Apomixis conserved heterozygosity and averts inbreeding depression in flowering plants
Authors: AKSHAY DANGE* ,DEEPAK SAPKAL** and JITENDRA WAYDE***
A sexual reproduction does not involve fusion of male and female gamentes. Asexual reproduction is of two types - vegetative reproduction and apomixes. New plants may develop from vegetative parts of the plant (vegetative reproduction) or may arise from embryos that develop without fertilization (apomixis). This phenomenon is firstly discovered by Leuwenhock (1719) in Citrus seeds. Apomixis was defined by Hans Winkler (1908) as development of an embryo without fusion of male and female gamates or Apomixis is the process in which embryos and seeds are developed in the ovule without meiosis and egg cell fertilization (Simply genetically controlled process). Apomictically produced progeny are genotypically identical to the parent in flowering plants; "apomixis" means agamospermy, i.e. seed are reproducing by clonal selection. Apomixis is widely spread among higher plants and it's observed in 298 species belonging to 35 families. Apomixisis most commonly found in Gramineae, Compositae, Rutaceae also in the major cereals like maize, wheat and pearl millet and there wild relative. In apomictic species, sexual reproduction is either suppressed or absent on the basis of that it is classified as two types (facultative and obligate). When sexual reproduction does occur, the apomixis is termed as facultative. But when sexual reproduction is absent, it is referred to as obligate. Many crop species show apomixis, but it is generally facultative. Apomixis is a nuisance when the breeder desires to obtain sexual progeny, i.e., selfs or hybrids. But it is of great help when the breeder desires to maintain varieties. Thus in breeding of apomictic species, the breeder has to avoid apomictic progeny when he is making crosses or producing inbred lines. But once a desirable genotype has been selected, it can be multiplied and maintained through apomictic progeny. This would keep the genotype of a variety intact. A sexually reproducing crop species are highly heterozygous and show severe inbreeding depression. Therefore, breeding methods in such species must avoid inbreeding. Apomixes phenomenon is very important for breeding point of view because by this technology we are getting seed by asexual means or by pass the sexual process. It will helpful to the farmer because there is no need to purchased hybrid seed every year.
A simplified classification of apomixis is given below. This classification is based on from which part of ovule embryo is developed.
Adventive Embryony :-
In this case, embryos develop directly from vegerative cells of the ovule, such as
nucellus, integument, and chalaza and its example are mango, citrus, etc.
Some vegetative cells of the ovule develop into unreduced embryo sacs after meiosis.The embryo may develop from egg cell or some other cell of this embryo sac. Examples are Hieraceum, Malus, Crepis, Ranunculus, etc.
Embryo sac is produced from the megaspore, which may be haploid or, moregenerally, diploid. Diplospory leads to parthenogenesis or apogamy.
Characteristics and features of apomixes:-
• Genetic study is difficult to conduct because embryo (seed) developed without sexual reproduction.
• It affects megagametogenesis and megasporengenesis.
• It is facultative when some progeny result from meiosis.
• It is also obligate when progeny is 99.99 percent seed developed from apomixes.
• In polyploidy it is very common feature.
• It declines the development and maintenance of A, B, R lines (A- male sterile, B- male fertile and R -restorer line)
About Author / Additional Info:
** Dr. Deepak R Sapkal, Assistant Professor (Genetis and Plant Breeding), Lovely Professional University, Jalandhar, State : Punjab (INDIA)
* AKSHAY DANGE ,M.Sc. Student (Genetics and Plant Breeding)
*** JITENDRA WAYDE, M.Sc. Student (Genetics and Plant Breeding)