Apomixsis- definition, classification, Agamospermy, vegetative reproduction

                                                                     APOMIXIS

 

The normal sexual cycle (amphimixis) involves two important processes (a) Meiosis, this transforms a diploid sporophytic cell (megaspore mother cell, MMC) into four haploid gametophytic cells, and (b) Fertilization, In which two haploid gametes of opposite sex fuse re-establishing the diploid sporophytic generation. Thus, in a sexual cycle a diploid generation (sporophytic) alternates with a haploid generation (gametophytic) In angiosperms the gametophytic generations are very short and are represented by embryo sac on the female side and microspores or pollen grains on the male side. In some plants meiosis and syngamy are interrupted and still a viable embryo is formed within the confines of the seed-coat. Such asexual seeds, which produce progeny identical to the female parent, are called apomictic seeds, and the phenomenon is known as apomixis (Apo = away from + mixis = act of mixing or mingling). The initial discovery of apomixis in higher plants is attributed to the observation that a solitary female plant of Alchornea ilicifolia from Australia continued to form seeds in Kew Gardens in England (Smith, 1841). apomixis is more common in polyploids than diploids. Following Winkler (1908), who coined the term apomixis to mean "substitution of sexual reproduction by an asexual multiplication process without nucleus or cell fusion", plant breeders restrict the scope of apomixis to asexual seed formation (Bashaw, 1980; Khush et al., 1994). However, many authors favour a broader concept, according to which apomixis refers to substitution of the usual sexual reproduction by a form of asexual reproduction which does not involve meiosis and syngamy. Accordingly, there are two main categories of apomixis.

 1. Vegetative Reproduction:

In this category the plants propagate by a part of their body other than the seed. The structural units employed for this purpose are called propagules.

2. Agamospermy:

 The plants belonging to this category have retained seed as the agent of propagation but the embryo is formed by some process in which normal meiosis and syngamy have been eliminated. Two different types of agamospermy are recognized, based on whether the embryo develops from a cell of the unreduced female gametophyte (gametophytic) or directly from diploid sporophytic cells of the ovule, such an nucellus and integument sporophytic or adventive embryony In gametophytic apomixis an unreduced embryo sac develops from the MMC by circumvention of meiosis (diplospory) or directly from a cell be their nucellus (apospory). The embryo is formed by the unfertilized egg (parthenogenesis) or some other cell of the embryo sac (apogamety). Curtain species of Potentila show both apospory and apogamy In gametophytic apomixis alternation of gametophytic and sporophytic generations maintained.

 VEGETATIVE REPRODUCTION

 Some form of vegetative reproduction is generally found associated with the seminiferous plants (reproducing by means of normal seeds). However, only those plants are regarded as apomictic where vegetative reproduction has replaced the sexual method completely or essentially so Vegetative propagation includes reproduction by means of bulbils, bulbs, runners, suckers, and so on. These propagules are formed by the sporophyte only Gustafsson (1946) has distinguished three types of vegetative reproduction in higher plants:

(1) The propagules are formed outside the floral regions and, despite the occurrence of functional sex organs, no fertilization or seed setting takes place Agave americana and Elodea canadensis are examples of this type.

(2) The propagules are formed outside the floral regions and the plants are sexually sterile. Fritillaria imperialis and Lilium bulbiferum are typical representatives of this group. They propagate by means of bulbils and bulblets. A species may have sexually sterile as well as sexually fertile races (F. imperialis) The former shows stronger tendency for vegetative propagation

 (3) The propagules are formed on the floral branches either in addition to or in place of the flowers. The phenomenon is commonly described as vivipary Since this term is also used in reference to a situation where sexually formed seeds germinate on the mother plant (as in mangroves), in the present context the term vegetative vivipary will be used.

 Vegetative vivipary is quite common in grasses (Deschampsia, Festuca Pas) and Allium It is actually an adaptation for the multiplication of a genotype under a set of environmental conditions which prevent or limit under the opportunity for normal pollination. This is suggested by the fact that i most of the vegetative viviparous races have not lost the capacity for flowering and normal seed setting For example, Deschampsia caespitosa reproduces sexually in Sweder, but shows vivipary when grown in California.Vegetative vivipary can also be induced artificially in Pau bulbosa the initiation of inflorescence is promoted by 16 hours light following vernalization for a week or more at 10⁰C Normal fonts develop under high temperatures 21-27⁰C and long days following the initiation of inflorescence However, if short days and low temperatures (20⁰Cor below) are given after the initiation of inflorescence, bulbils develop in place of normal florets.

AGAMOSPERMY

Adventive Embryony:

In this type of agamospermy the gametophytic generation is completely eliminated. In this respect it is close to the vegetative propagation, but differs from it essentially in two ways: (a) it has retained the seed habit, and (b) one or more diploid sporophytic cells undergo the stages of embryogeny and grow into mature embryos inside the normally developed sexual embryo sac. The zygotic embryo either degenerates or competes with the apomictic embryos.Adventive embryony usually leads to the formation of more than one embryo in a seed. Besides the well known examples of citrus and mango, adventive embryony is known to occur in Buxaceae. Cactacear. Euphorbiaceae, Myrtaceae and Orchidaceae

Gametophytic Apomixis:

Diplospory:

