Cytological Mechanism of Cytoplasmic Inheritance in Pinus tabulaeformis: II. Transmission of Male and Female Organelles During Fertilization and Proembryo Development

Abstract

In an earlier report the ultrastructure and nucleoid organelles of male gamete in Pinus tabulaeformis Carr. have been described. Presently, the ultrastructure of the cytoplasm of the egg cell and pollen tube—immediately before fertilization and during cytoplasmic transmission of male gametophyte—has been described for the same species. The fate of parental plastids and mitochondria in the proembryo has also been followed. The mature egg cell contains a large amount of mitochondria, but seems to lack normal plastids. Most plastids have transformed into large inclusions. Apart from the large inclusions, there are abundant small inclusions and other organelles in the egg cell. During fertilization, pollen tube penetrates into the egg cell at the micropylar end and thereafter the contents are released. Plastid and mitochondrion of male origin are lacking near the fusing sperm-egg nuclei. The second sperm nucleus—not involved in karyogamy—remains at a site near the receptive vacuole. This nucleus is surrounded by large amount of male cytoplasm containing mixed organelles from the sperm cell, tube cell, and egg cell. At the free nuclear proembryo stage, organelles of male and female origin are visible in the perinucleus-cytoplasmic zone. Most of the mitochondria have the same morphological features as those in the egg cell. Some of the mitochondria appear to have originated from the sperm and tube cells. Plastids are most likely of male gametophyte origin because they have similar appearance as those of the sperm and tube cell. Large inclusions in the egg cell become vacuole-like. Paternal plastids have been incorporated into the neocytoplasm of the proembryo. In the cellular proembryo, maternal mitochondria are more abundant. Plastids resembling those of the sperm and tube cell are still present. These cytological results clearly show that in P. tabulaeformis , plastids are inherited paternally and mitochondria bipaternally. The cytological mechanism of plastid and mitochondrion inheritance in gymnosperm is discussed.

在電鏡下觀察油松（Pinus tabulaeformis Carr.）傳粉後的胚珠臨近受精時的花粉管和卵細胞的細胞質、受精時雄配子體細胞質的傳遞、游離核和細胞原胚發育時期質體和線粒體的傳遞。在成熟卵細胞中含許多線粒體，缺少正常結構的質體，它們轉變為大內含體。此外，卵細胞還有豐富的小內含體和其他一些細胞器。花粉管在卵細胞的珠孔端釋放其內含物。精核與卵核融合時，核周圍未見來自精細胞的質體和線粒體。不參與融合的精核停留在接受液泡旁，在其周圍有大量的雄性細胞質，其中混合有精細胞、管細胞和卵細胞的細胞器。在游離核原胚時期，核週區的細胞質中可見雄性與雌性親本的細胞器相混合；其中許多線粒體與原來卵細胞中的線粒體有相同的形態，也有一些線粒體看來是來自精細胞和管細胞；質體是由雄配子體傳遞，形態與精細胞的或花粉管中的質體相似。卵細胞中變異的質體（即大內含體）在原胚發育時期變為液泡狀，而雄性質體參加到新細胞質中。在原胚細胞中，線粒體大多數為母本來源，質體則表現為精細胞或管細胞的質體形態。該研究確定了油松具父系質體和雙親線粒體遺傳的細胞學基礎。對裸子植物線粒體和質體遺傳的機理從細胞學的角度進行了分析。