Genetic dissection of late-life fertility in Caenorhabditis elegans

Abstract

The large post-reproductive life span reported for the free-living hermaphroditic nematode, Caenorhabditis elegans, which lives for about 10 days after its 5-day period of self-reproduction, seems at odds with evolutionary theory. Species with long post-reproductive life spans such as mammals are sometimes explained by a need for parental care or transfer of information. This does not seem a suitable explanation for C elegans. Previous reports have shown that C elegans can regain fertility when mated after the self-fertile period but did not report the functional limits. Here, we report the functional life span of the C elegans germ line when mating with males. We show that C elegans can regain fertility late in life (significantly later than in previous reports) and that the end of this period corresponds quite well to its 3-week total life span. Genetic analysis reveals that late-life fertility is controlled by conserved pathways involved with aging and dietary restriction.

Figure 1.

Methods and results of measuring late-life fertility of Caenorhabditis elegans hermaphrodites (N2 CGCb), depleted for self-sperm. (A) Experimental design showing method for assessment of late-life fertility in individual hermaphrodites (see Experimental Procedures for additional description). (B) Fertility and survival of wild-type hermaphrodites that were either mated on various days of adulthood or left unmated. Each horizontal bar shows daily fertility colorimetrically and survival of an individual hermaphrodite; matings and counts started at Day 8 of adulthood. (C) Summary data from several different experiments. Percent of hermaphrodites at various adult ages that were able to regain fertility and produce any cross-progeny (n = 10–25); for Day 15 no hermaphrodites produced any cross-progeny in three independent experiments (n = 60).

Figure 2.

DIC micrographs of young and old hermaphrodite gonads (N2 CGCb). (A) Typical self-fertile hermaphrodite on the second day of adulthood (A, B, and E) white scale bar is 50 μM. Black arrows indicate the flow of cells, starting with the germ cells in the germ line and ending with oocytes as labeled. (B) Shows 2× enlargement of the region of the gonad containing the oocytes and spermatheca. The white arrow points to the proximal oocyte, nearest the spermatheca (labeled “sp”) and the next oocyte to be fertilized. (C) Typical sperm-depleted gonad on the 11th day of adulthood. Small white arrows point to unengulfed cells remaining in the flaccid gonad sheath. (D) The same gonad as in (C) after 1 hour of mating. The flaccid space, previously occupied by unengulfed cells is now occupied by a proximal oocyte (white arrow) awaiting ovulation and fertilization. We observe this rapid “rejuvenation” under the microscope in 30%–60% of animals that are able to produce progeny (82 mated wild-type gonads were examined from 41 individuals). (E) Typical gonad of an unmated 11-day-old adult eat-2(ad465) mutant (30 unmated gonads, from 15 individuals; see also Supplementary Figure 1).

Figure 3.

Frequency and magnitude of late-life fertility after cessation of self-reproduction. (A) Percent of wild-type and Age (long-lived) mutant hermaphrodites that are able to produce cross-progeny after self-sperm depletion. (B) Mean (± SEM) number of cross-progeny produced for wild-type and Age mutants after mating on different days of adulthood. Progeny production was measured for at least 15 individuals for each data point.

Figure 4.

Late-life progeny production after ad libitum feeding or dietary restriction by bacterial deprivation (BD). (A) Progeny production (mean ± SEM) for wild-type and mutant animals mated on Day 11 of adulthood after being fed ad libitum or after a period of BD (see Experimental Procedures and Table 1). “Wild-type ts” denotes wild-type animals that were raised at 25°C and then shifted to 20°C, as controls for the temperature sensitive pha-4(zu225) allele. (B) Progeny production after mating on Day 14 of adulthood. Data represent all trials combined (see also Table 1 and Supplementary Excel File 1 for individual trial data).

Figure 5.

Graphical representation of genetic effects on life span and cross-fertility under AL and DR conditions. A colorimetric table summarizing the effects of genotype on life span and late-life cross-fertility is shown. The references for the reported life span effects are presented in Table 2.