Density dependence in Caenorhabditis larval starvation

Abstract

Availability of food is often a limiting factor in nature. Periods of food abundance are followed by times of famine, often in unpredictable patterns. Reliable information about the environment is a critical ingredient of successful survival strategy. One way to improve accuracy is to integrate information communicated by other organisms. To test whether such exchange of information may play a role in determining starvation survival strategies, we studied starvation of L1 larvae in C. elegans and other Caenorhabditis species. We found that some species in genus Caenorhabditis, including C. elegans, survive longer when starved at higher densities, while for others survival is independent of the density. The density effect is mediated by chemical signal(s) that worms release during starvation. This starvation survival signal is independent of ascarosides, a class of small molecules widely used in chemical communication of C. elegans and other nematodes.

Figure 1. Some Caenorhabditis species survive L1 starvation longer when starved at high density.

(a) Survival curves of C. elegans (N2) in L1 starvation at different worm densities. (b) Survival curves of C. briggsae (JU757) in L1 starvation at different worm densities. (c) Effect of worm density during starvation on 50% survival time for various wild isolates of C. elegans and C. briggsae. Slopes of the curves reflect the magnitude of the density dependence. (d) Effect of worm density during L1 starvation on 50% survival time for several representative Caenorhabditis species. (e) Magnitude of the density effect in L1 starvation for all tested Caenorhabditis species, including various strains of C. elegans and C. briggsae. Density dependence slopes were obtained from linear regressions of 50% survival time as a function of log₂(worm density). (f) Latitude distributions of four Caenorhabditis species constructed based on data from. Arrows in C. elegans and C. briggsae distributions indicate locations where worms tested in this work (Fig. 1c) had been isolated. Note log scale for x-axes in (c) and (d).

Figure 2. Conditioned medium (CM) from high density L1s extends survival of N2 worms at low density.

(a) CM from high density N2 L1 larvae extend survival of N2 L1s at low density (0.8 worms/μl) in a dose dependent manner. Shown in black is the survival curve in M9 buffer. Numbers in the legends of (a) and (b) correspond to density of worms during conditioning (worms/μl). (b) CM from daf-22 L1 worms has survival extension effects similar to those of N2 CM. (c) CM from C. briggsae (AF16) L1s also extends survival of C. elegans (N2) in L1 starvation, though the effect appears to be smaller compared to N2 CM. (d) In contrast, neither C. elegans (N2) nor C. briggsae (AF16) CMs has an effect on L1 survival of C. briggsae (AF16) at moderate CM densities. For (c) and (d) CM was prepared at 60 worms/μl. CMs collected at 10-fold higher densities extend survival of both C. elegans and C. briggsae (Fig. S15).

Figure 3. C. elegans chemosensory mutants have diminished density dependence compared to wild-type.

Magnitude of the density effect on L1 starvation for mutants with impaired sensing of soluble chemicals (red), thermosensory and olfactory mutants (blue), worms engineered to lack ASE or ASK chemosensory neurons (yellow), and wild type (black).

Figure 4. Active component is released after hatching and is necessary past 24 hours to affect survival.

N2 L1 worms were starved continuously at high density in the same medium (control HD), at high density but with a wash and fresh buffer after the first 24 hours of starvation (washed HD), continuously at low density (control LD), at high density for the first 24 hours and then diluted to low density (HD to LD).

Figure 5. CM fractions have a synergistic effect on starvation survival.

Survival curves of N2 L1s in M9, unfractionated CM, four individual CM fractions (flow-through and water rinse (FT + WR), 50% methanol, methanol, propanol-2), and a combination of all four fractions. The four fractions had no or little effect on starvation survival individually but had a strong positive effect similar to that of unfractionated CM when applied in combination.