On the Megasporogenesis inLarix Leptolepis(Sieb. et Zucc.) Gordon (Pinaceae)

Abstract

SUMMARYThe mature megasporocyte of various gymnosperms appears as a clearly polarized cell. With reference to the genus Larix Miller (Pinaceae), previous LM observations showed the presence of starch accumulations in the chalazal cytoplasm in L. occidentalis. A peculiar polarization, characterized by the presence of «reticular plasma» in the micropylar cytoplasm, and by a noteworthy abundance of starch in the chalazal region, has been observed in L. sibirica. The present TEM study examines the differentiation of the megasporocyte and the behaviour of the organelles during meiosis in L. leptolepis (Sieb. et Zucc.) Gordon. The most important results are the presence of abundant perinuclear RER complexes in the micropylar cytoplasm of the mature megasporocyte—such a finding can clarify the «reticular plasma» observed in L. sibirica—and the localization of the majority of the mitochondria and amyliferous reserves in the chalazal cytoplasm. It follows that the four megaspores produced by meiosis have different cytoplasmic contents and are of different sizes. The chalazal megaspore, which will give rise to the female gametophyte, is clearly privileged. This agrees with observations reported by other authors as regards other gymnosperms. We have found in L. leptolepis the segregation of the majority of the megasporocyte RER into the micropylar megaspores. These latter remain alive for a long period of time. Increases in RER and ribosomes and the localization of numerous RER elements around autophagic-type vacuoles, degenerating plastids and mitochondria suggest a transformation of micropylar megaspore contents into nutrients which may be used for the gametophyte development. In agreement with what has generally been observed in gymnosperms, the meiotic process involves a certain isolation of the megasporocyte cytoplasm from the surrounding cells. The megasporocyte wall thickening in L. leptolepis seems to be determined by an abundant deposition of components similar to those also present in the walls of the surrounding cells. Notwithstanding this, these components are thought to have a peculiar arrangement which is responsible for ultrastructural layering.