In angiosperms, the ovule is the precursor of the seed and the site of embryo sac development. In turn, female reproductive unit development, double fertilization, and embryo and endosperm growth occur in the embryo sac. Thus, the ovule has an important function in the reproductive system and life cycle of angiosperms (Hu, 2005; Shi and Yang, 2011; Skinner et al., 2004). Ovules are usually buried deep in the ovary, positioned on the placenta in different patterns. Before fertilization, the ovule contains basic parts, including the nucellus, integument, and micropyle, and is connected to the placenta via the funiculus (Hu, 2005). Formation of a mature and fertile ovule begins with ovule primordial division, with the primordial front end developing into the nucellus and the base into the funiculus. A ring-shaped protrusion developing into the integument emerges within the vicinity of the base of the nucellus. Ovules typically have two layers of integument, with the inner integument appearing first and the outer integument forming thereafter. The integument grows upward to surround the nucellus and forms an apical micropyle (Endress, 2011). The embryo sac arises in the nucellus concurrent with ovule development. Female gametophyte production is divided into two stages: megasporogenesis and female gametophyte development. The former is the prestage of female gametogenesis, comprising sporogenous cell differentiation and megaspore formation to the maturation of functional megaspores. The latter refers to the period from megaspore mitosis to cellularization and maturation of the female gametophytes (Reiser and Fischer, 1993). Any disruption in the process of ovule development can result in failure to form mature and fertile ovules. In general, the most prominent characteristics of sterile ovules are abnormalities or the absence of female germ units and failed development of the nucellus, with only the integument developing. This situation results in the lack of mature and functional synergid cells to guide the pollen tube into the embryo sac to release sperm cells, mature egg cells for sperm and egg fusion to produce fertilized eggs, which in turn develop into embryos, and mature central cells for fertilization to form a fertilized polar nucleus, which develops into the endosperm (Akhalkatsi et al., 1999; Casper and Wiens, 1981; Hu, 2005).
Camellia oleifera, originating in China, is a type of evergreen shrub or small tree in the Theaceae family. This important and unique woody tree species is a source of edible oil in southern China. Indeed, along with Cocos nucifera, Elaeis guineensis, and Olea europaea, C. oleifera is one of the world’s four major woody oil plants (Zhuang, 2008). The contents of unsaturated fatty acids, oleic acids, and linoleic acids in C. oleifera can be as high as 90%, 75% to 83%, and 7.4% to 13%, respectively. The oil of C. oleifera, which has properties of softening blood vessels and lowering blood pressure and blood lipids, is known as “the longevity oil” and “the king of oil”; it is among the highest quality edible oils worldwide and is termed “oriental olive oil” (Gao et al., 2015a, 2015b; Lee and Yen, 2006). Because mature seeds are the major source for oil extraction, the cultivation and breeding objective of C. oleifera is to enhance the production of mature and full seeds. Because C. oleifera is unique to China, research on its embryology is commonly reported in studies by Chinese scholars. For example, Yuan et al. (2011) systematically studied the development of the female gametophyte of C. oleifera, noting onion-type embryo sac development in normal ovules. The megasporocyte undergoes meiotic division, giving rise to two dyad cells; however, only the megaspore at the chalazal end has a biological function, forming 7-cell, 8-nucleus embryo sacs. In addition, Gao et al. (2015b) carried out a detailed study of double fertilization in fertile embryo sacs of C. oleifera and reported premitotic gametogony double fertilization in this species. Cao (1965) performed karyotype analysis of C. oleifera endosperm development and found that endosperm cells are absorbed during embryo formation; thus, the mature seeds have no endosperm. The results of Gao et al. (2015c) and Liao et al. (2014) show that C. oleifera self-incompatibility is due to a prezygotic late-acting reproductive barrier. In general, mature seeds within fruit are important indicators of seed set under good cross-pollination conditions, which is often associated with mature fertile ovules in the ovary. To a certain extent, the number of fertile ovules in the ovary also reflects the number of mature and potential seeds that can develop into fruit. Conversely, abortive seeds cannot be harvested for economic gains.
C. oleifera is a self-incompatible plant, and fruit set is greatly reduced following self-pollination. Under cross-pollination, fruit set is reestablished; nevertheless, some dry and aborted seeds still form. Therefore, in this study, we explore the nascent structures of aborted seeds prefertilization and postfertilization to determine the physiological differences between fertile and aborted ovules. To this end, we compared fertile and abortive ovules, examining the developmental characteristics of aborted ovules and the number of sterile ovules in a mature ovary. Such a study of the developmental characteristics and occurrence of sterile ovules is of great significance to C. oleifera reproductive biology research and can provide basic information on the evolution and breeding of C. oleifera, as well as lay a theoretical basis for selecting cultivars with a higher seed-setting proportion.
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