Advantages of Apomixis

Introduction

It has been ten years since this journal published a review on apomixis, or asexual seed formation (Koltunow, 1993). The emphasis in that article was on the similarities known among apomictic processes in terms of sexual reproduction events. Apomixis inheritance had been established in some species, and molecular mapping studies had begun. However, the molecular relationships between apomictic and sexual reproduction were unknown. With advances in both sexual and apomictic systems in the intervening years, subsequent reviews of apomixis in the literature have considered the economic benefits of providing apomixis to developing and developed agricultural economies (Hanna, 1995; Savidan, 2000a) and strategies to gain an understanding of apomixis through comparison with sexual systems (Hanna, 1995; Savidan, 2000a) (Koltunow et al., 1995a; Grimanelli et al., 2001a). The potential to “synthesise apomixis” in agricultural crops where it is currently absent by modifying sexual reproduction steps, as well as the potential ecological consequences of releasing “synthesized apomicts” in nature, have also been discussed (van Dijk and van Damme, 2000; Grossniklaus, 2001; Spillane et al., 2001).Recently, comparative developmental features of apomixis have been considered in light of the growing body of knowledge about ovule and female gametophyte development, as well as seed formation in sexual plants (Koltunow and Grossniklaus, 2003). We focus on the initiation and progression of apomixis in plants that naturally express the trait in this review. Since 1993, there has been a growing appreciation for the complexities that underpin apomixis; some contentious issues have been resolved, while others have been raised.

Apomixis

Apomixis is defined in flowering plants as the asexual formation of a seed from the maternal tissues of the ovule, avoiding the processes of meiosis and fertilisation, which leads to embryo development. The first observation of apomixis in higher plants was made when a solitary female plant of Alchornea ilicifolia (syn. Caelebogyne ilicifolia) from Australia was planted at Kew Gardens in England (Smith, 1841).Winkler (1908) defined apomixis as “the substitution of sexual reproduction by an asexual multiplication process lacking a nucleus and cell fusion.” As a result, some authors have chosen to use the term apomixis to describe all forms of asexual reproduction in plants, but this broad interpretation is no longer widely accepted. The term “apomixis” is now synonymous with the term “agamospermous” (Richards, 1997). Because seeds are only found in angiosperm and gymnosperm taxa, this definition of apomixis is limited to those taxa. Although phenomena similar to apomixis are known in lower plants, there is still debate about the use of this term in cases where the reproductive structures involved are different but are considered analogous (Asker and Jerling, 1992).

Prevalence of Apomixis

Although it is sometimes referred to as a botanical curiosity, apomixis is far from uncommon, being relatively common among angiosperms and with a distribution pattern that suggests it has evolved many times. It has been described in over 400 flowering plant taxa, including representatives from over 40 families (Carman, 1997), and it is found in both monocotyledonous and dicotyledonous plants; however, it appears to be absent in gymnosperms. These are almost certainly very conservative estimates. Apomixis must be confirmed unequivocally by examining both genetic and cytological evidence at the same time (Nogler, 1984a).Embryological examination of plant taxa for apomixis has not been exhaustive, and even when apomixis has been declared for a given plant, supporting genetic evidence is rare. It appears likely that as our understanding of this phenomenon expands and methods for determining its presence improve, many more angiosperm taxa will be discovered to have apomictic representatives, and some suspected cases will be revised. Pitman’s (2002) recent study backs up this prediction.                                          

Mechanisms of Apomixis

The cell type that gives rise to the unreduced embryo sac further subdivides gametophytic mechanisms. The progenitor cell for the unreduced embryo sac in diphosphorus types is the megaspore mother cell (MMC) or a cell with apomictic potential occupying its position. That cell may enter meiosis, but it will die, and development will continue through mitotic division to the formation of an embryo sac (meiotic diplospory). Alternatively, that cell could go through direct mitosis, resulting in an unreduced embryo sac (mitotic diplospory). One or more somatic cells of the ovule, known as aposporous initials, give rise to an unreduced embryo sac in aposporous apomicts. Aposporous initials can differentiate at different stages of ovule development. In the ovule, mitotically reduced and aposporous embryo sacs may coexist, or the aposporous embryo sac may continue to develop while the reduced sexual embryo sac degenerates. Gametophytic mechanisms have been further subdivided based on characteristics related to the involvement or avoidance of different phases of the meiosis, the number of mitotic divisions, and the eventual form of the embryo sac (2001).

