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.
Akhalkatsi, M., Pfauth, M. & Calvin, C.L. 1999 Structural aspects of ovule and seed development and nonrandom abortion in Melilotus officinalis (Fabaceae) Protoplasma 208 211 223
Casper, B.B. & Wiens, D. 1981 Fixed rates of random ovule abortion in Cryptantha flava (Boraginaceae) and its possible relation to seed dispersal Ecology 62 866 869
Deng, Y.Y., Yu, X.L. & Luo, Y.B. 2010 The role of native bees on the reproductive success of Camellia oleifera in Hunan Province, Central South China Acta Ecol. Sin. 30 4427 4436
Gao, C., Yuan, D.Y., Wang, B.F., Yang, Y., Liu, D.M. & Han, Z.Q. 2015a A cytological study of anther and pollen development in Camellia oleifera Genet. Mol. Res. 14 8755 8765
Gao, C., Yuan, D.Y., Yang, Y., Wang, B.F., Liu, D.M. & Zou, F. 2015b Pollen tube growth and double fertilization in Camellia oleifera J. Amer. Soc. Hort. Sci. 140 12 18
Gao, C., Yuan, D.Y., Yang, Y., Wang, B.F., Liu, D.M., Zou, F. & Tan, X.F. 2015c Anatomical characteristics of self-incompatibility in Camellia oleifera Scientia Silvae Sinicae 2 60 68
Gao, C., Yuan, D.Y., Yuan, J., Qiu, Y.Q. & Gan, G.J. 2011 Influences of sod culture on the physical and chemical characteristics of soil in hilly Pyrus pyrifolia orchard of eastern Hunan Hubei Agr. Sci. 8 1593 1595
Hu, S.Y. 2005 Reproductive biology of angiosperms. China Higher Educ. Press, Beijing, China
Jing, H.C., Bergervoet, J.H.W., Jalink, H., Klooster, M., Du, S.L., Bino, R.J., Hilhorst, H.W.M. & Groot, S.P.C. 2000 Cucumber (Cucumis sativus L.) seed performance as influenced by ovary and ovule position Seed Sci. Res. 10 435 445
Lee, C.P. & Yen, G.C. 2006 Antioxidant activity and bioactive compounds of tea seed (Camellia oleifera Abel.) oil J. Agr. Food Chem. 54 779 784
Liao, T., Yuan, D.Y., Zou, F., Gao, C., Yang, Y., Zhang, L. & Tan, X.F. 2014 Self-sterility in Camellia oleifera may be due to the prezygotic late-acting self-incompatibility PLoS One 9 e99639
Lillecrapp, A.M., Wallwork, M.A. & Sedgley, M. 1999 Female and male sterility cause low fruit set in a clone of the ‘Trevatt’ variety of apricot (Prunus armeniaca) Sci. Hort. 82 255 263
Rim, Y.W., Beuselinck, P.R., McGraw, R.L. & Somers, D.A. 1990 Megagametophyte development in Lotus corniculatus, L. conimbricensis, and their protoplast fusion hybrid Amer. J. Bot. 77 1084 1094
Rocha, O.J. & Stephenson, A.G. 1990 Effect of ovule position on seed production, seed weight, and progeny performance in Phaseolus coccineus L. (Leguminosae) Amer. J. Bot. 75 1320 1329
Wang, X., Li, X., Zhang, J.W., Feng, G.H., Zhang, S.Z., Huang, L.C., Zhuo, R.Y. & Jin, L. 2011 Characterization of nine alfalfa varieties for differences in ovule numbers and ovule sterility Austral. J. Crop Sci. 5 447 452
Yuan, D.Y., Zou, F., Tan, X.F., He, C.Y., Yuan, J. & Fan, X.M. 2011 Flower bud differentiation and development of male and female gametophytes in Camellia oleifera J. Central South Univ. For. Technol. 31 65 70
Zhuang, R.L. 2008 Camellia oleifera in China. China For. Press, Beijing, China