182 Age-related molecular variability in Caenorhabditis elegans, T. R. Golden et al. a systematic loss of genomic DNA with age (t-test 4 days vs. 19 days, P < 0.0001) (Figs 5d and 6d), to which the loss of tail nuclei we observed via DAPI staining may contribute (Fig. 1). The masses have an origin in the hermaphrodite germ line To determine the cellular source of the excess genomic DNA copies, and of the DNA masses, we examined male C. elegans. Males did not accumulate DAPI-stained masses or experience the increase in nuclear genomes observed in N2 (Figs 5c and 6c). As a potential genetic control for the dependence of this phenomenon on oocytes, we examined the germ line prolifer- ation mutant glp-4, which has few germ line nuclei at young adulthood at the restrictive temperature (Beanan & Strome, 1992). However, at the restrictive temperature almost no ampli- fication was seen from glp-4 DNA, and even at the permissive temperature, this strain appeared to have less genomic DNA than wild-type worms (Supplementary Fig. S4). Discussion A basic, unanswered question about aging is why individuals of a homogenous population experience large differences in lifespan. Understanding whether this reflects differences in individual aging rates, or the action of different mechanisms of aging in different subsets of a population is unknown and will require the characterization of aging at the level of the individual. Only recently have studies begun to examine individual aging worms and Drosophila. Gene expression profiling of individual Fig. 3 Masses of nucleic acid are not seen in daf-2(e1370) nematodes. (a) 4 days old. (b) 24 days old. (c) 45 days old. Worms were fixed and stained with DAPI to detect DNA. These individuals are representative of ten individuals of each age studied. nematodes has identified age-related gene expression changes (Golden & Melov, 2004). Study of the integrity of aged worms has uncovered a stochastic element to the deterioration asso- ciated with aging (Herndon et al., 2002). A study of Drosophila asked whether individual metabolic rates determined individual lifespan (Hulbert et al., 2004). In each case, important questions about aging were examined, which can only be answered by the use of individuals. Our results begin to address the patho- physiology of aging in C. elegans at the molecular and cellular level. We have identified dramatic changes in nuclear genome copy number and nuclear content in C. elegans with age. DAPI staining of DNA reveals that wild-type and long-lived strains of nematodes systematically lose three nuclei in the tail. DAPI staining also revealed an accumulation of masses of DNA in wild- type nematodes that filled the central body cavity by 14 days of age. Quantitation of the nuclear genome via quantitative real-time PCR revealed an increase in both mean copy number, as well as in intraindividual heterogeneity with respect to nuclear genome copy number, with age in wild-type C. elegans. The hypodermal nuclei of the tail, H9 and H10, disappear in the majority of wild-type individuals by 14 days of age, and in daf-2(e1368) nematodes by 24 days of age (Fig. 1). This likely reflects cell loss with age, but may also indicate a massive amount of DNA damage or degradation preventing DAPI bind- ing. The extent to which nuclear loss occurs in other parts of the worm is unknown at present, but this phenomenon likely contributes to the heterogeneity in nuclear DNA content we observed in middle age via qPCR (Fig. 5a). The systematic loss of nuclei with age in C. elegans is a novel observation; Herndon © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2007 No claim to original US government works