Pettitt J M
J Cell Sci. 1976 May;20(3):671-85. doi: 10.1242/jcs.20.3.671.
The megasporocyte of Isoetes englemanni at the leptotene-zygotene interval of meiosis contains 4 disk-shaped proplastids about 12 mum in diameter. The disposition of these organelles in the cell is such that each of the four megaspores delimited during cytokinesis contains a single proplastid. During prophase and following their incorporation into the spores, the proplastids are undergoing fission by budding. The buds are first discernible as low surface evaginations which contain a complement of granular somal material, some wefts of tubular membrane and osmiophilic globuli, in addition to a number of vesicles derived by invagination from the inner membrane of the proplastid envelope. As the evaginations emerge they enlarge and the link with the parent body is reduced to a narrow channel. At this stage one or more of the vesicles derived from the proplastid envelope comes into register with the lumen of the channel. One vesicle is transported into the lumen, elongating as it passes through. The passage of the vesicle into the channel destroys the connexion between the matrix of the evagination and the stroma of the proplastid. The occurrence in the cytoplasm around the proplastid of bodies not connected to the proplastid, but identical in structure to the evaginations and carrying a membranous tail suggests that the evaginations are released by abscission of the channel close to the surface of the parent body. After release the bodies undergo division by constriction. Regression of the tail follows division in those bodies which are regular in outline and in which the matrix is ultrastructurally similar to the stroma of the parent organelle. The process does not seem to occur in co-existing forms which have assumed an irregular outline and have a less-opque matrix. The more mature megaspore of Isoetes contains proplastids up to 4 mum in greatest dimension. The stroma in these is dense and granular and contains membrane-bound vesicles, osmiophilic globuli, starch granules and wefts of tubular membrane. There is no evidence that the large budding organelle persists to this later stage in development. The resemblance of the plastids in the more mature megaspore to the bodies produced by evagination earlier in development suggests a common identity. The observations and interpretations lead to the proposition that the plastids in Isoetes englemanni are autonomous. This situation contrasts with the one described for another heterosporous haploid dioecious pteridophyte, Marsilea vestita, where nucleocytoplasmic interaction has been interpreted as the de novo creation of plastids and mitochondria following the elimination by autophagy of the organelles inherited at meiosis. It is suggested that an explanation to account for the 2 different mechanisms might be sought in regard to the degree of developmental success enjoyed by the individual megaspores in the 2 plants. In Isoetes all 4 megaspores of every tetrad survive and develop, while in Marsilea the mature megasporangium contains a single functional megaspore.
在减数分裂的细线期-偶线期，恩格勒水韭的大孢子母细胞含有4个直径约12微米的盘状前质体。这些细胞器在细胞内的分布方式使得在胞质分裂过程中界定的4个大孢子每个都含有一个前质体。在前期以及它们并入孢子后，前质体通过出芽进行分裂。芽最初表现为低表面突起，除了一些由前质体包膜内膜内陷产生的小泡外，还含有一团颗粒状细胞质物质、一些管状膜的细丝和嗜锇小球。随着突起出现，它们会增大，与母体的连接会缩小为一个狭窄的通道。在这个阶段，一个或多个源自前质体包膜的小泡与通道的管腔对齐。一个小泡被转运到管腔内，在穿过时伸长。小泡进入通道会破坏突起基质与前质体基质之间的连接。在前质体周围细胞质中出现的与前质体不相连但结构与突起相同且带有膜状尾巴的物体表明，突起是通过在母体表面附近切断通道而释放的。释放后，这些物体通过缢缩进行分裂。在轮廓规则且基质在超微结构上与母体细胞器基质相似的物体中，尾巴在分裂后会退化。这个过程似乎不会发生在已经呈现不规则轮廓且基质不那么不透明的共存形态中。恩格勒水韭更成熟的大孢子含有最大尺寸达4微米的前质体。这些前质体的基质致密且呈颗粒状，含有膜结合小泡、嗜锇小球、淀粉粒和管状膜的细丝。没有证据表明大型出芽细胞器会持续到这个发育后期。更成熟大孢子中的质体与发育早期通过出芽产生的物体相似，这表明它们具有共同的身份。这些观察和解释导致了这样一个观点，即恩格勒水韭中的质体是自主的。这种情况与另一种异孢单倍体雌雄异株蕨类植物—— vestita（原文有误，应为Marsilea vestita）的情况形成对比，在那里，核质相互作用被解释为在减数分裂时通过自噬消除遗传的细胞器后质体和线粒体的重新产生。有人认为，对于这两种不同机制的解释可能要从这两种植物中单个大孢子所享有的发育成功程度方面来寻找。在恩格勒水韭中，每个四分体的所有4个大孢子都能存活并发育，而在水车前属植物中，成熟的大孢子囊只含有一个功能正常的大孢子。
