Dimensiones de gestión de calidad de servicio (Agrupado)

Two main types of apomixis are known: gametophytic apomixis (gametophytic agamospermy) and adventitious embryony (nucellar) (Figure 5.3). The two forms of apomixis are unrelated (Asker and Jerling, 1992). In gametophytic apomixis, an egg cell develops parthenogenetically from an unreduced embryo sac, producing only maternal-type offspring. In contrast, adventitious embryony does not have a gametophytic stage but only a sporophytic stage, and it is connected with parthe- nogenesis. Embryos develop from somatic cells of the nucellus or integument. Thus, alternation of generation is eliminated (Grant, 1981).

98 PLANT CYTOGENETICS, SECOND EDITION

a. Gametophytic Apomixis

Gametophytic apomixis has been divided into diplospory and apospory (Figure 5.3). Both forms maintain the alternation of generations, but the gametophytic and sporophytic stages contain the same level of ploidy (Nogler, 1984). Meiosis and fertilization are circumvented in both forms, and an unreduced egg cell develops asexually by parthenogenesis. Although the predominant mode of reproduction in gametophytic apomixis is obligate, it may sometimes be combined with sexuality. Such a type is known as facultative apomixes, and frequency is influenced by several factors.

i. Diplospory — Diplospory (generative apospory) occurs in plants where meiosis is absent or development of a linear tetrad of megaspores does not occur. It is subdivided into two forms named after the genera in which they were first discovered (Figure 5.3):

Taraxacum: In this type, the megaspore mother cell enters the meiotic prophase-I, but chromosomes remain as univalents resulting in restitution nuclei with full somatic chromosome complement. The second division is normal producing dyads with unreduced chromosome number. Usually the chalazal dyad cell gives rise to an eight-nucleate embryo sac after three mitoses (Figure 5.4). There are four divisions of nuclei and cells instead of the five observed in normal embryogenesis (Fig- ure 5.4). The Taraxacum type of apomixis, in which embryo and endosperm develop without fertilization, is known as mitotic diplospory or autonomous apomixis and is found among genera of the Compositae (Taraxacum, Erigeron, Chondrilla), rarely in Arabis holboellii and Agropyron scabrum, and in certain Paspalum species (Nogler, 1984; Asker and Jerling, 1992).

Antennaria: The megaspore mother cell does not undergo meiosis but directly undergoes three mitotic divisions, producing an eight-nucleate unreduced (diploid) embryo sac (Figure 5.4). The Antennaria

type is also called mitotic diplospory and has a wide taxonomic distribution (Nogler, 1984). Other types of diplospory: Deviation from the Taraxacum and Antennaria types of diplospory has been

observed.

Eragrostis: It is similar to the Antennaria type, but with the difference that the embryo sac formed has four nuclei, because the third mitosis is omitted. Thus, cells and nuclei have two divisions (Figure 5.4).

Ixeris: An asyndetic meiotic chromosome pairing produces a restitution nucleus, as observed in the Taraxacum type, which then undergoes a second meiotic division without cytokinesis.

Figure 5.3 Classification of the mode of reproduction in plants.

MODE OF REPRODUCTION

MODE OF REPRODUCTION IN PLANTS 99

This results in two unreduced instead of four reduced nuclei. The two mitoses produce an eight-nucleate embryo sac. The Ixeris type of diplospory is found in Ixeris dentata.

Allium: Premeiotic chromosome doubling by endomitosis or endoreduplication in the female pro- duces unreduced nuclei. Meiosis is normal and yields a tetrad with unreduced nuclei of parental genotypes. Two subsequent mitoses in the chalazal dyad result in an eight-nucleate embryo sac. It is observed in Allium nutans and A. odorum.

ii. Apospory — Apospory, earlier termed “somatic apospory” (Nogler, 1984), is characterized by the development of unreduced embryo sacs directly from the somatic (vegetative) cells located in the center of the nucellus. There may be multiple embryo sacs in an ovule, but only one of them matures into an aposporous embryo sac (Nogler, 1984; Asker and Jerling, 1992). Aposporous apomixis is of two main types: the bipolar type Hieracium and the monopolar Panicum type (Figure 5.3).

Hieracium: The initial cell produces an eight-nucleate bipolar embryo sac by mitotic divisions along with the development of the Polygonum type observed in the related sexual taxa. Thus, an ovule may contain two or more eight-nucleate embryo sacs, one is reduced which may degenerate, and others are unreduced (Khokhlov, 1976). Apospory is widely distributed in several families, e.g., in the genera of Hypericum, Poa, Ranunculus, Crepis, Hieracium, Hierochloe, and Beta (Nogler, 1984).

Panicum: The unreduced being four-nucleate monopolar embryo sac is produced after the second mitosis, and the third mitosis is absent. This type of apomixis was discovered in Panicum maximum

(Warmke, 1954) and is found in grasses belonging to Panicoideae and Andropogoneae (Nogler, 1984; Asker and Jerling, 1992). It is widely distributed in the genera of the Bothriochloa–Dichanth-

ium–Capillipedium complex, Cenchrus, Chloris, Digitaria, Eriochloa, Heteropogon, Hyparrhenia, Panicum, Paspalum, Pennisetum, Sorghum, Themeda, and Urochloa (Nogler, 1984). An exceptional

case is recorded within the Panicoideae. The unreduced aposporous sacs are usually four-nucleate, and the reduced ones are eight-nucleate and bipolar.

b. Adventitious Embryony

The terms nucellar or adventive embryony are synonymous with adventitious embryony (Asker and Jerling, 1992). Adventitious embryony is an asexual reproduction in which adventitious embryos develop from one or more of the nucellar and integumentary cells of the ovule.

Adventitious embryony differs from diplospory and apospory in that it lacks embryo sacs and produces viable seeds. Thus, these embryos do not contain polar nuclei or endosperm. Double fertilization is apparently absent and, even if present, it is influenced by environmental conditions (Naumova, 1993). Meiosis is usually normal with reduced chromosome numbers in embryo sacs. Adventitious embryos can be induced in plants experimentally through tissue culture. The production of somatic embryos (embryoids, i.e., embryo-like structures) in vitro is a prerequisite for regeneration of plants; however, adventitious embryos and embryoids are not always produced in a similar way (Naumova, 1993).

Adventitious embryony is widespread in economically important crops belonging to the genera

Citrus, Euphorbia, Mangifera, Malus, Ribes, Beta and several genera of grasses. It has been identified

in more than 250 species of 121 genera belonging to 57 families of flowering plants (Naumova, 1993). Polyembryony: Polyembryony, the occurrence of more than one embryo in a seed, may be simple or multiple (Figure 5.5). Simple polyembryony is divided into sexual and asexual. Polyem- bryony from sexual reproduction may be zygotic–zygotic polyembryony, develop from suspen- sor–suspensor polyembryony or synergid cells — synergid polyembryony (Lakshmanan and Ambe- gaokar, 1984). Polyembryony is controlled by several genes. It may be autonomous where no induction is necessary, or induced when pollination is required. Asexual polyembryony may arise from gametophyte or sporophyte. The gametophyte polyembryony can be haploid or diploid, and the sporophytic form is from nucellar and integumentary cells (Figure 5.5).