 In this category of apomicts, the MMC differentiates as in sexual ovules but it does not enter meiosis. A special feature of apomictic megasporocytes is that they lack a callose wall around them unlike the megasporocytes undergoing meiosis in sexual individuals of the same species (Savidan, 2001). It produces diploid embryo sac through mitotic divisions As early as 1896, Juel described diplospory in Antennaria al Since then this phenomenon has been reported in several species. Of the plants known to exhibit gametophytic apomixis, 75% belong to three families, the Asteraceae, Rosaceae and Poaceae (Bicknell and Koltunow, 2004). Four types of diplosporic embryo sac development have been described, and named after the genera in which they were first observed

 (i) Taraxacum type:

 The MMC enters meiotic prophase and pairing of homologous chromosomes occurs to some extent. However, due to precocious desynapsis univalents are scattered over the spindle at metaphase-1 A restitution nucleus is formed after the first meiotic division. The MMC with the restitution nucleus divides mitotically to form a dyad with somatic (2n) chromosome number Usually the micropylar cell of the dyad degenerates while the chalazal cell undergoes further mitotic divisions to form an 8-mucleate embryo sac. Besides Taraxacum, type of diplospory occurs in Erigeron sp., Aralus halbellis and Agropyron scabra

(ii) Ixeris type:

In the triploid races of Ixeris dentata the MMC divides by a semi-heterotypic (asyndetic) meiotic prophase, in which there is no pairing and a restitution nucleus results. The restitution nucleus undergoes 3 mitotic divisions leading to the formation of an 8-nucleate embryo sac.

(iii) Antennaria type:

 MMC does not enter meiosis and acts as an unreduced megaspore After a long interphase, it increases in size and shows pronounced vacuolation to appear like functional megaspore Through mitotic divisions it forms an unreduced 8 nucleate embryo sac of Polygonum type This type of diplospory is of wide taxonomic distribution. It has been reported in Tripsucum dactyloides, T. zopilotense and Eupatorium glandulosum (Leblanc and Savidan, 1994).

(iv) Allium Type:

In this type of premeiotic chromosome doubling is the cause of unreduced embryo sac formation (Hakansson and Levan 1957; Kojima and Nagato, 1992). Chromosome number in MMC is doubled by endomitosis and the ensuing meiosis results in a dyad of unreduced cells. Two subsequent mitoses in the chalazal dyad cell results in an 8-nucleate embryo. sac Allium nutans and A. odorum show Allium type of diplospory. Kojima and Nagato (1992) reported 76%-98% diplospory in six cultivars of A. tuberosum, based on the percentage of reduplicated MMC.

Apospory:

 Apospory is by far the most common mechanism of apomixis in higher plants (Bashaw, 1980). It was first reported by Rosenberg (1907) in Hieracium sp. Within the genus Hieracium, subgenus Hieracium. H murorum is autonomous diplosporous apomict whereas the subgenus Pilosella species H pracaltum exhibits autonomous apospory, Obligate ucellar apospory with pseudogamy has been observed in seven species of Pennisetum P macerum meum , Poritair P pedicellation plantace, and Pamulatum (Dujardim and Hanna, 1 199) In aposporous plants a normal MMC differentiates in the sa manner as in sexual ovules and begins to enter meiosis which may or m not complete both divisions to form a linear tetrad.  At some stage during this period one or more adjacent nucellar cells become meristematic appear like MMC These cells undergo mitotic divisions to form unreduced mitotic embryo sacs. In obligate apospory the sexual MMC or its products ab and the sporous embryo sac occupies the entire area of the ovule The aposporous of degeneration timing of initiation of the sexual embryo sac depends on the time of aposporous embryo sacs The sooner an aposporous initial induced earlier the degeneration of the developing megaspore occurs. There in considerable variation, even within the same species, with respect to de number of fully developed aposporous embryo sacs formed within an ovule. In the aposporous members of the Asteraceae, only one nucellar cell acts the mother cell and gives rise to structurally normal, 8-nucleate embryo sac In grasses, on the other hand, more than one embryo sac may develop in the same nucellus with 4-nucleate monopolar organization. In aposporous H. aurantiacum of the subgenus Pilasella, multiple embryo sacs in an ovule fuse into one due to degeneration of their walls during their development (Koltunin et al, 1998, 2000) Occasionally, aposporous and sexual embryo sacs may co-exist Apospory is mainly of two types

 (i) Hieracium type:

 In Hieracium, subg Pilosella, the MMC undergoes the usual meiotic division and forms a tetrad. At this stage a nucellar cell at the chalazal end of the tetrad becomes activated and starts developing into an aposporous, unreduced embryo sac All the four megaspores gradually degenerate and the aposporic embryo sac matures. Occasionally, aposporous embryo sac may develop in addition to the reduced embryo sac. This type of aposporic embryo sacs are 8-nucleate.

(i) Panicum type:

 In Panicum and some other grasses, the aposporous embryo sac is 4-nucleate, organized into a 3-celled egg apparatus and a polar nucleus Antipodals are absent In Poa pratensis the ovules contain multiple embryo sacs but only one of these gives rise to an embryo in the mature seed (Savidan, 2001). Generally the largest embryo sac in an ovule, which is closest to the micropyle, is fertilized and forms seed.

REFERENCE :

THE EMBROLOGY OF ANGISPERMS 6^th EDITION Author; SS BHOJWANI, SP BHATNAGAR ,PK DANTU.