Current studies of the pteridophyte life cycle
Current studies of the pteridophyte life cycle
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DOI：
10.1007/bf02858324
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发表时间：
1987-10
期刊：
影响因子：
--
通讯作者：
E. Sheffield;P. Bell
中科院分区：
文献类型：
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作者：
E. Sheffield;P. Bell
本文结合配子体发生、孢子发生、孢子体和单孢子生殖的相关知识，对蕨类植物配子体/孢子体/配子体循环的控制因素进行了重新研究。配子体发生的超微结构和细胞化学特征表明，卵的发生与从配子体到孢子体的生殖阶段变化有着特别重要的关系。卵细胞具有丰富的细胞质，它在一个封闭的腔室（卵母管）中成熟，在这个腔室中有机会吸收由卵母管中其他细胞裂解释放的营养物质。成熟配子的细胞质具有良好的细胞器和核糖体，并含有大量的RNA和DNA。后者主要存在于细胞器中，但少量可能游离于细胞质中，并伴有组蛋白。染色质精细分散，没有可检测到的Feulgen反应，染色体不能被识别。这表明卵细胞细胞质的独特富集引起了负责孢子生长的基因的再激活；随之而来的信使RNA在受精前就已经开始出现了。对这一假设的支持来自于卵细胞在尿苷类似物存在下成熟的实验。相比之下，成熟的精子只不过是一个带有运动器官的细胞核。染色质高度浓缩，转录似乎处于休眠状态。除了为受精卵提供孟德尔基因的补充外，核缩制的主要功能可能是带来雌性细胞核的重组。雄性染色质的去浓缩伴随着雌性染色质的再浓缩，相反的过程在第一次分裂的前期共同终止。关于孢子体向配子体的转变，特别重要的是围绕孢子母细胞的壁增厚。标记实验表明，这是对复杂分子的有效屏障。据设想，孢子发生发生在一个封闭和缺乏营养的环境中。由于减数分裂核的需要，细胞质进一步耗损，最后几乎没有机会得到补充，由四分体的四个孢子共享。虽然单倍体与卵细胞相似，但孢子的基因组是由耗尽的细胞质支撑的，这与卵细胞不同。在这种情况下，孢子体基因的激活不能持续，因此配子体得以表达。这种对孢子发生在周期中的意义的解释得到了对孢子的实验研究的支持。从亲本的相关影响中分离出来，并使有关细胞受到代谢应激，恢复细胞分裂的能力，但将随后的生长限制为配子体形态。蕨类植物的异孢子循环可以与这些观点相协调。在大孢子发生过程中，减数分裂的作用被生存的大孢子成熟的特别丰富的环境所逆转。孤雌生殖的大量记录表明，随后产生的卵细胞具有强烈的孢子性倾向，而在同孢子的蕨类植物中，这种现象没有得到证实。具有较大孢子和迅速成熟的配子体的专性无配子体蕨类植物占据中间位置。
The factors controlling the gametophyte/sporophyte/gametophyte cycle in the Pteridophyta are re-examined in the light of current knowledge of gametogenesis, sporogenesis, apospory and apogamy.The ultrastructural and cytochemical features of gametogenesis point to oogenesis as being particularly significant in relation to change of reproductive phase from gametophyte to sporophyte. The egg cell is richly endowed with cytoplasm, and it matures in a closed chamber (the archegonium), in which there is opportunity to take in nutrients released by the lysis of the other cells in the archegonial canal. The cytoplasm of the mature gamete is well provided with organelles and ribosomes, and contains substantial quantities of RNA and DNA. The latter is principally in the organelles, but a small amount may be free in the cytoplasm, accompanied by histones. The chromatin is finely dispersed, giving no detectable Feulgen reaction, and chromosomes cannot be recognized. It is suggested that the unique enrichment of the cytoplasm of the egg cell causes reactivation of the genes responsible for sporophytic growth; the consequent messenger RNA’s are already beginning to appear before fertilization. Support for this hypothesis comes from experiments in which egg cells are allowed to mature in the presence of uridine analogues.The mature spermatozoid by contrast is little more than a nucleus provided with a motor apparatus. The chromatin is highly condensed, and it seems likely that transcription is dormant. Other than contributing a complement of Mendelian genes to the zygote, the main function of karyogamy may be to bring about the reorganization of the female nucleus. Decondensation of the male chromatin is accompanied by recondensation of the female, the opposed processes co-terminating in prophase of the first division.With regard to the change from sporophyte to gametophyte, particular significance is seen in the thickened wall which surrounds the spore mother cell. Labelling experiments indicate that this is an effective barrier to complex molecules. Sporogenesis is envisaged as taking place in a confined and nutritionally deprived environment. The cytoplasm is further impoverished by the demands of the meiotic nucleus and is ultimately, with little opportunity for replenishment, shared between the four spores of the tetrad. Although haploid like that of the egg cell, the genome of the spore is supported by a depleted cytoplasm, unlike that of the egg cell. In these circumstances the activation of the sporophytic genes cannot be sustained, and the gametophytic are consequently expressed.This interpretation of the significance of sporogenesis in the cycle receives support from the experimental investigation of apospory. Isolation from the correlative influences of the parent, together with subjecting the cells concerned to metabolic stress, restore the ability of the cell to divide but restrict the ensuing growth to gametophytic morphology.Heterosporous cycles in the Pteridophyta can be reconciled with these views. In megasporogenesis the effects of meiosis are reversed by the particularly rich environment in which the surviving megaspore matures. The strong sporophytic tendencies of the egg cells subsequently produced are indicated by the numerous records of parthenogenesis, a phenomenon of which there are no well-established instances in the homosporous Pteridophyta. The obligately apogamous ferns, with their larger spores and rapidly maturing gametophytes, occupy an intermediate position.