**1. Introduction**

Angiosperms have two reproduction routes: gametophytic (sexual) and apomictic (asexual). The first form of reproduction leads to seeds with an embryo from the union of the egg cell nucleus with one of the generative nuclei from the male gamete producing a plant with genetic characteristics different from the female parent. This process promotes genetic diversity through hybridization and the adaptation of plants that allows the conservation of species [1]. We will start this chapter by describing the apomictic process and the evolutionary mechanism of alternate pathways that have allowed cloning plants using seeds [2], including most citrus species.

Various mechanisms intervene to generate asexual embryos, eluding the fundamental aspects of sexual reproduction: meiosis and fertilization [3]. According to the mechanism of embryogenesis, apomixis comprises gametophytic apomixis (apospory and diplospory) and adventitious embryony. In these, apomictic embryos have maternal inheritance, but each mechanism is associated with a different probability of producing sexual offspring, varied selection pressure to maintain male fertility, and expected levels of genetic diversity within populations [4]. In apospory, the initial cells form unreduced embryo sacs from direct mitosis and can coincide with sexual embryogenesis: if endosperm forms, the process is pseudogamic, or autonomous if it does not [5]. This second route, which occurs in less than 1% of angiosperms, comprises the development of embryos from somatic cells.

Similarly, in diplospory, the megaspore does not undergo meiosis and through mitosis forms an unreduced embryo sac with cells arranged as in the Polygonum type (sexual reproduction); but the sexual process is wholly compromised [5]. Whereas in adventitial apomixis, the development of the sexual embryo sac seems to develop normally, and after fertilization and development of the sexual embryo, somatic embryos develop from nucellar or integumental cells [6, 7]. In citrus, adventitious embryonic development does not occur without zygote formation (apomixis is facultative), depends on endosperm formation, and is simultaneous with the development of sexual embryos [6, 8, 9]. Therefore, when the embryo sac expands, the embryogenic


#### **Table 1.**

*Percentage of polyembryony and embryos per seed in apomictic genotypes of citrus.*

#### *Citrus Polyembryony DOI: http://dx.doi.org/10.5772/intechopen.105994*

cells of the nucellus enter the endosperm, competing for space and nutrients with the zygotic embryos that may or may not complete their development [10].

Adventitial embryony or sporophytic apomixis frequently occurs in Rutaceae, Celastraceae, and Orchidaceae; furthermore, it is common in tropical and subtropical trees and shrubs [4]. Polyembryony in citrus is a type of sporophytic apomixis that occurs in most of its species and hybrids (**Table 1**). For example, in 12 genotypes (Sikkim, Thorny, Kinnow and Cleopatra mandarin, Calamondin, Nova×Orlando hybrid, Minneola tangelo, Thornton, Parramatta, Parson Brown, Smooth Flat Seville, and rough lemon rootstock) rates of polyembryonic seeds ranged from 69.8% to 91.4%, with up to 14 embryos per seed [8]. The following genotypes do not show polyembryony, thus classified as non-apomictic: *Citrus hongheensis*, *C. macropera*, *C. medica*, *C. maxima*, *C. clementina*, *Chirita mangshanensis*, and some mandarin hybrids [16, 17]. Although polyembryony is not a common biological phenomenon among angiosperms, it is a form of reproduction that has been studied for 14 decades. According to Cook [18], the oldest record of polyembryony in orange seeds (*Citrus sinensis* L. Osbeck) occurred in 1719 by Leeuwenhoek. Later, Strasburger [19] worked with different species and established that in sour orange (*Citrus aurantium*), all the embryos not derived by the cross originated from nucellar cells, and he called them "adventitious embryos." Since then, polyembryony in citrus has become a form of vegetative propagation, with the problem that for genetic improvement, seedlings from hybrid embryos cannot be differentiated from seedlings from adventitious embryos. Then, hybrid embryos can be differentiated by molecular methods or by crossing unifoliate materials with trifoliate orange (*Poncirus trifoliata*) pollen.
