Macro-level Modeling of the Response of <i>C. elegans</i> Reproduction to Chronic Heat Stress

A major goal of systems biology is to understand how organism-level behavior arises from a myriad of molecular interactions. Often this involves complex sets of rules describing interactions among a large number of components. As an alternative, we have developed a simple, macro-level model to describe how chronic temperature stress affects reproduction in C. elegans. Our approach uses fundamental engineering principles, together with a limited set of experimentally derived facts, and provides quantitatively accurate predictions of performance under a range of physiologically relevant conditions. We generated detailed time-resolved experimental data to evaluate the ability of our model to describe the dynamics of C. elegans reproduction. We find considerable heterogeneity in responses of individual animals to heat stress, which can be understood as modulation of a few processes and may represent a strategy for coping with the ever-changing environment. Our experimental results and model provide quantitative insight into the breakdown of a robust biological system under stress and suggest, surprisingly, that the behavior of complex biological systems may be determined by a small number of key components.</p

Macro-level Modeling of the Response of C. elegans Reproduction to Chronic Heat Stress

A major goal of systems biology is to understand how organism-level behavior arises from a myriad of molecular interactions. Often this involves complex sets of rules describing interactions among a large number of components. As an alternative, we have developed a simple, macro-level model to describe how chronic temperature stress affects reproduction in C. elegans. Our approach uses fundamental engineering principles, together with a limited set of experimentally derived facts, and provides quantitatively accurate predictions of performance under a range of physiologically relevant conditions. We generated detailed time-resolved experimental data to evaluate the ability of our model to describe the dynamics of C. elegans reproduction. We find considerable heterogeneity in responses of individual animals to heat stress, which can be understood as modulation of a few processes and may represent a strategy for coping with the ever-changing environment. Our experimental results and model provide quantitative insight into the breakdown of a robust biological system under stress and suggest, surprisingly, that the behavior of complex biological systems may be determined by a small number of key components

Balanced Trade-Offs between Alternative Strategies Shape the Response of <i>C. elegans</i> Reproduction to Chronic Heat Stress

To ensure long-term reproductive success organisms have to cope with harsh environmental extremes. A reproductive strategy that simply maximizes offspring production is likely to be disadvantageous because it could lead to a catastrophic loss of fecundity under unfavorable conditions. To understand how an appropriate balance is achieved, we investigated reproductive performance of C. elegans under conditions of chronic heat stress. We found that following even prolonged exposure to temperatures at which none of the offspring survive, worms could recover and resume reproduction. The likelihood of producing viable offspring falls precipitously after exposure to temperatures greater than 28°C primarily due to sperm damage. Surprisingly, we found that worms that experienced higher temperatures can recover considerably better, provided they did not initiate ovulation. Therefore mechanisms controlling this process must play a crucial role in determining the probability of recovery. We show, however, that suppressing ovulation is only beneficial under relatively long stresses, whereas it is a disadvantageous strategy under shorter stresses of the same intensity. This is because the benefit of shutting down egg laying, and thus protecting the reproductive system, is negated by the cost associated with implementing this strategy – it takes considerable time to recover and produce offspring. We interpret these balanced trade-offs as a dynamic response of the C. elegans reproductive system to stress and an adaptation to life in variable and unpredictable conditions.</p

Sex Pheromones of C. elegans Males Prime the Female Reproductive System and Ameliorate the Effects of Heat Stress.

Pheromones are secreted molecules that mediate animal communications. These olfactory signals can have substantial effects on physiology and likely play important roles in organismal survival in natural habitats. Here we show that a blend of two ascaroside pheromones produced by C. elegans males primes the female reproductive system in part by improving sperm guidance toward oocytes. Worms have different physiological responses to different ratios of the same two molecules, revealing an efficient mechanism for increasing coding potential of a limited repertoire of molecular signals. The endogenous function of the male sex pheromones has an important side benefit. It substantially ameliorates the detrimental effects of prolonged heat stress on hermaphrodite reproduction because it increases the effectiveness with which surviving gametes are used following stress. Hermaphroditic species are expected to lose female-specific traits in the course of evolution. Our results suggest that some of these traits could have serendipitous utility due to their ability to counter the effects of stress. We propose that this is a general mechanism by which some mating-related functions could be retained in hermaphroditic species, despite their expected decay

Sex Pheromones of <i>C. elegans</i> Males Prime the Female Reproductive System and Ameliorate the Effects of Heat Stress

Pheromones are secreted molecules that mediate animal communications. These olfactory signals can have substantial effects on physiology and likely play important roles in organismal survival in natural habitats. Here we show that a blend of two ascaroside pheromones produced by C. elegans males primes the female reproductive system in part by improving sperm guidance toward oocytes. Worms have different physiological responses to different ratios of the same two molecules, revealing an efficient mechanism for increasing coding potential of a limited repertoire of molecular signals. The endogenous function of the male sex pheromones has an important side benefit. It substantially ameliorates the detrimental effects of prolonged heat stress on hermaphrodite reproduction because it increases the effectiveness with which surviving gametes are used following stress. Hermaphroditic species are expected to lose female-specific traits in the course of evolution. Our results suggest that some of these traits could have serendipitous utility due to their ability to counter the effects of stress. We propose that this is a general mechanism by which some mating-related functions could be retained in hermaphroditic species, despite their expected decay

ascr#10 and ascr#3 have different effects on the <i>C</i>. <i>elegans</i> reproductive system.

