×10⁻⁵/genome/generation. We detected no mutations to a weak Mrt phenotype. 5/95 wild isolates had a strong Mrt phenotype, and although quantification of the weak Mrt phenotype is inexact, the weak Mrt phenotype is not rare in nature. We estimate a strength of selection against mutations conferring the strong Mrt phenotype , similar to selection against mutations affecting competitive fitness. The appreciable frequency of weak Mrt variants in nature combined with the low mutation rate suggests that Mrt may be maintained by balancing selection.