Apomixis Importance

We must first understand what apomixis is and why it is so important. It provides plants with a number of benefits that make them more efficient and resilient. As an example:

• Apomixis ensures that a plant reproduces even when pollinators are absent or extinct
• Apomixis saves a lot of maternal energy that would otherwise be wasted producing a non-viable/unfit progeny; thus, the cost of meiosis is reduced and less energy is expended
• Apomixis also saves male energy in some plants by avoiding the cost of pollen production

Types of Apomixis

Recurrent Apomixis

Without fertilisation, the embryo sac develops from the diploid egg mother cell or from other diploid cells of the embryo sac in this type of apomixis. As a result, the egg has the normal diploid number of chromosomes, the same as the mother plant. The embryo develops from the egg nucleus without fertilisation. This sequence of events is known to occur in some Crepis, Poa Taraxacum, and Allium species without the stimulus of pollination, whereas in others, such as Parthenium, Rubus, and Malus (apple), the stimulus of pollination appears to be required for the development embryo or the production of a viable endosperm.

Non-Recurrent Apomixis

The embryo develops directly from the haploid egg cell (haploid parthenogenesis) or some other haploid cells of the embryo sac (haploid apogamy) without fertilisation in this type of apomixis, and as a result, the embryo developed is also haploid. This type of apomixis is extremely rare and is mostly of genetic interest. The plants that are created are haploid and sterile. It is found in Solanum nigrum and Lilius species, among others.

Adventitious Embryony or Nuclear Embryony

Nuclear embryony, or nucellar budding, is another name for adventitious embryony. The embryo does not develop from the cells of the embryo sac in this case, but rather from a cell or a group of cells from the nucleus or the integuments. Because the cells of the integument or nucellus are diploid, the resulting embryo is also diploid. In addition to the regular embryo, such embryos usually develop outside the embryo sac. Adventitious embryony occurs in a wide range of plant species, but it is most common in tropical and subtropical tree species such as citrus and mango.

Vegetative apomixis or bulbils

In this type, instead of flowers, the inflorescence produces vegetative buds or bulbils. While these buds or bulbils are still attached to the mother plants, they may sprout into new plants. This type of apomixis is common in Allum, Agave, Poa, and Diascorea, as well as some grasses.

Advantages

• In the absence of pollinators, such as in harsh environments, reproduction is guaranteed
• Maternal energy is not wasted in unfit offspring (meiosis cost)
• Some apomictic plants (but not all) avoid the male energy cost of pollen production

Disadvantages

• Cannot prevent the accumulation of harmful genetic mutations
• Usually restricted to specific ecological niches a lack of adaptability to changing environments

Conclusion

It has been ten years since this journal published a review on apomixis, or asexual seed formation (Koltunow, 1993). The emphasis in that article was on the similarities known among apomictic processes in terms of sexual reproduction events. Apomixis inheritance had been established in some species, and molecular mapping studies had begun. Apomixis is defined in flowering plants as the asexual formation of a seed from the maternal tissues of the ovule, avoiding the processes of meiosis and fertilization, which leads to embryo development. ). Because seeds are only found in angiosperm and gymnosperm taxa, this definition of apomixis is limited to those taxa. Although phenomena similar to apomixis are known in lower plants, there is still debate about the use of this term in cases where the reproductive structures involved are different but are considered analogous. The cell type that gives rise to the unreduced embryo sac further subdivides gametophytic mechanisms. The progenitor cell for the unreduced embryo sac in diphosphorus types is the megaspore mother cell (MMC) or a cell with apomictic potential occupying its position. Without fertilisation, the embryo sac develops from the diploid egg mother cell or from other diploid cells of the embryo sac in this type of apomixis. As a result, the egg has the normal diploid number of chromosomes, the same as the mother plant.