Evolutionary shifts in copepod acid tolerance in an acid-recovering lake indicated by resurrected resting eggs

We employed zooplankton resting eggs to track population-level shifts in acid tolerance over the last century in a boreal shield lake recovering from acidification. Shifts in mean and variance of ecological tolerances of species that occupy ecosystems recovering from anthropogenic stress are important to consider because of their potential to influence eco-evolutionary dynamics at community and ecosystem levels. In a laboratory experiment, we compared juvenile survival of Leptodiaptomus minutus copepods hatched from resting eggs from three time periods (80- to 100-year- old: pre-industrial, 20- to 50-year- old: lake acidification, and 8- to 10-year- old: recovery of lake-water back to pH ≥ 6.0) under several pH treatments. Mitochondrial DNA was used to confirm species identity and to test for population bottlenecks as a possible mechanism to explain our results. We expected that nauplii hatched from eggs deposited prior to industrialization (lake-water pH ≈ 6.0) and from the period of pH recovery (lake-water pH ≥ 6.0) would have lower mean and more variable survival at acidic pH compared to nauplii hatched from the period of peak lake acidification (lake-water pH ≈ 4.7). Our results, which are likely a combination of both genetic and environmental effects, suggest support for this hypothesis. Nauplii hatched from eggs deposited during the period of acidification in George Lake had reduced variation in pH tolerance compared to the recovery period. This was likely driven by strong selection rather than genetic drift because we found no evidence of a population bottleneck. However, we could not detect differences in the variance of naupliar survival between pre-industrial and acidic time periods. Trends in mean naupliar survival from different time periods matched findings from other field-based studies that detected a relationship between lake acidification history and acid tolerance in L. minutus.     

Membrane potential of fertilized eggs of Ilyanassa obsoleta during polar lobe formation and cytokinesis

The membrane potential of fertilized eggs of Ilyanassa obsoleta does not change significantly during the cell shape changes involved in formation and resorption of the third polar lobe and in cytokinesis. The membrane potential is predominantly K⁺-dependent.     

The action of salt-solutions on the unfertilized and fertilized eggs of Arbacia,and of other animals

Summary The last two experiments show that after 4 1/2 hours in the solutions employed the unfertilized eggs divided excellently when returned to sea-water. The last experiment shows that potassium chloride, 1.5%, gives the same results as the other salts.     

CDC25B acts as a potential target of PRKACA in fertilized mouse eggs

Protein kinase A (PRKACA) has been documented as a pivotal regulator in meiosis and mitosis arrest. Although our previous work has established that PRKACA regulates cell cycle progression of mouse fertilized eggs by inhibiting M-phase promoting factor (MPF), little is known about the intermediate factor between PRKACA and MPF in the mitotic cell cycle. In this study, we investigated the role of the PRKACA/CDC25B pathway on the early development of mouse fertilized eggs. Overexpression of unphosphorylatable CDC25B mutant (Cdc25b-S321A or Cdc25b-S229A/S321A) rapidly caused G2-phase eggs to enter mitosis. Microinjection of either Cdc25b-WT or Cdc25b-S229A mRNA also promoted G2/M transition, but much less efficiently than Cdc25b-S321A and Cdc25b-S229A/S321A. Moreover, mouse fertilized eggs overrode the G2 arrest by microinjection of either Cdc25b-S321A or Cdc25b-S229A/S321A mRNA, which efficiently resulted in MPF activation by directly dephosphorylating CDC2A-Tyr15, despite culture under conditions that maintained exogenous dibutyryl cAMP. Using a highly specific antibody against phospho-Ser321 of CDC25B in Western blotting, we showed that CDC25B-Ser321 was phosphorylated at the G1 and S phases, whereas Ser321 was dephosphorylated at the G2 and M phases in vivo. Our findings identify CDC25B as a potential target of PRKACA and show that PRKACA regulates G2/M transition by phosphorylating CDC25B-Ser321 but not CDC25B-Ser229 on the first mitotic division of mouse fertilized eggs.     

