Purging Deleterious Mutations under Self Fertilization: Paradoxical Recovery in Fitness with Increasing Mutation Rate in Caenorhabditis elegans

Background: The accumulation of deleterious mutations can drastically reduce population mean fitness. Self-fertilization is thought to be an effective means of purging deleterious mutations. However, widespread linkage disequilibrium generated and maintained by self-fertilization is predicted to reduce the efficacy of purging when mutations are present at multiple loci. Methodology/Principal Findings: We tested the ability of self-fertilizing populations to purge deleterious mutations at multiple loci by exposing obligately self-fertilizing populations of Caenorhabditis elegans to a range of elevated mutation rates and found that mutations accumulated, as evidenced by a reduction in mean fitness, in each population. Therefore, purging in obligate selfing populations is overwhelmed by an increase in mutation rate. Surprisingly, we also found that obligate and predominantly self-fertilizing populations exposed to very high mutation rates exhibited consistently greater fitness than those subject to lesser increases in mutation rate, which contradicts the assumption that increases in mutation rate are negatively correlated with fitness. The high levels of genetic linkage inherent in self-fertilization could drive this fitness increase. Conclusions: Compensatory mutations can be more frequent under high mutation rates and may alleviate a portion of the fitness lost due to the accumulation of deleterious mutations through epistatic interactions with deleterious mutations. Th

EMS induced extinction rates.

<p>Replicate populations of PX385 were continually exposed to a range of different EMS concentrations and driven to extinction. We calculated the mean time to extinction for each EMS concentration. Treatment with 100 mM EMS required more generations of exposure to induce extinction than all other EMS treatments (P<0.001, Tukey's HSD). Control populations, with no EMS exposure, did not go extinct during the course of the experiment. Error bars represent two standard errors of the mean.</p

EMS dose response curve.

<p>Replicate populations of PX384 were exposed to five generations of mutagenesis across a range of different EMS concentrations. Mean fecundity generally decreased with increasing EMS concentration, however, exposure to 100 mM significantly elevated fecundity relative to much lesser concentrations of EMS (20 mM) (P<0.001, Tukey's HSD). Error bars represent two standard errors of the mean.</p

EMS induced mortality rates.

<p>Replicate populations of PX385 were exposed to different EMS concentrations. Following mutagenesis the populations were scored for live and dead worms and the mean mortality rate calculated for each EMS concentration. Overall, the EMS induced mean mortality rate, or toxicity, increased linearly (R² = 0.95, F_1,38 = 682.1, P<0.001) with increasing EMS concentration. Error bars represent two standard errors of the mean.</p

Dose response curve experimental design.

<p>Dose response curve experimental design.</p

Time series EMS dose response curve.

<p>Replicate populations of PX385 were exposed to five generations of mutagenesis across a range of different EMS concentrations. Mean fecundity was assessed prior to mutagenesis, after three generations of mutation, and after five generations of mutation. The mutated populations (solid lines) exhibited reduced mean fecundity, relative to the control populations (dashed line), after three generations (F_1,216 = 35.64, P<0.001). Then, after five generations, the populations exposed to 100 mM exhibit a substantial increase in mean fecundity while all of the other mutated populations exhibit further reductions in mean fecundity (P<0.001, Tukey's HSD). Error bars represent two standard errors of the mean.</p

Intraspecific and interspecific variation in thermotolerance and photoacclimation in Symbiodinium dinoflagellates

Light and temperature are major drivers in the ecology and biogeography of symbiotic dinoflagellates living in corals and other cnidarians. We examined variations in physiology among 11 strains comprising five species of clade A Symbiodinium. We grew cultures at 26oC (control) and 32oC (high temperature) over a duration of 18 days while measuring growth and photochemical efficiency (Fv/Fm). Responses to thermal stress ranged from susceptible to tolerant across species and strains. Most strains exhibited a decrease in cell densities and Fv/Fm when grown at 32oC. Tolerance to high temperature (T32) was calculated for all strains, ranging from 0 (unable to survive at high temperature) to 1 (able survive at high temperature). There was substantial variation in thermotolerance across species and among strains. One strain had a T32 close to 1, indicating that growth was not reduced at 32oC for only this one strain. To evaluate the combined effect of temperature and light on physiological stress, we selected three strains with different levels of thermotolerance (tolerant, intermediate and susceptible) and grew them under five different light intensities (65, 80, 100, 240 and 443 μmol quanta m-2 s-1) at 26 and 32oC. High irradiance exacerbated the effect of high temperature, particularly in strains from thermally sensitive species. This work further supports the recognition that broad physiological differences exist not only among species within Symbiodinium clades, but also among strains within species demonstrating that thermotolerance varies widely between species and among strains within species

Relative population growth rates, therotolerance, and relative photochemical efficiency;Fv/Fm changes between three strains of Clade A under different growth lights and temperatures from Intraspecific and interspecific variation in thermotolerance and photoacclimation in <i>Symbiodinium</i> dinoflagellates

Light and temperature are major drivers in the ecology and biogeography of symbiotic dinoflagellates living in corals and other cnidarians. We examined variations in physiology among 11 strains comprising five species of clade A <i>Symbiodinium</i>. We grew cultures at 26°C (control) and 32°C (high temperature) over a duration of 18 days while measuring growth and photochemical efficiency (Fv/Fm). Responses to thermal stress ranged from susceptible to tolerant across species and strains. Most strains exhibited a decrease in cell densities and Fv/Fm when grown at 32°C. Tolerance to high temperature (<i>T</i>₃₂) was calculated for all strains, ranging from 0 (unable to survive at high temperature) to 1 (able survive at high temperature). There was substantial variation in thermotolerance across species and among strains. One strain had a <i>T</i>₃₂ close to 1, indicating that growth was not reduced at 32°C for only this one strain. To evaluate the combined effect of temperature and light on physiological stress, we selected three strains with different levels of thermotolerance (tolerant, intermediate and susceptible) and grew them under five different light intensities (65, 80, 100, 240 and 443 μmol quanta m⁻² s⁻¹) at 26 and 32°C. High irradiance exacerbated the effect of high temperature, particularly in strains from thermally sensitive species. This work further supports the recognition that broad physiological differences exist not only among species within <i>Symbiodinium</i> clades, but also among strains within species demonstrating that thermotolerance varies widely between species and among strains within species