Please read the article below and answer these questions based on the article an

Question

Please read the article below and answer these questions based on the article and USE YOUR OWND WORDS!

(1) Why does thermogenesis not occur during the transition into torpor?

(2) In the study by Toien et al., how was the metabolic rate of hibernating black bears measured?

(3) Bears typically come out of hibernation leaner (i.e., less fat) than when they entered. Why is this?

(4) Suggest a human application based on your answer to the previous question.

Both the AM and the NF symbioses are based on the shared activity of a set of plant genes, SYM genes (3–5). This indicates that bacte- ria hijacked the signal transduction pathway that fungi had used to gain entry into plant tissues and cells. Op den Camp et al. provide evidence that in Parasponia, the only nonle- gume partner of rhizobia, a single receptor can recognize both the fungal and bacterial signals and induce the common SYM path- way to promote the intracellular accommoda- tion of the partner microorganisms.

Op den Camp et al. used Rhizobium strains, some of which were able to form nodules, and some of which were unable to form nodules, to prove that both Parasponia and legumes use lipochito-oligosaccharides called Nod factors to induce nodule develop- ment. They also showed that Nod factors act similarly in both symbioses via a common signaling cascade; in Parasponia, the intro- duction of a dominant active form of calcium/ calmodulin-dependent kinase (CCaMK), a key element of this pathway, resulted in spon- taneous nodulation, as in legumes.

Op den Camp et al. also provide insight into how bacterial Nod factor receptors (NFRs) evolved from receptors involved in plant-fungi partnerships. The most-studied legumes recognize rhizobia—or, more accu- rately, the bacterial Nod factors—via a pair of LysM-type receptor-like kinases, NFR1/ LYK3 and NFR5/NFP (6, 7). Because these NFRs are specif ic to bacterial symbiosis, investigators had hypothesized that they evolved either by duplication of the mycor- rhiza-specific receptors, which then gained new functions, or by the recruitment of new

receptors that turned on the common signal- ing pathway. Op den Camp et al.’s analysis indicates that receptor duplication was not essential for plants to acquire the ability to form a symbiotic relationship with NF bac- teria. Instead, the presence of a single NFR5- like receptor in Parasponia, and its indispens- able role in both symbioses, strongly suggests that rhizobia entered symbiotic interactions with plants through the same entrance used by mycorrhizal fungi. It also means that the molecular “keycard” that opens the door to plant partnerships for both bacteria and fungi—the bacterial Nod factor and mycor- rhizal (Myc) factor—must be very similar. Indeed, Maillet et al. (8) recently described the Myc factors of AM fungi as lipochito-oli- gosaccharide molecules that are very similar to Nod factors.

These results raise several questions: Why is the appearance of nitrogen-f ixing nod- ules, especially rhizobial ones, restricted to a small fraction of mycorrhizal plants? How do plants discriminate between symbiotic fungi and bacteria? Was it necessary for host plants to distinguish between the microbes to create different niches? Studies of genes from related plants suggest that plant fami- lies establishing rhizobial or actinorhizal (Frankia) symbioses belong to the same large lineage. This raises the possibility that, dur- ing the evolution of flowering plants, a pre- disposition for symbiotic nodule formation originated only once (9). Did this predispo- sition occur by changing the activity of one or more component(s) of the common sym- biotic pathway, for example, by enabling it to provide different outputs?

Both bacteria and mycorrhizal fungi induce changes in intracellular calcium (Ca2+-) concentrations (termed calcium spik- ing). However, the frequency and duration of the oscillations, as well as the speed of Ca movement, are different in the two symbio- ses (10). Early elements of the common SYM pathway, such as the LysM-type receptors and another receptor protein, the symbiosis receptor kinase (SYMRK), are required for the induction of the calcium spiking, which is then deciphered by CCaMK. It will be inter- esting to compare calcium spiking upon rhi- zobial and fungal inoculations in species that possess dual-functioning receptors.

There is not yet enough systematic data from different plant lineages to determine exactly how molecules like SYMRK and CCaMK contributed to the evolution of a predisposition to nodule formation. The real challenge is to find out why lineages with predisposition for nodulation (for exam- ple, certain legumes) are unable to establish NF symbiosis.

Do bears really hibernate? Their high body temperature during winter dor- mancy has raised some doubt about this behavior, as it is unlike the pronounced decreases observed in small mammals that enter this nonactive state. On page 906 of this issue, Tøien et al. (1) show that bears do indeed hibernate. Through continuous mea- surement of oxygen consumption, body tem- perature, and heart, muscle, and brain activ- ities, the authors show that black bears display unusual patterns of metabolic and ther- mal regulation during hibernation as well as when they emerge from this resting state in the spring.