<p>(A) Sperm guidance in heat-stressed hermaphrodites on plates with individual ascarosides and ascaroside cocktails. Sperm guidance on plates with male cocktail (<i>P</i> = 3.1 x 10⁻³, Kolmogorov-Smirnov test Bonferonni corrected for four comparisons) and ascr#10 (<i>P</i> = 4.3 x 10⁻³, Kolmogorov-Smirnov test Bonferonni corrected for four comparisons) were significantly different from control. Red lines mark the median values. Means are: control = 1.1, male cocktail = 3.2, hermaphrodite cocktail = 1.2, ascr#10 = 2.8, and ascr#3 = 1.3. (B) Representative photographs of heat-stressed hermaphrodites during recovery. Hermaphrodites were monitored during recovery for the clearance of large concretions formed in the uterus during heat stress. Worms were examined at 48, 72, and 96 hours of recovery–that is, the time corresponding to when most of recovery occurs (See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s008" target="_blank">S8 Fig</a>). Worms on plates with ascr#10 were significantly worse at clearing large concretions from the uterus (<i>P</i> = 8.7 x 10⁻³, binomial test Bonferroni corrected for two comparisons). In both (A) and (B) individual ascarosides were at 10 fmol. Male and hermaphrodite ascaroside cocktails were as in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.g002" target="_blank">Fig 2</a> (1.92 fmol ascr#3 + 7.2 fmol ascr#10 and 6.0 fmol ascr#3 + 1.68 fmol ascr#10, respectively). See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s014" target="_blank">S2 Table</a> for numbers of independent trials and number of worms tested in each trial.</p

Experience Modulates the Reproductive Response to Heat Stress in <i>C. elegans</i> via Multiple Physiological Processes

Natural environments are considerably more variable than laboratory settings and often involve transient exposure to stressful conditions. To fully understand how organisms have evolved to respond to any given stress, prior experience must therefore be considered. We investigated the effects of individual and ancestral experience on C. elegans reproduction. We documented ways in which cultivation at 15°C or 25°C affects developmental time, lifetime fecundity, and reproductive performance after severe heat stress that exceeds the fertile range of the organism but is compatible with survival and future fecundity. We found that experience modulates multiple aspects of reproductive physiology, including the male and female germ lines and the interaction between them. These responses vary in their environmental sensitivity, suggesting the existence of complex mechanisms for coping with unpredictable and stressful environments

Effects of male scent on brood sizes of selfing hermaphrodites at 20°C and after recovery from heat stress.

<p>(A) Self-brood sizes of <i>C</i>. <i>elegans</i> hermaphrodites raised at 20°C with or without <i>C</i>. <i>elegans</i> male scent. Broods from 24 hermaphrodites were determined for each condition. Red lines mark the median values. Means are: control = 285.3 and male-scented = 277.3. They are not significantly different (<i>P</i> = 0.99, Kolmogorov-Smirnov test). (B) Brood sizes for hermaphrodites recovering from heat stress alone (i.e. recovery is due to self-fertilization) on control or male-scented plates. The animals whose brood sizes were counted were the same as whose recovery is shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.g001" target="_blank">Fig 1B</a> (see <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s013" target="_blank">S1 Table</a> for raw experimental data). Only worms with offspring were considered. Red lines mark the median values. Means are: control = 2.3 and male-scented = 2.6. They are not significantly different (<i>P</i> = 0.27, Kolmogorov-Smirnov test). See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s014" target="_blank">S2 Table</a> for numbers of independent trials and worms tested in each trial.</p

The male pheromone signal is conserved and affects brood size of gonochoristic Caenorhabditis nematodes.

<p>(A) Fraction of <i>C</i>. <i>elegans</i> hermaphrodites that recover self-fecundity after heat stress on plates with male scent of three related species. See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s013" target="_blank">S1 Table</a> for raw experimental data including numbers of independent trials and worms tested in each trial. Recovery on control and <i>C</i>. <i>elegans</i> male-scented plates are given for comparison (shown as white bars); these data are the same as shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.g001" target="_blank">Fig 1B</a>. Error bars are ±SD among separate trials. (B) Brood sizes of <i>C</i>. <i>remanei</i> females exposed to male scent for 16 hours were significantly higher than naïve females after 10-minute matings (at 20°C) (<i>P</i> = 9.5 x 10⁻³, Kolmogorov-Smirnov test). Red lines mark the median values. Means are: control = 160.1 and male-scented = 205. See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005729#pgen.1005729.s014" target="_blank">S2 Table</a> for numbers of independent trials and worms tested in each trial.</p

Mutants with altered ovulation dynamics recover from heat stress differently than the wild type.

<p>(A) Schematic depiction of several key components of Ca²⁺ signaling that regulates ovulation. Modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105513#pone.0105513-Allman1" target="_blank">[53]</a>. (B) Recovery of fecundity (after a 24-hour heat stress at 31°C) in a mutant with up-regulated Ca²⁺ signaling is lower than in a wild type control (binomial test <i>p</i> = 4.3×10⁻⁹). (C) Recovery of fecundity (after a 24-hour heat stress at 29°C) of mutants or chemical treatment that down-regulate Ca²⁺ signaling is higher than in a wild type control: <i>itr-1</i> (binomial test <i>p</i> = 7.7×10⁻²⁸), <i>crt-1</i> (<i>p</i> = 2×10⁻²⁵), and dantrolene (<i>p</i> = 1.6×10⁻⁸). Error bars are s.d.</p