Membrane permeabilities determining resting,action and mechanoreceptor potentials inStentor coeruleus

Summary In response to mechanical stimuli the protozoan,Stentor coeruleus, contracts in an all-or-none fashion and simultaneously reverses the direction of its ciliary beat. These behaviors have previously been shown to be correlated with the presence of a mechanoreceptor potential and all-or-none action potential (Wood 1970, 1973a). In the studies reported below the ionic bases of the resting, receptor and action potentials ofStentor were determined by use of intracellular microelectrodes penetrating animals chilled to 8.5–10 °C. The resting potential is most dependent on the extracellular concentration of KCl but some dependence on CaCl₂ concentration was also observed. If allowance is made for the large increases in membrane conductance observed in solutions containing 2–8 mM KCl it is found that the resting potential data are well described by a modified form of the Goldman equation whereP _Ca/P _K = 0.068 andP _Cl/P _K = 0.072. The intracellular ionic activities (K _i ⁺ = 13.1 mM, Cl _i ^– = 9.9 mM, Ca _i ⁺ = 0 mM) which provide the best fit of this equation to the resting potential data are in close agreement with the intracellular concentration values measured by flame microspectrophotometry (K_i=12.4 mM, Cl_i = 9.4 mM) except in the case of Ca_i where most of the intracellular concentration is presumed to be bound. 65 to 75 mV action potentials are produced by suprathreshold depolarizations but contractions were not generally seen in these chilled animals, only ciliary reversals. The action potential peak varies with CaCl₂ concentration with a slope of 12.6 mV/10 fold change but varies only slightly with KCl or Cl^– concentration. These peak potentials are well described by assuming that theP _Ca/P _k = 7.9 andP _Cl/P _K=1.0 at the time of the action potential peak. Depolarizing receptor potentials and brief inward receptor currents were observed for all forms of punctate and gross bodily mechanical stimulation employed. No evidence was found for any form of hyperpolarizing mechanoreceptor potentials as observed in some other ciliates. The reversal potential of the mechanoreceptor current varied with CaCl₂ concentration in a manner similar to that of the action potential peak. As in the case of the action potential both theP _Ca/P _k andp _cl/p _k ratios appear to increase as a result of mechanical stimulation to 9.3–15 and 1.2–1.95 respectively. Mechanoreceptor currents are voltage dependent being increased when the membrane is depolarized above resting potential and decreased when the membrane is hyperpolarized. In general the electrophysiological characteristics ofStentor appear similar to those ofParamecium andStylonychia, but its resting membrane appears more selectively permeable to K⁺, it produces only depolarizing receptor potentials when mechanically stimulated and the initial action potential elicited by depolarizing current pulses can be all-or-none even in culture medium.     

The cellular resting and action potentials: interpretation based on the association-induction hypothesis

The Hodgkin, Huxley, and Katz theories of resting and action potentials are based on the membrane theory, which holds that cell K+ and water exist in the free state. Reviewed here are these theories of cellular potential along with the results of experimental testings. Reviewed also is Ling's association-induction (AI) hypothesis, which holds that all K+ is absorbed selectively and singly on anionic protein sites and that cell water is absorbed in multilayers on extended chains of "matrix proteins." In the development of the AI model, molecular mechanisms of cell permeation and electric potentials were presented according to which the potentials are surface-adsorption phenomena. Thus they resemble those suggested by Baur rather than the membrane potentials proposed by Ostwald and Bernstein. In the present review it is shown that the AI version of the surface adsorption model can account for evidence supporting the Hodgkin, Huxley, Katz approach as well as evidence against it-including extensive recent confirmation of the absorbed state of K+ in muscle.     

