Fine-structural aspects of fertilization in Chlamydomonas reinhardi.
Fine-structural aspects of fertilization in Chlamydomonas reinhardi.
复制标题
莱茵衣藻受精的精细结构方面。
作者：
I. Friedmann;A. L. Colwin;L. Colwin
瓢虫配子缺乏营养细胞所具有的细胞壁。在细胞顶端下方，配子形成一个受精小管，长2µm，直径0.2µm；其质膜和先端质膜有细长的管状突起。在受精小管的底部有规律地排列着类胆管体，一种环状细胞质细胞器；覆盖在身体上的质膜呈现为一个电子密集的环。配子除了两个鞭毛的基体外，还具有两个“自由”基体。在结合的初始阶段，接合细胞由受精小管连接，受精小管的质膜与两个接合细胞的质膜连续。在小管的一端有一个明显的啸管体，但在另一端有一个小的结构，可能是啸管体的同源物。随后，受精小管变短变宽，直到最终没有小管存在，两个原生质体的顶端相连。然后，合并的细胞像千斤顶刀一样弯曲，原生质体发生横向排列。当前配子的鞭毛基部似乎彼此接近，鞭毛根的纤维成分接触时，这个有四个鞭毛的合子开始运动。在活动的受精卵中，细胞核不融合，而是包裹在两个配子的杯状质体中。在淀粉含量高和低的条件下，正（+）和负（-）菌株的交配表明，只有正（+）交配型的配子含有类卵体。由于含有卵管体的配子似乎也产生受精小管，因此可以推断只有正（+）交配型配子才产生受精小管。如果进一步的调查支持这一推论，就可以确定，将正（+）交配类型指定为雄性，将负（-）交配类型指定为雌性存在结构基础。受精涉及配子膜通过一个特殊结构（受精管）的融合，在这方面与动物门中受精的某些方面有相似之处。讨论了其他种类衣藻的受精小管与原生质桥的关系。
Gametes of C. reinhardi lack the cell wall which vegetative cells possess. Just below the cell apex gametes form a fertilization tubule which is up to 2 µ long and 0.2 µ in diameter; its plasma membrane and that of the apex have slender tubular projections. At the base of the fertilization tubule regularly lies the choanoid body , a collar-shaped cytoplasmic organelle; the plasma membrane overlying the body appears as an electron-dense ring. Gametes possess two ‘free’ basal bodies in addition to the basal bodies of the two flagella. In the initial stage of union the conjugating cells are connected by the fertilization tubule whose plasma membrane is continuous with that of both copulants. At one end of the tubule lies a conspicuous choanoid body, but at the other end is a small structure which possibly is a homologue of the choanoid body. Subsequently, the fertilization tubule shortens and widens until finally no tubule exists and the apical ends of the two protoplasts adjoin. The merging cells then bend like a jack-knife and lateral alignment of the protoplasts occurs. This four-flagellated zygote becomes motile at about the time when the flagellar bases of the former gametes seem to approach each other and when fibrillar elements of the flagellar roots come into contact. In the motile zygote the nuclei do not fuse but remain ensheathed in the cup-shaped plastids of the two gametes. A mating of plus (+) and minus (-) strains cultured, respectively, for high and low starch content suggested that gametes of only the plus (+) mating type contain the choanoid body. Since it appears that the gamete containing the choanoid body also produces the fertilization tubule, it is inferred that gametes of only the plus (+) mating type produce the fertilization tubule. Should further investigation support this inference, it would be established that there is a structural basis for designating the plus (+) mating type as male and the minus (-) type as female. Fertilization involves fusion of the gamete membranes through the mediation of a specialized structure (the fertilization tubule) and in this respect there are similarities to certain aspects of fertilization in animal phyla. The relation of the fertilization tubule to the protoplasmic bridge of other species of Chlamydomonas is discussed.