Evolution of germ-line signals that regulate growth and aging in nematodes

Abstract

We show that a signal from the germ line represses growth in the nematode Caenorhabditis elegans. Laser-microbeam ablation of cells that give rise to the germ line causes adults to become giant. Ablation of these cells in self-sterile mutant worms also causes gigantism, suggesting that the germ line represses growth because it is the source of a growth-antagonizing signal rather than because of a sink of resources required for reproduction. The C. elegans germ line also emits a signal that represses longevity. This longevity-repressing signal requires the activity of DAF-16, a forkhead/winged-helix transcription factor, but we find that that the growth-repressing signal does not. The growth-repressing signal also does not require the activity of DBL-1, a transforming growth factor beta-related protein that promotes growth in worms. By ablating the germ-line precursors of other species of free-living nematodes, we also found that both the growth-repressing and longevity-repressing signals are evolutionarily variable. Some species have both signals; others have just one or the other. We suggest that variation in germ-line signaling contributes to body size and life-history diversity in the nematodes.

Figure 1

Growth and survival of wild-type N2 grown at 20°C after ablation of germ-line and somatic precursor cells in hatchling nematodes. (a) The four precursor cells seen in a hatchling. Z1 and Z4 give rise to the somatic gonad and Z2 and Z3 give rise to the germ line. Growth (b, volume, mm³) and survival (c, days) of N2 hermaphrodites in which the germ-line precusors, Z2 and Z3 (solid line) were ablated or else left intact (♦). ■, Z2/3(−); ▴, Z1–4(−). n, total number of nematodes observed, followed by the number of independent experiments performed in parentheses; v, mean maximum body size (±SE); l = mean longevity (±SE). P values are based on a comparison of ablated and intact animals. (b) Intact control, n = 194(3), v = 0.0058 mm³ (± 0.00008); Z2/3(−), n = 65(2), v = 0.0084 mm³ (± 0.0002), P < 0.0001. (c) Intact control, n = 366(3), l = 9.9 (± 0.2); Z2/3(−), n = 67(2), l = 12.9 (± 0.5), P < 0.0001. (d) A wild-type hermaphrodite with intact gonads (Upper) and a germ-line-ablated hermaphrodite (Lower), both grown at 20°C for 11 days from hatching.

Figure 2

Responses of fer-6, daf-16, and dbl-1 mutants to ablation of germ-line precursors, Z2/3. Intact control (⧫) and Z2/3(−) (solid line). (a and b) fer-6 (hc6), control, and Z2/3(−) nematodes were reared at 25°C. Intact control, n = 29(1), v = 0.0031 mm³ (± 0.0002); Z2/3(−), n = 26(1), v = 0.0044 mm³ (± 0.0003), P < 0.0001; intact control, n = 29(1), l = 8.7 (± 0.4); Z2/3(−), n = 29(1), l = 11.0 (± 0.6), P = 0.0003. A wild-type N2 experiment was run alongside at 25°C; intact control, n = 35(1), v = 0.0074 mm³ (± 0.0002); Z2/3(−), n = 55(1), v = 0.0084 mm³ ± (0.0002), P < 0.0001; intact control, n = 29(1), l = 9.9 ± 0.4; Z2/3(−), n = 55(1), l = 15.3 (± 0.8), P < 0.0001. (c and d) daf-16 (mu86), intact control, n = 51(2), v = 0.0086 mm³ (± 0.0004); Z2/Z3(−), n = 59(2), v = 0.0114 mm³ (± 0.0002), P < 0.0001; intact control, n = 51(2), l = 10.8 (± 0.4); Z2/3(−), n = 59(2), l = 11.7 (± 0.4), P = 0.3. In one experiment, all of the gonadal precursor cells Z1-Z4(−) were ablated as well: n = 28(1), v = 0.0108 mm³ (± 0.0003), l = 11.6 (± 0.6). Z2/3(−) versus Z1-Z4(−), P = 0.8 for body size and P = 0.8 for survival. (e and f) dbl-1 (nk3), intact control, n = 44(2), v = 0.0035 mm³ (± 0.0001); Z2/3(−), n = 50(2), v = 0.0055 mm³ (± 0.0001), P < 0.0001; intact control, n = 48(2), l = 14.3 (± 1.1); Z2/3(−), n = 55(2), l = 20.2 (± 1.4), P < 0.001.

Figure 3

Response of selected free-living nematode species to ablation of germ-line precursor cells, Z2/3. Intact control worms (♦); Z2/3(−) worms (▪). (a and b) Caenorhabditis briggsae (AF16), intact control n = 35(2); v = 0.0044 mm³ (± 0.0001); Z2/3(−), n = 44(2), v = 0.0056 mm³ (± 0.0001), P < 0.0001; intact control, n = 45(2), l = 18.6 ± (0.8); Z2/3(−), n = 44(2), l = 26.2 (± 1.2), P < 0.0001. (c and d) Rhabditis sp. (PS1191), intact control, n = 25(1), v = 0.0138 mm³ (± 0.0005); Z2/3(−), n = 27(1), v = 0.0158 mm³ (± 0.0005), P = 0.007; intact control, n = 34 (1), l = 12.9 (± 0.8); Z2/3(−), n = 31(1), l = 14.2 (± 0.7), P = 0.2. (e and f) O. sp. (CEW1), intact control, n = 15(1), v = 0.0017 mm³ (± 0.0003); Z2/3(−), n = 20(1), v = 0.0033 mm³ (± 0.0008), P < 0.0001; intact control, n = 15(1), l = 9.2 (± 0.4); Z2/3(−), n = 20(1), l = 9.9 (± 0.4), P = 0.2. (g and h) O. myriophila (BW290), intact control, n = 54(1), v = 0.0200 mm³ (± 0.0008); Z2/3(−), n = 30(1), v = 0.0260 mm³ (± 0.00009), P < 0.0001; intact control, n = 54 (1); l = 12.9 (± 0.4); Z2/3(−), n = 30(1); l = 13.4 (± 0.5), P = 0.6. (i and j) P. pacificus (PS1843), intact control, n = 42(2), v = 0.0095 mm³ (± 0.0002); Z2/3(−), n = 21(2), v = 0.0083 mm³ (± 0.0004), P < 0.0001; intact control, n = 42(2), l = 17.1 (± 0.9); Z2/3(−), n = 21(2), l = 27.1 (± 2.5), P < 0.0001. (k and l) P. maupasi (PS321), intact control, n = 30(2), v = 0.0119 mm³ (± 0.0003); Z2/3(−), n = 36(2), v = 0.0097 mm³ (± 0.0002), P < 0.0001; intact control, n = 30(2), l = 25.6 (± 1.4); Z2/3(−), n = 36(2), l = 36.3 (± 1.5), P < 0.0001.

Figure 4

Intact (Upper) and Z2/3(−) ablated (Lower) 11-day-old nematodes of O. sp. (CEW1; a and b) and P. pacificus (c and d).