George Acquaah - Principles of Plant Genetics and Breeding

The benefits of apomixis may be examined from the perspectives of the plant breeder and the crop producer.

Apomixis, essentially, is a natural mechanism of cloning plants through seed. If it could be developed to become a breeding tool, it would allow plant breeders to develop hybrids that can retain their original genetic properties indefinitely with repeated use, without a need to reconstitute them. In other words, hybrid seed could be produced from hybrid seed. The plant breeder would not need to make crosses each year to produce the hybrid. This advantage would accelerate breeding programs and reduce development cost of hybrid cultivars. Apomixis would be greatly beneficial when uniformity of product is desired. Breeders could use this tool to quickly fix superior gene combinations. That is, vigor could be duplicated, generation after generation without decline. Furthermore, commercial hybrid production could be implemented for species without fertility control mechanisms (e.g. male‐sterility system); neither would there be a need for isolation in F 1hybrid seed production. There would be no need to maintain and increase parental genotypes. Cultivar evaluation could proceed immediately following a cross. This advantage is applicable to clonal propagation in general.

In addition to these obvious benefits, it is anticipated that plant breeders will divert the resources saved (time, money) into other creative breeding ventures. For example, cultivars could be developed for smaller and more specific production environments. Also, more parental stock could be developed to reduce the risk of genetic vulnerability through the use of a few elite genetic stocks as parents in hybrid development.

There are some plant breeding concerns associated with apomixis. Species that exhibit partial apomixis are more challenging to breed because they produce both sexual and apomitic plants in the progeny. Complete apomicts are easier to breed by conventional methods.

The most obvious benefit of apomitic cultivars to crop producers is the ability to save seed from their field harvest of hybrid cultivars for planting the next season. Because apomixis fixes hybrid vigor, the farmer does not need to purchase fresh hybrid seed each season. This would especially benefit the producer in poor economies, who often cannot afford the high price of hybrid seed. Apomixis, as previously indicated, could accelerate plant breeding. This could translate into less expensive commercial seed for all producers. Realistically, such benefits will materialize only if commercial breeders will make acceptable profit from using the technology.

Some speculate that apomixis has the potential to reduce biodiversity because it produces clonal cultivars and hence uniform populations that are susceptible to disease epidemics. However, others caution that the suspected reduction in biodiversity would be minimal since apomixis occurs naturally in polyploids, which occurs less frequently than diploids.

Apomixis arises by a number of mechanisms, of which four major ones that differ according to origin (cell that undergoes mitosis to produce the embryo) are discussed next. Seed formation without sexual union is called agamospermy, the mechanism that may be summarized into two categories – gametophytic apomixisand adventitious apomixis. There are two types of gametophytic apomixis – aposporyand diplospory.

This is the most common mechanism of apomixis in higher plants. It is a type of agamospermy that involves the nucellar. The somatic cells of the ovule divide mitotically to form an unreduced (2 n ) embryonic sac. The megaspore or young embryo sac aborts, as occurs in species such as Kentucky bluegrass ( Poa pratensis ).

Unreduced megaspore mother cell produces the embryo sac following mitosis instead of meiosis. This cytological event occurs in species such as Tripsacum .

Unlike apospory and diplospory in which an embryo sac is formed, no embryo sac is formed in adventitious embryony. Instead, the source of the embryo could be somatic cells of the ovule, integuments, or ovary wall. This mechanism occurs commonly in Citrus but rarely in other higher plants.

This mechanism produces an outcome that is essentially equivalent to haploidy. The reduced ( n ) egg nucleus in a sexual embryo sac develops into a haploid embryo without fertilization by the sperm nucleus. In this case the embryo has not an identical genotype as the plant on which it is formed.

Other less common mechanisms of apomixis are androgenesis(development of a seed embryo from the sperm nucleus upon entering the embryo sac), and semigamy(sperm nucleus and egg nucleus develop independently without uniting, leading to one haploid embryo). The resulting haploid plants contain sectors of material from both maternal and paternal origin, and is therefore chimeric.

There are other tissue culture‐based applications of interest to plant breeders besides micropropagation.

Somatic embryogenesis has potential commercial applications, one of which is in the synthetic seedtechnology (production of artificial seeds). A synthetic seed consists of somatic embryos enclosed in protective coating. There are two types currently being developed:

1 Hydrated synthetic seed – This kind of seed is encased in hydrated gel (e.g. calcium alginate).

2 Desiccated synthetic seed – This kind of seed is coated with water soluble resin (e.g. polyoxethylene).

To develop synthetic seed, it is critical to achieve a quiescent phase, which is typically lacking in somatic embryogenesis (i.e. without quiescence, there is continuous growth, germination, and eventually death, but no stationary stage as in embryos in mature seeds). The application will depend on the crop. Lucerne ( Medicago sativa ) and orchardgrass ( Dactylis glomerata ) are among the species that have received much attention in artificial seed development. Potential application of artificial seed is in species that are highly heterozygous and in which conventional breeding is time‐consuming. Trees can be cloned more readily by this method. In some tropical species that are seed propagated but in which seeds have short duration of viability, artificial seed production could be economical, because of the high economic value of these crops (e.g. cacao, coconut, oil palm, coffee). Also, hybrid synthetic seed could be produced in species in which commercial hybrid production is problematic (e.g. cotton, soybean).

Whereas the prospect of commercial synthetic seed is appealing, several factors make this impractical at this time. Problems may occur at maturation, germination, rooting, shot apex formation, or acclimatization.

Large‐scale production of high quality viable propagules remains a key challenge.

A major limitation is the poor conversion of apparently normal propagules into normal plantlets.

Improper development and maturation of somatic embryos causes poor germination and conversion problems.