🧬 Apomixis and Its Classification
Learn the meaning, types, and breeding importance of apomixis.
Apomixis is one of the most interesting topics in plant breeding because it offers the possibility of producing seed without normal fertilization. If this mechanism can be used effectively, it could help preserve desirable genotypes and even fix hybrid vigor.
What Is Apomixis?
Apomixis is a type of asexual reproduction through seed in which embryo formation occurs without normal fusion of male and female gametes.
In simple terms:
- sexual organs or related structures may be present
- but the seed develops without normal fertilization
So the offspring formed through apomixis are genetically derived mainly from the maternal tissue or maternal lineage without the usual sexual recombination process.
Important forms by occurrence
- obligate apomixis: the plant reproduces only through apomixis
- facultative apomixis: both sexual and apomictic reproduction may occur in the same plant
Why Apomixis Is Important
Normally, sexual reproduction causes recombination and segregation, which change the genetic composition of progeny.
Apomixis is important because it can preserve the existing genotype from generation to generation.
This has major breeding significance because it may:
- maintain uniformity
- preserve heterozygosity
- reduce segregation
- help fix hybrid vigor
Distribution of Apomixis
Apomixis occurs in many higher plants and is especially known in some members of:
- Gramineae
- Compositae
- Rosaceae
- Rutaceae
Some economically important crop groups have wild or related apomictic forms, which is why the topic attracts strong breeding interest.
Classification of Apomixis
A practical classification often includes the following major types:
- recurrent apomixis
- non-recurrent apomixis
- adventive embryony
- vegetative apomixis
1. Recurrent Apomixis
In recurrent apomixis, the embryo develops from a diploid egg cell without fertilization.
Because the egg remains diploid, the resulting embryo is also diploid and can continue the line effectively.
This type is especially important in plant breeding because it can perpetuate useful genotypes across generations.
Examples of mechanisms associated with recurrent apomixis include:
- diploid parthenogenesis
- apospory
- related diploid embryo-sac pathways
This is one of the most useful categories from the breeder’s viewpoint.
2. Non-Recurrent Apomixis
In non-recurrent apomixis, the embryo develops from a normal haploid egg cell without fertilization.
As a result:
- the embryo is haploid
- the system usually does not maintain itself stably generation after generation
Therefore, this form is more important academically than practically for routine crop improvement.
3. Adventive Embryony
In adventive embryony, embryos arise directly from somatic tissues such as:
- nucellus
- integuments
This means the embryos are produced outside the normal embryo sac.
This can result in polyembryony, where more than one embryo occurs in a seed.
This type is well known in crops like citrus.
From a breeding perspective, adventive embryony is useful because it can preserve maternal types very effectively.
4. Vegetative Apomixis
In vegetative apomixis, vegetative structures such as bulbils or related propagules are produced in place of normal flowers or seeds in the reproductive region.
Though some taxonomic or theoretical discussions separate this from strict seed apomixis, it is often discussed along with apomictic phenomena in breeding courses because it also bypasses normal sexual recombination.
Advantages of Apomixis in Plant Breeding
Apomixis is attractive to breeders for several reasons.
1. Fixation of heterosis
If apomixis can be combined with hybrid breeding, hybrid vigor could theoretically be maintained generation after generation without repeated hybrid seed production.
2. Maintenance of genetic uniformity
The same superior genotype can be multiplied without segregation.
3. Rapid multiplication of elite types
Once a desirable genotype is obtained, apomixis can help preserve it efficiently.
4. Conservation of maternal influence
Where maternal effects are useful, apomixis may help maintain them in the progeny.
Limitations and Practical Challenges
Although apomixis is highly attractive in theory, its practical use is limited by:
- difficulty of identification
- complexity of inheritance
- challenge of transferring usable apomictic systems into major sexual crops
- need for reliable detection and breeding integration
So apomixis remains both a practical breeding interest and a long-term research opportunity.
Detection of Apomixis
Detection of apomixis usually requires careful study of:
- embryo-sac development
- embryo origin
- fertility pattern
- progeny uniformity
This often involves detailed cytological and developmental observation, making it a technically demanding process.
Which Types Are Most Useful for Breeding?
From the breeder’s point of view, the most useful forms are those that produce viable diploid embryos and can maintain desirable genotypes.
That is why recurrent apomixis and some cases of adventive embryony are of greatest importance in crop improvement.
Summary Cheat Sheet
- Apomixis is asexual seed formation without normal fertilization.
- It may be obligate or facultative.
- Major types include recurrent apomixis, non-recurrent apomixis, adventive embryony, and vegetative apomixis.
- Recurrent apomixis is most useful in breeding because it produces diploid embryos that can perpetuate the genotype.
- Adventive embryony involves embryo formation from somatic tissues such as nucellus or integuments.
- Apomixis is important because it can maintain uniformity and potentially fix heterosis.
- Its major limitation is the difficulty of effective detection and utilization in breeding programmes.
References
2 sources • [1] [2]
References
Principles of Plant Breeding class notes
BookStandard BSc Agriculture plant breeding practical handbook
BookLesson Doubts
Ask questions, get expert answers