Apomixes: Type and importance

असंग जनन (Apomixis):- जब जनक पौधे से भ्रूण व बीज का निर्माण बिना निषेचन के होता है तो इसे असंग जनन कहते हैं। बीजांड की कोई भी कायिक कोशिका ज़ाइगोट के समान कार्य करने लगती है। इसके फलस्वरूप बने संतति पौधे अपने जनकों के क्लोन होते हैं। यह तीन प्रकार का होता है - 

(When embryo and seed are produced from the parent plant without fertilization, it is called asexual reproduction. Any vegetative cell of the ovule acts as a zygote. The resulting seedlings are clones of their parents. It is of three types -)

a. अपस्थानिक भ्रूणता (Adventive Embryony)

b. अपबीजाणुकता (Apospory)

c. द्विबीजाणुकता (Diplospory)

a. अपस्थानिक भ्रूणता (Adventive Embryony):- जब भ्रूण का विकास बीजाण्ड की किसी कायिक कोशिका (अध्यावरण या बीजाण्डकाय या निभाग) से बिना भ्रूणकोष बने होता है तो इसे अपस्थानिक भ्रूणता कहते हैं। जैसा कि नीचे diagram में प्रदर्शित किया गया है। उदाहरण:- नींबू, आम आदि।

(When the embryo develops from a vegetative cell of ovule (integument or nucellus or challaza) without the formation of embryo sac, it is called adventive embryony. As shown in the diagram below. Examples: Lemon, mango etc.)

b. अपबीजाणुकता (Apospory):- जब भ्रूणकोष का विकास बीजाण्ड की किसी कायिक कोशिका (अध्यावरण या बीजाण्डकाय या निभाग) से बिना अर्धसूत्री विभाजन हुए होता है तो इसे अपबीजाणुकता कहते हैं। अर्थात यहाँ गुरुबीजाणु का निर्माण नहीं होता है। भ्रूणकोष की प्रत्येक कोशिका द्विगुणित होती है। भ्रूणकोष की किसी भी एक कोशिका से भ्रूण बन जाता है। जैसा कि नीचे diagram में प्रदर्शित किया गया है। उदाहरण:- Crepis

(When the development of the embryo sac occurs without meiosis from any vegetative cell of ovule (integument or nucellus or challaza), it is called as apospory. It means megaspore is not produced here. Each cell of the embryo sac is diploid. An embryo is developed from any one cell of the embryo sac. As shown in the diagram below. Example: - Crepis)

c. द्विबीजाणुकता (Diplospory):- जब भ्रूणकोष का विकास द्विगुणित गुरुबीजाणु से होता है तो इसे द्विबीजाणुकता कहते हैं। गुरुबीजाणु का निर्माण बिना अर्धसूत्री विभाजन के होने के कारण यह द्विगुणित होता है। अत: भ्रूणकोष की प्रत्येक कोशिका भी द्विगुणित होती है। जैसा कि नीचे diagram में प्रदर्शित किया गया है।

(When the embryo sac develops from a diploid megaspore, it is called as diplospory. Megaspore is diploid because it is formed by mitosis instead of meiosis. Therefore, each cell in the embryo sac is also diploid. As shown in the diagram below.)

 भ्रूण का विकास निषेचन के बिना होता है। भ्रूण का विकास भ्रूणकोष की कौनसी कोशिका से होता है, इस आधार पर द्विबीजाणुकता 2 प्रकार की होती है-

(Embryo development occurs without fertilization. Depending upon the embryo develops from which cell of the embryo sac, there are 2 types of diplospory -)

i. अनिषेकजनन (Parthenogenesis)

ii. अपयुग्मन (Apogamy)

i. अनिषेकजनन (Parthenogenesis):- जब भ्रूण का विकास अण्ड कोशिका से होता है तो इसे अनिषेकजनन कहते है। जैसा कि नीचे diagram में प्रदर्शित किया गया है। उदाहरण:- Taraxacum (घास)

[When the embryo develops from the egg cell, it is called parthenogenesis. As shown in the diagram below. Example: - Taraxacum (grass)]

ii. अपयुग्मन (Apogamy):- जब भ्रूण का विकास प्रतिमुखी कोशिका या सहायक कोशिका से होता है तो इसे अपयुग्मन कहते हैं। जैसा कि नीचे diagram में प्रदर्शित किया गया है। उदाहरण:- प्याज

(When the embryo develops from the antipodal cell or the synergid, it is called apogamy. As shown in the diagram below. Example: - Onion)

Importance of apomixis:-

i. Development of Purelines:- Apomixis is an effective means of rapid production of pure lines. Haploid parthenogenesis and haploid apogamy give rise to haploid plants which after colchicine treatment will produce diploid purelines. Such purelines can be used in breeding programmes for developing high yielding cultivars and hybrids.

ii. Maintenance of Purity:- The obligate apomixis breeds true for the characteristics of mother plant. Thus it is useful in maintaining the genetic purity from generation to generation. It can maintain a genotype for unlimited number of generations. In other words, apomictic lines always breed true irrespective of homozygosity and heterozygosity.

iii. Conservation of Heterosis:- The obligate recurrent apomixis is useful in conserving heterosis or hybrid vigour for unlimited generations. Apomixis does not permit segregation. Hence heterosis can be easily conserved.

iv. Easy Hybrid Seed Production:- Apomixis provides an easy way of hybrid seed production. The hybrid seed is automatically produced by apomictic means. There is no need of crossing. In other words, in apomictic genotypes crossing is not required for hybrid seed production. Once the hybrid is developed using apomictic line as one of the parents, the hybrid seed production will occur automatically. Thus it permits commercial seed production of hybrids.

v. Cheaper Hybrid Seed:- Use of apomixis is the cheaper way of hybrid seed production. It does not require crossing. Hence it helps in saving lot of money which is required for engaging labourers for crossing purpose in conventional hybrids for production of hybrid seed.

vi. Easy for Handling:- The handling of apomictic genotypes is easy. When the hybrid is developed using apomixes, we have to handle one genotype only. When the hybrid is developed using cytoplasmic genie male sterility, the breeder has to handle three type of material i.e. A, B and R lines. The purity of apomictic lines/genotypes is maintained automatically, whereas A, B and R lines require lot of labour and care for maintenance of their genetic purity. In other words, when obligate apomixis is available, there is no need for using male sterility for hybrid development.

vii. No Need of Isolation:- In case of apomixis, there is no need of isolation distance, where as in case of hybrid seed production by conventional method by using male sterility system, the proper isolation distance is required for maintaining the genetic purity of parental lines.

viii. No Outcrossing:- The apomixis is genetically controlled. It is governed by one or few dominant or recessive genes. Hence no seed setting occurs with cross pollination. Thus it does not permit outcrossing. In other words, contamination of apomictic genotypes through outcrossing is not possible.

ix. No Segregation:- Apomictic lines always breed true (as stated above) to the characteristics of mother plant. The hybrid seed is produced automatically. There is no need of making crosses every year to produce hybrid seed. This is the cheaper method of hybrid seed production. The apomictically propagated hybrid can be used as a variety, because it does not permit segregation.

x. Useful to Farmers:- Apomictically propagated cultivars and hybrids are useful to the farmers, because farmers can produce their own seed. Thus farmers need not to purchase fresh seed every year from seed companies.