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  3. Cellular slime molds are extraordinary life forms that exhibit features of both fungi and protozoa, although often classed for c

Cellular slime molds are extraordinary life forms that exhibit features of both fungi and protozoa, although often classed for c

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问题     Cellular slime molds are extraordinary life forms that exhibit features of both fungi and protozoa, although often classed for convenience with fungi. At one time they were regarded as organisms of ambiguous taxonomic status, but more recent analysis of DNA sequences has shown that slime molds should be regarded as inhabiting their own separate kingdom.
    Their uniqueness lies in their unusual life cycle, which alternates between a feeding stage in which the organism is essentially unicellular and a reproductive stage in which the organism adopts a multicellular structure. At the first stage they are free-living, separate amoebae, usually inhabiting the forest floor and ingesting bacteria found in rotting wood, dung, or damp soil. But their food supplies are relatively easily exhausted since the cells’ movements are restricted and their food requirements rather large.
    When the cells become starved of nutrition, the organism initiates a new genetic program that permits the cells to eventually find a new, food-rich environment. At this point, the single-celled amoebae combine together to form what will eventually become a multicellular creature. The mechanism by which the individual members become a single entity is essentially chemical in nature. At first, a few of the amoebae start to produce periodic chemical pulses that are detected, amplified, and relayed to the surrounding members, which then move toward the pulse origin. In time, these cells form many streams of cells, which then come together to form a single hemispherical mass. This mass sticks together through the secretion of adhesion molecules.
    The mass now develops a tip, which elongates into a finger-like structure of about 1 or 2 millimeters in length. This structure eventually falls over to form a miniature slug, moving as a single entity orienting itself toward light. During this period the cells within the mass differentiate into two distinct kinds of cell. Some become prestalk cells, which later form into a vertical stalk, and others form prespore cells, which become the spore head.
    As the organism migrates, it leaves behind a track of slime rather like a garden slug. Once a favorable location has been found with a fresh source of bacteria to feed on, the migration stops and the colony metamorphoses into a fungus-like organism in a process known as "culmination." The front cells turn into a stalk, and the back cells climb up the stalk and form a spherical-shaped head, known as the sorocarp. This final fruiting body is about 2 millimeters in height. The head develops into spores, which are dispersed into the environment and form the next generation of amoebae cells. Then the life cycle is repeated. Usually the stalk disappears once the spores have been released.
    The process by which the originally identical cells of the slime mold become transformed into multicellular structures composed of two different cell types — spore and stalk — is of great interest to developmental biologists since it is analogous to an important process found in higher organisms in which organs with highly specialized functions are formed from unspecialized stem cells. Early experiments showed which parts of the slime mold organism contributed to the eventual stalk and which parts to the head. Scientists stained the front part of a slug with a red dye and attached it to the back part of a different slug. The hybrid creature developed as normal. The experimenters then noted that the stalk of the fruiting body was stained red and that the spore head was unstained. Clearly, the anterior part of the organism culminated in the stalk and the posterior part in the spore head. Nowadays, experiments using DNA technology and fluorescent proteins or enzymes to label the prespore and prestalk cells have been undertaken. This more molecular approach gives more precise results than using staining dyes but has essentially backed up the results of the earlier dye studies.
Why does the author refer to the fungus-like organism as a fruiting body?
选项 A、Because it has become one entity.
B、Because it is 2 millimeters in height.
C、Because it now has a stalk and head.
D、Because it has reached its reproductive stage.
答案D
解析 推理判断题。第五段第三句提到，前面的细胞形成茎(a stalk)，后面的细胞爬上茎形成球形的冠(a spherical-shaped head)，接着第四句说这种最终形态的fruiting body大约高2毫米，第五、六句接着讲到了其繁殖过程，故可以推测fruiting body应该是其开始进入繁殖时期时的状态，故答案为[C]项。fruiting body的中文名为“子实体”，即高等真菌的产孢结构。
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