Approaches for fixation of Heterosis

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Approaches for fixation of Heterosis


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Approaches for fixation of Heterosis:

Approaches for fixation of H eterosis Geetika Mehta


Heterosis Heterosis can be defined as the superiority of the F 1 hybrid over its parents. Types:- Relative heterosis Heterobeltiosis Economic or Standard heterosis


History Hybrid vigour in artificial tobacco ( Nicotiana spp. ) h ybrids was first reported by Koelreuter in 1673. Hybrid vigour & Inbreeding depression were also reported by Charles R. Darwin in 1876 in his book “The effects of Cross and Self fertilization in vegetable kingdom”. Heterosis was rediscovered by G.H.Shull & E.M.East independently in Maize in 1908.


The term Heterosis is used as synonym for Hybrid vigour. The term Heterosis was coined by Shull in 1914.

Understanding Heterosis:

Understanding Heterosis To understand the phenomenon of heterosis following theories have been given. Dominance theory Over-dominance theory Epistatic theory

Dominance theory:

Dominance theory Given by Davenport (1908). It suggests that:- Dominant alleles have favourable ,while recessive allele have unfavourable effect. In heterozygotes the harmful effect of recessive allele is masked by its dominant allele.

Over-dominance theory:

Over-dominance theory Given by East & Shull Independently (1908). According to it:- Heterozygosity at some loci are superior to both relevant homozygotes. It is also known as single gene heterosis.

Epistatic theory:

Epistatic theory In 1952 Gowen suggested that influence of one locus on the expression of another may be involved in heterosis. A majority of heterotic crosses show significant epistasis but all heterotic crosses do not show epistasis and all crosses that show epistasis are not heterotic. In general it can said that , heterosis up to a large extent is due to dominance gene action but epistasis and over dominance are also involved.


Apart from them , recent studies have also revealed the role of :- Small RNAs. Epigenetic regulation including Circardian – mediated metabolic pathways. Organelle interactions in Heterosis.

Small RNAs:

Small RNAs Small RNAs includes :- Micro RNAs (miRNAs) Small interfering RNAs (siRNAs) Trans-acting siRNAs (tasiRNAs) Small RNAs are associated with nonadditive gene action of target genes in allotetraploids. miRNAs & tasiRNAs controls the expression of genes that encodes transcription factors and proteins that are important in growth & development.

Organelle heterosis:

Organelle heterosis

Methods of fixation of Heterosis:

Methods of fixation of Heterosis Apomixis Vegetative Propagation Balanced lethal system Polyploidy


Apomixis The term Apomixis coined by Winkler(1908) When meiosis and syngamy are interrupted and still a viable embryo is formed, such asexual seeds which produce progeny identical to the female parent. Reported in 400 species in 40 families but rare in crop plants. (Bicknell and Koltunow ,2004; Koltunow ,1993) Common families are Asteraceae, Poaceae, Rosaceae and Rutaceae (Bashaw,1980;Hanna and Bashaw.1987) Winkler(1908)


Particulars Apomixes Amphimixis 1.Term first used or coined by 2.Sexual Fusion or union of male and female gamete 3.Gene combination or gene flow 4.Segregation 5.Rapid development of Inbred lines 6.Conservation of heterosis 7.Type of reproduction 8.Types 9.Frequency in plants Winkler 1906 Does not involve Not possible Does not occur Possible Possible Asexual Parthenogenesis, apogamy , Apospory , Adventive embryony Low Weismann 1891 Involved Possible Occurs Not possible Not possible Sexual Autogamy , Allogamy High


CLASSIFICATION BASED ON SITE OF ORIGIN Parthenogenesis a) The term was coined by Owen in 1849. b) The embryo develops from egg cell without fertilization.. c) It may be either haploid or diploid. Haploid Parthenogenesis: The development of embryo from the haploid egg cell. Useful in developing pure lines Pure lines obtained by chromosome doubling e.g.. Hordeum bulbosum , Solanum nigram . Diploid Parthenogenesis: The development of embryo from the diploid egg cell. Sometimes embryo sac develops without reduction division Useful for conserving heterosis reported in grasses like Taraxacum , rice, Tobacco d) It is genetically controlled . e) It occurs in nature as well as can be induced artificially

Causes of Parthenogenesis:

Causes of Parthenogenesis Inability of the pollen tube to discharge the contents inside the embryo sac. Insufficient attraction between male and female gametes Early degeneration of the sperm Very long style Short pollen tube Slow rate of pollen tube growth Stimulation of pollination in the absence of pollen tube Self incompatibility and cross incompatibility


2. Apogamy The term apogamy was coined by Winkler in 1908. In apogamy the embryo originates either from synergids or antipodal cells. It may be either haploid or diploid. Embryo develops from the cells of normal embryo sac. It occurs in nature and can also be induced by artificial.


