The Role of Diapause
If conditions within an organism’s environment occasionally or regularly become harsh, it may be advantageous for an organism to have a resistant stage built into the life cycle. In such a life history strategy, the organism suspends any growth, reproduction, or other activities for a period of time so that they may occur at a later, more hospitable time. This genetically determined resting stage, characterized by the cessation of development and protein synthesis and suppression of the metabolic rate, is called diapause. Many other kinds of resting stages, with different levels of suppression of physiological activities, are known. Some of these resistant stages can be extremely long-lived. In one case, seeds of the arctic lupine, a member of the pea family recovered from ancient lemming burrows in the Arctic, germinated in three days even though they were carbon-dated at more than 10,000 years old!
Unfavorable conditions that are relatively predictable probably pose a simpler problem for organisms than do unpredictable conditions. Adaptations to the regular change of seasons in the temperate and polar regions may be relatively simple. For example, many seeds require a period of stratification, exposure to low temperatures for some minimum period, before they will germinate. ■ This is a simple adaptation to ensure that germination occurs following the winter conditions rather than immediately prior to their onset.
■ In contrast, unfavorable conditions that occur unpredictably pose considerable problems for organisms.
■ In fact, unpredictability is probably a greater problem than is the severity of the unfavorable period. ■ How can organisms cope with the unpredictable onset of good or poor conditions?
Many adaptations to this general problem are based on a resting stage that awaits favorable conditions.
We will consider two examples from the vertebrates. The first is the red kangaroo. This marsupial inhabits the deserts of central Australia where the onset of rains and the resulting sudden growth of vegetation are extremely unpredictable. Obviously, it is advantageous for a kangaroo female to produce young at a time when plant productivity is sufficient to support her offspring. For such a relatively large mammal, however, gestation (the period of development during pregnancy) is so long that if a female waited to mate and carry the young until after the rains came, the favorable period might be past. The kangaroo’s life history adaptation to this problem involves the use of embryonic diapause during gestation (development in the uterus).
After a 31-day gestation period, the female gives birth to a tiny helpless young typical of marsupials. The newborn crawls into the mother’s pouch and attaches to a teat where it continues to grow and develop. After 235 days it leaves the pouch but remains with the mother and obtains milk from her. Two days after giving birth, the female mates again. The fertilized egg enters a 204-day period of embryonic diapause during which it remains in the uterus but does not attach. It then implants, and 31 days later, birth of the second young occurs. Note that the first young leaves the pouch at just this time. Again, the female mates, fertilization occurs, and another diapause follows. The eventual result is that at any one time, the female has three young at various stages of development: one in diapause, one in the pouch, and one outside the pouch. Among other benefits, this allows her to freeze the development of an embryo during times of drought and food shortage until the offspring in the pouch is able to leave.
A similar strategy—accelerated development combined with a resting stage—has also allowed amphibians to inhabit deserts. The spadefoot toads, such as Couch’s spadefoot toad, inhabit some of the most severe deserts in North America. Adults of this species burrow deeply into the substrate where it is cooler and perhaps more moist. Here they enter into a resting state in which they are covered with a protective layer of dead skin. When it rains, the adults emerge and congregate to mate at temporary ponds. Development is greatly accelerated: the eggs hatch within 48 hours, and the tadpoles change into toads at 16-18 days. Consequently, they can complete the life cycle during the brief window of favorable conditions, then return to the resistant resting stage to await the next rainfall. Resting stages thus comprise a series of adaptations that allow the species to avoid the most difficult conditions for life.
1. According to paragraph 1, why do some organisms have a resting stage during their life cycle?
A. To recover from injuries suffered during harsh conditions
B. To devote all of their energy to a period of growth and reproduction
C. To wait for local conditions to become favorable for important life events
D. To prepare to move to a different environment if conditions become harsh
2. Why does the author mention “seeds of the arctic lupine”?
A. To argue that members of the pea family are extremely resistant to cold temperature
B. To provide information about what ancient lemmings ate during their long resting periods
C. To provide an example of an organism with a resting stage that has many different levels of suppression of physiological activities
D. To support that some resting stages last an extremely long time.
3. According to paragraph 2, why do many seeds require a period of stratification?
A. To slowly build up a tolerance for lower and lower temperatures
B. To guarantee that the seeds grow after and not before unfavorable weather
C. To make sure that the seeds can deal with unpredictable conditions
D. To give the seeds enough time to germinate before winter /
4. The word “severity” in the passage is closet in meaning to
5. Why did the author mention red kangaroo’s diapause in paragraph 3?
A. To serve as an example of the explanation of diapause proposed at the beginning of paragraph 3
B. To provide an exception to animal’s diapause theory
C. To argue that not all of the animals need to diapause
D. To prove that red kangaroo’s gestation is unnecessary
6. According to paragraph 4, all of the following statements are true about the young offspring of the red kangaroo EXCEPT:
A. After birth, a newborn crawls into the mother’s pouch where it grows and develops.
B. After a young kangaroo leaves its mother’s pouch, it still needs its mother’s milk.
C. A mother usually gives birth to three baby kangaroos at the same time.
D. During unfavorable conditions, the mother kangaroo can stop the development of her embryo
7. Paragraph 4 supports all of the following statements about the red kangaroo of central Australia EXCEPT:
A. A female kangaroo mates again shortly after her newborn enters her pouch.
B. During diapause, a young kangaroo stays in the female’s pouch and growth of a second fertilized egg inside the uterus is delayed.
C. To put different young kangaroos at various stages of development is advantageous for the female kangaroo to handle them at the same time.
D. The pause of the development of an embryo has more benefits for preparing it to avoid the harsh times than for competing with its siblings.
8. What is the main purpose of paragraph 4 in the passage?
A. To give the details of an adaptation mentioned in paragraph 3
B. To describe an adaptation different from the one explained in paragraph 3
C. To introduce an adaptation that is described in detail in paragraph 5
D. To discuss an adaptation that is set as successful as the one mentioned before.
9. The word “congregate” in the passage is closet in meaning to
10. The word “comprise” in the passage is closet in meaning to
A. consist of
B. bring about
C. are similar to
D. take the place of
11. According to paragraph 5, how do amphibians such as spadefoot toads survive the severe heat conditions in the North American deserts?
A. They dig down into the ground and go into a resistant resting state.
B. They remain in the ponds that develop after it has rained.
C. They lose their outer layer of skin.
D. Their eggs live dormant until the desert air becomes cooler and more moist.
12. According to paragraph 5, which of the following occurs during the life cycle of the spadefoot toad?
A. The female’s eggs hatch under the surface of the desert.
B. The adults mate during the dry period.
C. The newborn grows into an adult before unfavorable conditions return.
D. The newborn enters a resting stage before it becomes an adult.
13. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
Such adaptations to predictable conditions can also be made by animals, such as by hibernating during the coldest months.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
A. The diapause stage evolved very early and is most common in species that first appeared more than 10,000 years ago.
B. Unpredictable conditions are more problematic for organisms than are fairly predictable changes such as the seasons.
C. Some seeds may germinate in three days even if they have been exposed to very low temperatures for a long time.
D. Some marsupials can care for three newborns in their pouch at the same time, allowing the young to leave the pouch only when conditions are favorable for their growth.
E. The female red kangaroo adapts to unfavorable conditions by delaying the development of the embryo in the uterus.
F. Some amphibians adapt to arid environments by completing accelerated development with resting stages deep underground.
Q1C Q2D Q3B Q4B Q5A Q6C Q7D Q8A Q9B Q10A
Q11A Q12C Q13B Q14 BEF