Principles of Plant Genetics and Breeding. George Acquaah
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7.15 Apomixis
Seed production in higher plants that are sexually propagated species normally occurs after a sexual union in which male and female gametes fuse to form a zygote, which then develops into an embryo. However, some species have the natural ability to develop seed without fertilization, a phenomenon called apomixis. The consequence of this event is that apomictically produced seeds are clones of the mother plant. That is, apomixis is the asexual production of seed. Unlike sexual reproduction, there is no opportunity in apomixis for new recombination to occur to produce diversity in the offspring. However, the seeds have the same advantages as non‐apomictic seeds: they can be stored and sown, and have less chance to be virus infected than whole plants or plant organs.
7.15.1 Occurrence in nature
Apomixis is widespread in nature, and occurs in several unrelated plant families. About 10% of the estimated 400 plant families and a mere 1% of the estimated 40 000 plant species they comprise exhibit apomixis. The plant families with the highest frequency of apomixis are Gramineae (Poaceae), Compositae, Rosaceae, and Asteraceae. Many species of Citrus, mango, perennial forage grasses, and guayule reproduce apomictically.
Some species can produce both sexual and apomictic seeds and are called partial apomicts (e.g. bluegrass, Poa pratensis). Species such as bahiagrass (Paspalum notatum) reproduce exclusively or nearly so by apomixis and are called complete apomicts. There are several indicators of apomixis. When the progeny from a cross in a cross‐pollinated species fails to segregate, appearing uniform and identical to the mother plant, this could indicate apomixis. Similarly, when plants expected to exhibit high sterility (e.g. aneuploids, triploids) instead show significantly high fertility, apomixis could be the cause. Obligate apomicts may display multiple floral features (e.g. multiple stigmas and ovules per floret, double or fused ovaries) or multiple seedlings per seed. Facultative apomixis may be suspected if the progeny of a cross shows an unusually high number of identical homozygous individuals that resemble the mother plant in addition to the presence of individuals that are clearly different (hybrid products).
The indicators suggested are by no means conclusive evidence of apomixis. To confirm the occurrence of apomixis and discovery of its mechanisms requires an additional progeny test as well as cytological tests of megasporogenesis and embryo sac development.
7.15.2 Benefits of apomixis
The benefits of apomixis may be examined from the perspectives of the plant breeder and the crop producer.
Benefits to the plant breeder
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 F1 hybrid 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.
Benefits to the producer
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.
Impact on the environment
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.
7.15.3 Mechanisms of apomixis
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 apomixis and adventitious apomixis. There are two types of gametophytic apomixis – apospory and diplospory.
Apospory
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 (2n) embryonic sac. The megaspore or young embryo sac aborts, as occurs in species such as Kentucky bluegrass (Poa pratensis).
Diplospory
Unreduced megaspore mother cell produces the embryo sac following mitosis instead of meiosis. This cytological event occurs in species such as Tripsacum.
Adventitious embryo
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.
Parthenogenesis
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