3. Apospory The term was coined by De Very and Bower in 1886 . The embryo develops from the diploid egg cell of another embryo sac which originates from the cell of ovule lying outside the normal embryo sac. It gives rise to diploid embryos. The another embryo sac may develop from the cell of archesporium , nucellus or integument. It is of two types:- a). Generative Apospory : The development of embryo from the embryo sac that has originated from the cell of archesporium . eg . parthenium . b). Somatic Apospory : When the embryo develops from the embryo sac that has originated from the cell of either nucellus or integument. E.g. Malus , Rubus , Allium , Crepis , Poa , Opuntia .


4. Adventive Embryony The term adventive embryony was first used by Strasburger in 1878. The embryo develops directly from the diploid cells of either nucellus or integument. It always give rise to diploid embryos. In this case another embryo sac is not formed. It is a type of sporophytic budding. It has been reported in Citrus .


INDUCTION OF APOMIXIS Apomixis particularly the parthenogenesis can be artificially induced by various ways as given below: By the stimulation of widely related pollen or foreign. By low temperature treatment to ovule. By pollinating by X-ray irradiated pollen. By treatment with certain Chemicals like belviton .


GENETICS AND CAUSES OF APOMIXIS It is genetically controlled which is governed by one or few genes that can be dominant, recessive or epistatic in expression. The close relationship with sexual reproduction suggests that key steps in the sexual program are deregulated in space and/or time, giving rise to apomixis ( Grimanelli et al., 2001; Grossniklaus , 2001; Koltunow, 1993; Koltunow and Grossniklaus , 2003; Spielman et al., 2003) . During apomictic reproduction, deregulated sexual development results from ( i ) Circumvention of meiotic reduction ( apomeiosis ). (ii) Activation of the egg without fertilization (parthenogenesis). (iii)Production of functional endosperm.


Genes responsible for apomixis SERK (Somatic embryogenesis receptor like- Kinase) : Strongly activated during premeiosis and meiosis stage.e.g. in Rice located on chromosome 3. APOSTART : Lipid binding START domain containing protein strongly activated at post meiosis phase. e.g. in Arabidopsis on chromosome 5 in inverted position. (Albertini et al 2005)


Contd. The term ASGR (for apospory -specific genomic region) was coined to describe the locus in Pennisetum/ Cenchrus . Later , Partial sequencing of ASGR-linked BACs revealed some candidate apomixis genes such as  ASGR-BBML ( Conner et al 2008 ). Apomixis in Dandelions ( Taraxacum officinale s.l .) is controlled by a few dominant genes, Two unlinked dominant apomixis loci have been identified, controlling: The avoidance of meiotic reduction ( DIPLOSPOROUS-Dip). Parthenogenesis of the embryo ( PARTHENOGENESIS-Par). (Dijk PV 2009)


Contd. A novel model with five major genes required to control asexual seed formation: Apospory initiator ( Ait ) gene, Apospory preventer ( Apv ) gene, Megaspore development ( Mdv ) gene, Parthenogenesis initiator (Pit) gene,and Parthenogenesis preventer ( Ppv ) gene in Poa pratensis . (Fritz Matzk , 2005).

Molecular markers Associated with Apomixis:

Molecular markers Associated with Apomixis The defining of molecular markers closely linked to apomixis could allow the coding region to be cloned and inserted into other crop species. RAPD marker OPC4-600 in Brachiaria & Pennisetum . ( Albertini et al 2005) ugt 197 a RFLP marker is conserved in all 11 Pennisetum , apomictic species but are absent in sexually reproducing 8 species.( Lubbers et al 1994).