Calcium-sensitive action potential of long duration in the fertilized egg of the ctenophore Mnemiopsis leidyi

The membrane properties of fertilized eggs of the ctenophore Mnemiopsis leidyi were studied using standard microelectrode techniques. The resting potential was approximately -80 mV, and was dependent on the extracellular K concentration. Depolarizing current injections elicited an action potential with an initial peak amplitude of +20 to +40 mV (duration about 5 sec) and a long lasting (duration 3 to 10 min) plateau phase. The depolarizing phase and the plateau phase appeared to have different ionic mechanisms. The entire action potential could be prevented by removal of extracellular Ca, but only the amplitude of the depolarizing phase, not the plateau phase, was dependent on the extracellular Ca concentration. The plateau phase was not observed in the absence of Ca, but in the presence of Ca its duration was dependent on the external Ca concentration. The data suggest that the plateau phase is activated as a consequence of Ca influx during the initial depolarizing phase. Removal of external Na resulted in only minor changes in the waveform of repolarization. The action potential was resistant to low concentrations of Mn and Cd in the presence of Ca. The role of this action potential in ctenophore development is not known, but in its waveform and duration it resembles the sperm-gated potentials that have been seen in eggs of other phyla. These experiments show ctenophore embryos to be excitable at very early stages, and suggest their utility in the study of the differentiation of cellular electrical properties.     

Membrane potential, action potential and activation potential of eggs of the sea urchin, Lytechinus variegatus

Unfertilized Lytechinus variegatus eggs in sea water in their normal physiological state have membrane potentials that approximate ?70 to ?80 mV. This conclusion is based on microelectrode measurements and on computation from the Na⁺ and K⁺ fluxes. The ?8 to ?15 mV values for the membrane potential previously reported and which are generally measured are the consequence of depolarization by impalement. The activation potential in inseminated eggs with an initial membrane potential more negative than ?60 mV is a compound event involving sperm-induced as well as voltage dependent conductance changes. The sperm-induced mechanism is a two-phase conductance increase which involves both Na⁺ and Ca²⁺ during the first phase, and Na⁺ alone during the second phase. In addition, the sperm-induced depolarization at the beginning of the first phase activates a voltage dependent Ca²⁺-conductance mechanism resulting in generation of an action potential.     

Effects of attention and slow potential shifts on self-regulation of event-related potentials

Research on the effects of self-regulation of slow potentials (SP) and event-related potentials (ERP) has failed to look at the possible interactions of these two kinds of brain potentials. The present study investigated such interactions by recording both ERP and SP potential changes in an operant ERP conditioning paradigm. Ten subjects participated in two conditions that were designed to differentially manipulate attention to the stimuli. In the operant conditioning task, subjects received auditory feedback as they attempted to increase the ERP amplitude at 180 msec poststimulus (P180), which was elicited by a subpainful shock stimulus to the forearm over 250 trials. In the distraction task, subjects were instructed not to attend to stimuli or feedback tones, but rather received and were tested on reading materials. Attention, as manipulated by these tasks, was not a determinant of changes in ERP amplitude since there were no significant differences in the size of P180 between attention conditions. While no significant change in the mean ERP amplitude occurred, subjects were able to produce ERPs above criterion threshold significantly more often during trials in the conditioning task than in the reading task. Thus, there was evidence of some learning. The difference in wave forms between hit and miss trials indicates a latency shift (with misses having a later ERP peak). This may indicate that latency, rather than, or in addition to, amplitude, is shaped during conditioning procedures. In addition, the CNV that developed between the shock stimulus and the feedback signal during conditioning was significantly larger in amplitude than in the distraction condition. This is taken as evidence of increased attention during conditioning. Since hit trials demonstrated larger contingent negative variation (CNV) amplitudes, production of CNVs may be instrumental in mediating hits. Therefore, attentional mechanisms may play a role in successful ERP self-regulation. No correlations were found involving P180, CNVs, or tonic slow potential shifts. Changes in tonic DC levels showed a suggestive trend between conditions. Although both conditions began with a negative shift, during conditioning the negativity increased, while during distraction the tonic level went to positivity. These trends support the hypothesis that attention and arousal increased during conditioning. The possible reasons for the lack of significant correlations between ERP and tonic or phasic slow potential changes in this paradigm are discussed.