Hibernation is a powerful behavior that reduces energy costs in mammals. However, in small mammals, it is frequently interrupted by arousals (2, 3), thereby reducing its effec- tiveness. Generally, after entrance into torpor, deep torpor is maintained for 1 or 2 weeks with body temperature close to the freezing point of body fluids, and is terminated by an arousal for about 1 day. During arousal, body temperature rises to a normal 36°C by endogenous heat production. Collectively, the arousal episodes require about 80% of the entire energy cost of the animal during the hibernation season. The reasons for the repeated arousals are still a mystery, but they may allow for the repair of neuronal damage induced by prolonged hypometabolism and brain inactivity at low temperature (4, 5).

Spontaneous hibernation behavior is dif- ficult to observe in captive animals, so its study has mostly relied on field studies of subjects in their natural habitat, or on ani- mals kept in conditions similar to their nat- ural environment. Studying large mammals (at least 10 kg) is particularly difficult because of the challenges of continuous and long-term monitoring. Tøien et al. observed five Alaskan black bears (Ursus americanus) (two females and three males ranging in body mass from 34.3 to 103.9 kg) that were kept in outdoor enclosures in a forest near Fairbanks, Alaska. The bears hibernated in isolated wooden nest boxes, which allowed continu- ous observation and measurement of oxygen consumption and body temperature as well as monitoring of physiological activities from implanted transmitters. The authors observed that during the hibernation period (November to March), the bears did not display repeated arousals, but instead showed multiday oscil- lations of body temperature between 30° and 36°C. Such a lack of periodic arousals dur- ing hibernation has so far only been observed in one small mammal [fat-tailed lemur (6)]. However, Tøien et al. found that the hiber- nating bears reduced their metabolic rate to 75% below basal metabolic rate (BMR). The observed minimum metabolic rate in hiber- nating bears (0.056 ml O2 g1 hour1) is within the range of those observed in small hibernat- ing mammals (0.02 to 0.06 ml O g1 hour1) (2, 3). This implies that bears use the entire mammalian scope of metabolic inhibition in torpor and are true hibernators. This reduction of metabolic rate to 75% below BMR is sub- stantially less prominent than that for small mammals (98% below BMR). The difference is largely due to the allometric scaling of BMR, indicating that hibernation is more effective in small mammals below 1 kg body mass.

Tøien et al. also observed that when the bears emerged from their dens in mid- April, they had a normal body temperature of 36.6°C. Yet, they maintained a low meta- bolic rate that was 47% below their BMR, and it took several weeks for it to rise to that of the active season (2.76 ml O2 g1 hour1). It is generally assumed that BMR is a species- specific constant that is necessary to maintain the vital physiological functions of an endo- thermic mammal resting at thermoneutrality. The findings of Tøien et al. show that bears can maintain their vital functions with a met- abolic rate that is reduced to nearly half of that normally required in an active state, indi- cating that BMR is not a constant but a physi- ologically controlled variable.

Transition into the torpid state includes three processes. Thermoregulatory heat pro- duction (by shivering or nonshivering thermo- genesis) is inhibited because thermoregula- tion is adjusted to a lower body temperature. Metabolic rate is depressed below the BMR at normothermic body temperature (active metabolic inhibition). This inhibition can be assisted by temperature effects on metabolic rate. Tøien et al. make the surprising finding that in hibernating bears, metabolic depres-sion is largely achieved by active metabolic inhibition, whereas temperature effects play only a minor role. In small mammals that hibernate, active inhibition and temperature- related metabolic depression, on average, may each be responsible for about 50% of total metabolic depression (3, 7, 8).

The molecular mechanisms and biochemi- cal pathways that underlie metabolic adjust- ment in torpor are still unclear. In general, torpor metabolism involves inhibition of pro- cesses that generate adenosine 5-triphosphate such as glycolysis (metabolism is rerouted to lipid utilization instead) and mitochondrial respiration, as well as energy-consuming pro- cesses such as transcription, translation, and protein degradation (9–11). This ultimately impairs cell proliferation and differentiation. However, entrance into torpor also requires increased expression of hibernation-specific genes to support lipid metabolism, gluconeo- genesis, cytoprotection, and other measures required to maintain cells (12, 13).

In most mammalian orders, one or several species use torpid metabolic depression. The greatest numbers are found among marsupi- als, rodents, and bats, but also in small num- bers in insectivores, primates, and elephant shrews; and it is likely that more such exam- ples will be discovered in large mammals (14). Although long considered an adaptation to cold, hibernation is also found in tropical animals and desert species, and, as in bears, can occur without substantial drops in body temperature. Perhaps we will find that a hypometabolic state is the primary means by which most, if not all mammals, can reduce their energy expenditures for prolonged peri- ods of time.

Explanation / Answer

1)thermoregularity is observed because it is aadjusted to a low body temperature.

2)metabolic rate of hibernating black bears is measured by cytoprotection .

3)bears can reduce their energy expenditures for prolonged period of time

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