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

C. elegans strains with the heat-sensitive mortal germline (Mrt) phenotype
become progressively sterile over the course of a few tens of generations
when maintained at temperatures near the upper range of C. elegans'
tolerance. Mrt is transgenerationally-heritable, and proximately under
epigenetic control. 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 UMrt≈ 6 10-5/genome/generation. We
detected no mutations to a weak Mrt phenotype. 6/95 wild isolates have 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 ≈0.1%, 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.