Fig. 2

Effect of individual and parental age on gonad size. a Experimental setup, indicating relationship between old worms (O), young worms (Y), offspring of old worms (O-offspring), and offspring of young worms (Y-offspring). Arrows indicate aging and fission events. Stars indicate the four groups of worms that were fixed for in situ hybridization analysis. b–e Gonad region of representatives from the four groups of worms analyzed for PRIle-piwi1 in situ hybridization. Black arrowheads indicate the presence of gonad expression; white arrowheads indicate the absence of gonad expression; t and o indicate testis and ovary, respectively. Asterisk in b indicates the second (right) testis that is also in focus in this image. f Gonad size in the four groups of worms. The oldest group (O) has significantly smaller gonads than the youngest group (Y-offspring) (W = 0, p = 0.0016), the offspring of old parents (O-offspring) have significantly smaller gonads than the offspring of young parents (Y-offspring) (W = 1, p = 0.0031), and O-offspring have significantly larger gonads than Y individuals (which are older offspring of the same parents) (W = 86, p value = 0.0028). O and Y were not significantly different from each other (W = 89, p = 0.2991). Gonad size was measured as the combined area of one testis and one ovary (either left or right) for each worm