a, Schematic illustration of generation of SQC0002 strain. CB4856 was crossed with Pbas-1::bas-1::gfp transgenic worms (with N2 genetic background), and about a quarter of aged F2 progeny showed an elevated level of BAS-1::GFP expression. The F2 progeny with high expression levels of BAS-1 were backcrossed with N2 worms eight times; the resulting strain was named SQC0002. b, Left, expression of BAS-1::GFP in SQC0002 and N2 worms at day 9 of adulthood. Scale bar, 10 μm. Representative of n = 5 independent experiments. Right, quantification of BAS-1::GFP fluorescence intensity. GFP fluorescence was normalized to average fluorescence intensity of age-matched N2 worms. c, Mating efficiency of SQC0002, CB4856, and N2 males at a range of ages. d, Whole-genome sequencing of SQC0002 worms identified a 328-kb region in chromosome I that possessed enriched CB4856 alleles. e, Age-dependent changes in mating efficiency in N2, SQC0002, and SQC0002 males with single-copy transgene of empty vector (SQC0002;EV) or rgba-1 (SQC0002;rgba-1). All data shown are mean ± s.e.m. *P < 0.05, **P < 0.01, ***P < 0.001 (b, two-sided t test; c, e, ANOVA with Dunnett’s test). For b, c and e, each data point represents the result of one independent experiment. The numbers of tested worms (b) and mating plates (c, e) are shown beneath the bars.