Ultrastructural Studies of Gametophytic Apomicts in Guinea Grass (Panicum maximum) II. Characteristics of Aposporous Initial Cell-derived Embryo Sac

Abstract

SummaryThe ultrastructural analysis using ultra-thin section and transmission electron microscope was carried out to characterize the features of the reproductive cells in aposporous initial cell (AIC)-derived embryo sac (AES), i.e. egg, synergid and polar cells, in ovule of facultative apomictic Panicum maximum. At anthesis, the first AES located in micropylar end consists of egg apparatus with 2 synergids and 1 egg cell, and the central cell with 1 nucleus. The other AESs contain less than four cells. The egg cell contains high dense of cytoplasm, and around the nucleus plastids and mitochondria are remarkable. There exist lipid bodies, few rough endoplasmic reticulum (rER), dictyosomes, and big vacuoles distributed in cytoplasm. The polar cell is occupied almost by one big vacuole, the lower dense cytoplasm than that of egg cell wraps 1 or 2 nuclei and is distributed around egg apparatus. Only in micropylar end of the AES the cell wall of the central cell with one nucleus exists with ingrowth of cell wall, indicating that this structure transports the nutrition for the embryo development. Synergid usually contains lowest dense of cytoplasm, and lipid bodies, plastid and mitochondria are distributed along the filiform apparatus in micropylar end. Filiform apparatus is located in the top place of the 2 synergids that is same to that of sexual embryo sac. Extreme rER and vacuoles distributed in chalazal end are the characteristics of the synergids. These vacuoles and rER might contain abundant inorganic substances that are absorbed as a part of nutrition for the developing embryo.

Ultrastructural Mechanisms of Aposporous Embryo Sac Initial Cell Appearance and Its Developmental Process in Gametophytic Apomicts of Guinea Grass (Panicum maximum)

“…However, it is considered that nucellar cells located in chalazal end inherit and develop dominantly when they sense some signals from sexual ones which will be the fate of abortion and degeneration, and/or lose capacity of division. At the same time, one of nucellar cell-derived AIC cells appears, sexual cells and other nucellar cells provide nutrients for AIC differentiation (Guan et al 2006(Guan et al , 2007(Guan et al , 2008. In some meanings, the life of the ovule (or ovary) will be kept by whatever either sexual or asexual (nucellar) cell becomes to embryo sac.…”

Female germ unit in Genlisea and Utricularia, with remarks about the evolution of the extra-ovular female gametophyte in members of Lentibulariaceae

“…Recently, very interesting hypothesis was proposed that some apomictic species from the Compositae family (genera Chondrilla , Taraxacum ) have synergids only in the juvenile phase, which lacks a filiform apparatus, and that this is connected with apomixis (Kościńska-Pająk 2006 ; Kościńska-Pająk and Bednara 2006 ). There are, however, arguments against this idea because other studies of apomictic species from other families showed the occurrence of the filiform apparatus (e.g., Jankun 1994 ; Guan et al 2007 ).…”

Establishment of Efficient System for Isolation and Manipulation of Single Protoplasts Containing Aposporous Embryo Sac Initial Cell in Guinea Grass (Panicum maximum)

“…The authors have reported that the most different event between the sexual and apomictic plants was the appearing of aposporous embryo sac initial cell (AIC) following the degeneration of the mature haploid megaspore in guineagrass (Panicum maximum). This was observed by Normarski differential interference-contrast microscopy (DIC), and transmission electron microscopy (TEM) [5][6][7][8]. And according to the above results, the authors have obtained the apomixis-specific gene, named as ASG-1, from facultative apomict of guineagrass, using the differential screening method based on the ovary length as an index [9,10].…”