PARTHENOCARPY, APOMIXIS & POLYEMBRYONY

 STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

PARTHENOCARPY, APOMIXIS & POLYEMBRYONY  

PARTHENOCARPY

• Parthenocarpy ( Gk . parthenos - virgin , karpos - fruit ) Production and development of seedless fruits is called parthenocarpy . 
• Mechanism of obtaining parthenocarpie fruits is becoming increasingly important because of three reasons : 
• Seeds are imitants during eating of the fruit . 
• Processing of fruits by food industry requires the removal of seeds which is quite difficult . Therefore , seedless fruits are preferred by food industry 
• There is an increasing tendency to grow fruit hearing plants inside green houses . Reliable insect pollinators cannot be ensured in each and every case . 

• Parthenocarpy is of two type
• vegetative and stimulative ( Westwood , 1993 ) . 
• In vegeta tive parthenocarpy the seedless fruits can develop even without the stimulus of pollination , c.g. , Pear . 
• In stimulative parthenocarpy 
• The stimulus of pollination is required without the actual process of fertilisation or seed setting , e.g.. Grapes . 

• A number of fruit varieties ( banana . Navel orange , pineapple , grapes ) have been altered genetically to undergo parthenocarpic development . 
• Hormonal treatment enables flowers to develop seedless fruits without the stimulus of pollination.
• The two commonly used hormones are auxins and gibberellins . 
• Tomato produces seedless fruits if treated with auxin while grape - vine forms seedless fruits on being treated with gibberellin . 
• Gibberellins are especially useful for inducing parthenocarpy in pomes . Low temperature , frost and fog are found to induce parthenocarpy in pear , olive , chillies and tomato .

• Apomixis and Polyembryony 
• Apomixis ( Gk . apo - without , miris , mixing ) is a mode of reproduction which does not involve formation of zygote through gametic fusion . 
• It is , therefore , akin to asexual repro duction . 
• In plants apomixis commonly mimics sexual reproduction but produces seeds without fertilisation , e.g. , some species of Asteraceae and grasses . 
• There are several methods of apomictic development in seeds . 
• The two common ones are recurrent agamospermy and adventive embryony .

• Recurrent Agamospermy 
• Agamospermy ( Gk , a - without , gamos - marriage , sperma seed ) is the formation of seed that has an embryo formed without meiosis and syngamy . 
• It is of two types , noncurrrent and recurrent . 
• In noncurrent agamospermy , the embryo is haploid . 
• Therefore , the seed having it is nonviable . 
• In recurrent agamospermy all the cells of embryo sac are diploid as it is formed directly either from a nucellar cell ( apospory ) or diploid megaspore mother cell ( diplospory ) . 
• The diploid egg as well as other diploid cells of embryo sac can grow into normal embryos . Formation of embryo directly from diploid egg without fertilization is called diploid parthenogenesis , e.g. .. Rubus , Apple  

• Adventive Embryony ( Sporophytic Budding ) . 
• An embryo develops directly from a diploid cell other than egg like that of nucellus and integument , e.g.,  Citrus , Opuntia . 
• It gives rise to a condition called polyembryony or the phenomenon of having more than one embryo . 
• There may be more than one egg cell in an embryo sac or more than one embryo sac in an ovule .
• All the egg cells may get fertilized . 
• Synergids and antipodal cells may also form embryos . 

• In gymnosperms polyembryony can also occur due to cleavage of growing embryo . called cleavage polyembryony . Occurrence of polyembryony due to fertilization of more than one egg is called simple polyembryony . 
• Formation of extra embryos through saprophytic budding is called adventive polyembryony . 
• Polyembryony is quite common in Onion , Groundnut , Mango , Lemon . Orange . In some of these cases stimulus of pollination may be required . 
• In Citrus  a seed has 2-40 embryos , one normal and the rest adventive , mostly nucellar . 
• In Allium odorum , there are 5 embryos , all developed by different methods - one from zygote , one from synergid , 2 from antipodal cells and one from integument of ovule . 

• Importance

• Hybrid varieties provide higher and better yield . 
• They are , therefore , preferred . A number of cereals and vegetables are being raised by using hybrid seeds . However , there is one major drawback .
• Hybrid seeds have to be produced every year because seeds collected from hybrid plants , if sown subsequently , do not maintain hybrid characters due to segre gation of traits . 
• Production of hybrid seeds every year is costly thus increasing the cost of crop production . This can be avoided if apomixis can be introduced in hybrid seeds . 
• Apo mixis is genetically controlled . Therefore , scientists are busy in identifying genes for apomixis so that they can be introduced in hybrid varieties .
• Adventive embryos are better clones than cuttings . 
• Embryos formed through apomixis are generally free from infections .

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