Apomixis: A Detailed Overview PDF

Unit V Apomixis Fr G Victor Roch SJ I. Apomixis Apomixis, derived from two Greek word "APO" (away from) and "mixis" (act of mixing/mingling). Apomixis is a type of reproduction in which sexual organs of related structures take part but seeds are formed without union of gametes. The first...

Unit V Apomixis Fr G Victor Roch SJ I. Apomixis Apomixis, derived from two Greek word "APO" (away from) and "mixis" (act of mixing/mingling). Apomixis is a type of reproduction in which sexual organs of related structures take part but seeds are formed without union of gametes. The first discovery of this phenomenon is credited to Leuwen Ock as early as 1719 in Citrus seeds. The genotype of the embryo and resulting plant will be the same as the seed parent. This is a clonal seed production. Definitions Apospory - the embryo sac originates from the any diploid cell except megaspore mother cell either directly by mitosis and/or after interrupted meiosis. Androgenesis - the embryo sac originates from generative nuleus of pollen tube cells. The most common mechanism of apomixis in higher plants characterized by the presence of multiple embryo sacs. Gametophytic apomixis - If the unfertilized cells give rise to a megagametophyte.  Sporophytic apomixis: If the unfertilized cells give rise directly to an embryo. Haploid parthenogenesis : Embryo developed from unfertilized egg cell. Haploid apogamy (pseudogamy) : embryo developed from synergids or antipodal cells. Adventitious embrony ( sporophytic budding ): embryo directly develop from nucellus or integuments (no production of embryosac) e.g. mango, citrus. Apomixis During sexual reproduction fertilization occurs due to fusion of male and female gamete to form a zygote. During this process, combination of genetic information from two individuals of different types (sexes) occurs. Apomixis can never lead to genetic recombination and variability in population. The offspring produced by apomixis do not undergo gene flow, hence rapid development of pure lines can be achieved by haploid parthenogenesis i. e single set of chromosomes. In higher plants, Apomixis is very common. Plants of more than 35 families are known to be apomictic.  Types of apomixis Apomixis can be classified based on following 1. Cell involved 2. Occurrence 3. Frequency 1. Cell Involved It can be further classified in following types a) Parthenogenesis : where embryo develops from an egg cell b) Diplospory : Either by mitotic division or by interrupting meiosis, the embryo sac is derived from the megaspore mother cell. There are diplospory apomixis : Taraxacum, Ixeris, and Antennaria. i. Taraxacum: meiotic prophase starts but then the phase is aborted which results in two unreduced dyads one of which gives rise to the embryo sac by mitosis ii. Lxeris : Two further mitotic divisions of the nuclei give rise to an octa- nucleate embryo sac which follows an equational division following meiotic prophase. iii. Antennaria: Megagametophyte is formed from the total of three mitotic divisions. c. Apogamy apomixis : The embryo originates from synergids or antipodal cells and develops from the normal embryo sac cells. d. Apospory: Nuclear cells give rise to the apomictic embryo sac. This embryo sac is known as aposporos initials which are distinct from the ameiotic megasporocyte. Aposporos initials differentiate and undergo mitosis to produce an embryo sac. It is mostly found in higher plants. Note : The initiation apospory embryo sac can occur alongside a sexual one or it can inhibit sexual embryo sac formation. e. Adventive embryony : Also known as sporophytic apomixis, In adventive embryonic embryos are produced directly from the nucellus or the integument of the ovule. Some prominent examples which it is observed are species of Garcinia, Mangifera indica, etc.  2. Occurrence On the basis of occurrence, it is classified into recurrent apomixis and non-recurrent apomixis. i. Recurrent Apomixis: Also known as gametophytic apomixis , in this, both the egg-cell and embryo are diploid and the embryonic is developed from the megaspore mother cell. ii. Non-Recurrent Apomixis: In non-recurrent apomixis, both the egg-cell and embryo are haploid and the embryo is developed directly from an egg-cell without fertilization. 3. Frequency i. Obligate apomixis: In Obligate apomixis , the reproduction takes place by apomictic means. ii. Facultative apomixis: In Facultative apomixis , the process of reproduction is carried out by both apomictic and sexual means. As mentioned above, there are 35 plant families in which apomixis is found. Examples: Sorbus, brambles, hawthorns, shadbush, and blackberries, hawkweeds, meadow grasses, mat grass, etc. Applications The apomixis has application in the produce sector. Some are mentioned as follows : 1. Maintaining purity : In case of obligate apomixis, the characteristics of the mother plant are retained. This helps to carry the same genotype through several generations. 2. Conservation of heterosis : In apomixis, there is no segregation. Also, obligate recurrent apomixis conserves hybrid vigor. So, heterosis is easily conserved. 3. Pure lines development : Haploid plants are produced through haploid apogamy and parthenogenesis. These plants, after being treated by colchicine, produce pure lines. These help in getting higher yields. 4. Hybrid Seed Production : Apomixis makes hybrid seed production a very easy process. Without any crossing, seeds are produced automatically through apomictic means. After the development of a hybrid, the hybrid seed production occurs automatically. Advantages of Apomixis The following are the advantages of Apomixis: 1. Heterozygotic advantages are preserved for many generations in apomictic plants as the genetic structure is conserved. This helps in maintaining uniformity in generations for both homo and heterozygosity. 2. Genetically uniform progenies can be produced rapidly without segregating them. 3. Apomixis also affects efficient exploitation of the maternal characteristics in the resulting progenies. Significance of Apomixis Apomixis has a huge significance in the plant breeding industry as this method helps in developing seeds without fertilization. The hybrid seeds produced by this method are cost efficient and also give large scale production in a lesser amount of time. This helps to increase the yield. Also, apomixis does not involve cross-fertilization, so it helps to retain the good characteristics of the crop plants. Benefits of Apomixis Apomixis produces new generations of plants which are very identical to the parent plants. This is very cost efficient as well as time efficient in faster production of feed and fiber around the world. This preserves good character over generations and also allows the introduction of other desirable characters. It helps in hybrid seed production. It is one of the most cost-effective methods to produce seeds. Apomixis prevents the loss of specific characters in the hybrid.  Limitations of apomixis 1. Ineffective selection : Due to the lack of heritable variation, the plants have the same genotype, that is either homozygous or heterozygous. 2. Frequency is low : The apomictic lines are found in very few crop plants with low frequency. However, frequency is more in case of fruit crops. 3. Genetic diversity : Low genetic diversity is found in obligate apomictic lines. However, apomictic lines are highly uniform genetically. 4. Lack of gene flow : Due to the occurrence of recombination in apomictic populations, there is a lack of gene flow.