01 Reproduction in Lower and Higher Plants - part 11 - Apomixis ,Parthenocarpy and Polyembryony

01 Reproduction in Lower and Higher Plants - part 11 - Apomixis ,Parthenocarpy and Polyembryony


Apomixis : 
  • Phenomenon of formation of embryo(s) through asexual method of reproduction without formation of gametes and the act of fertilization. 
  • Unusual sexual reproduction where there is no meiosis and syngamy. 
  • Embryo develops in the ovule and ovule developes to form seed . 
  • In apomixis, when a gametophyte organ or cell produces embryo like structure without fertilization, it is termed as apogamy. 
  • Similarly when diploid sporophyte cell produces a diploid gametophyte without undergoing meiosis is called apospory, 
  • e.g. Orange, Mango.
Main categories of apomixis are: 

a. Recurrent apomixis : 
  • In this type, the embryo sac generally rise either from anarchesporial cell or from some other part of the nucellus. 
  • In diplospory, the unreduced embryo sac is derived from the diploid megaspore mother cell e.g. Taraxacum. 
  • In apospory, the nucellar cells give rise to apomictic embryo sac. 
b. Non-recurrent apomixis : 
  • In this type, megaspore mother cell undergoes usual meotic division and a haploid embryo sac is formed. 
  • Here, the embryo arises either from the egg by parthenogenesis or from some other haploid cells of gametophyte through apogamy. 
  • Plants produced by this method are generally sterile and do not reproduce sexually, e.g. Nicotiana. 
c. Adventive Embryony : 
  • In this type, embryos may develop from somatic nucellus or integuments along with normal zygotic embryo. 
  • It is common in Mango, Orange, Lemon, etc. 
  • It gives rise to a condition called polyembryony.
  • Genetically identical plants can be produced effectively and rapidly by apomixis. 
Parthenocarpy :
  • This term is coined by Noll (1902). 
  • It is the condition in which fruit is developed without the process of fertilization. 
  • It occurs naturally in some varities of Pineapple, Banana, Papaya,etc. 
  • In these plants, it seems that the placental tissue in the unfertilized ovary produces auxin IAA (Indole-3 Acetic Acid) which is responsible for enlargement of ovary into fruit. 
  • The fruit resembles the normally produced fruit but it is seedless.
Polyembryony :
  • It is the development of more than one embryos, inside the seed and the condition is described as polyembryony. 
  • It was first noticed by Leeuwenhoek (1719) in the seeds of Citrus genus. 
  • It is the occurrence of more than one embryo in a seed which consequently results in the emergence of multiple seedlings. 
  • The additional embryos result from the differentiation and development of various maternal and zygotic tissues associated with the ovule of seed. 
  • Polyembryony may be true or false depending upon whether many embryos arise in the same embryo sac or in different embryo sacs in the same ovule. 
  • In adventive polyembryony, an embryo develop directly from the diploid cell of nucellus and integuments as in Citrus. 
  • In cleavage polyembryony, zygote proembryo sometimes divides (cleaves) into many parts or units. 
  • Each unit then developes into an embryo.
  • Polyembryony increases the chances of survival of the new plants. 
  • Nucellar adventive polyembryony is of great significance in horticulture.
  1. Parthenogenesis is the development of embryo directly from egg cell or a male gamete. It is a kind of apogamy. 
  2. Agamospermy : Here plants produce seeds. But embryo, inside it, is produced without (omitting) meiosis and syngamy. 
  3. Parthenocarpy can be induced artificially by - spraying of gibberellins, delaying pollination, use of foreign pollens, etc.
  4. Genetically uniform parental type seedlings are obtained from nucellar embryos.

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