Contd. A SCAR marker OPF08-600 of 600bp was found associated with apomixis in Buffelgrass Cenchus ciliaris .(Dwivedi et al 2006). In Boechera apomictic and sexual ovules displayed heterochronic gene expression patterns. ( Sharbel et al 2010). Apomixis is controlled by a single dominant “locus” in both Pennisetum and Cenchrus .

Conventional Breeding:

Conventional Breeding The apomictic genes can be transferred from donor sources to the agronomic bases through backcross technique. Five or six backcrosses are sufficient for the transfer.

Transfer of Apomixis through Genetic Engineering:

Transfer of Apomixis through Genetic Engineering With the help of molecular tools apomixis can be transferred to the sexual crops. The genetically modified plants thus produced should be Autonomous apomictic plants. It may be however difficult to engineer such plant as most of the apomictic grasses which needs pollination for the development of endosperm .( Grossniklaus 2001; Spilane et al.,2004)


Caution……. Problems in Transgenic apomixis:- Less endosperm development. Crossing with sexual plants and emergence of “Superweed”. Crossing with other outcrossing plants reduce the genetic variation. Gene flow to wild relatives will deteriorate their diversity which have potential for future breeding. (Dijk And Damme 2000)

Remedies ….:

Remedies …. Ploidy Barrier - Most of the natural apomicts are polyploids whereas their sexual relatives are diploids. Mate sterility can be induced. Stable transformation of Chloroplast can be done i.e. maternally inherited. Manipulation of genes that involved in floral development that ensures self pollination e.g. Cleistogamy.


Contd…….. Pollinating the transgenic plants with non transgenic plants. Conditional apomixis technology ,allowing the trait to be turned on and off at will, could not only allow the indefinite fixation of Heterosis but may also be used to prevent the escape of transgenes.


Case studies:- Pearl Millet The genus Pennisetum includes many species that reproduce through apomixis. A few of them have been tested and successfully hybridized with pearl millet, Pennisetum glaucum . Such hybridizations, however, produced mostly male-sterile hybrids, which could not promote the transfer of a nearly obligate type of apomixis.


Maize Different research groups have conducted studies regarding the inheritance and introgression of apomixis in maize through a wild relative Tripsacum dactyloides . The group headed by Petrov in Russia (1979,1984) crossed a tetraploid maize (2n=4x=40m) with an apomictic tetraploid Tripsacum (2n=4x=72t).


Five male sterile F 1 hybrid were obtained which had 2n=56=20m+36t chromosomes. Only one of the five F 1 hybrids was obviously facultative apomictic and was backcrossed using maize as pollinator. Later it was attempted by kindiger et al in 1996.


Wheat In an attempt to transfer apomixis to wheat, Carman et al. (1985) collected wild Elymus rectisetus genotypes from Australia and Tasmania finding some of them to be highly apomictic. Direct hybridization between wheat and E. rectisetus proved difficult, but a few hybrids were produced. Carman ’ s group experienced the same problem with wheat as CIMMYT ’ s group in maize.


Crop Technique used Reported by Year Cenchrus apomixis Bashaw and Hussey 1992 2. Pennisutum squamulatum transferring apomixis Hanna et al., 1993,1996 3. Maize transfer genes controlling apomixis Kindiger et al, Leblanc et al ., 1996 4. Wheat transfer genes controlling apomixis Carman and Wang 1992

Advantages of Apomixis:

Advantages of Apomixis Apomixis would make a major contribution in vegetatively propagated crops. Tree sp. where unique genotypes could be fixed and easily propagated. Problems with virus transmission in vegetatively propagated crops such as potato would be eliminated because the apomictic seeds go through a single cell stage.


Contd … Any elite genotype could be transformed directly into a cultivar. Polygenic traits would breed true. Labour-intensive tissue culture in ornamentals could become superfluous. Apomictic crops would breed true for other transgenic traits, avoiding the production of undesired non-transgenic offspring through the segregation of heterozygotes.


Polyploidy Heterosis can be fixed by chromosome doubling or polyploidization, especially in Interspecific and Intergeneric hybrids. There are some classical examples of natural events of hybridization followed by chromosome doubling . These crops showed significant Heterosis after hybridization. e.g. Tetraploid and then hexaploid Wheat. Tetraploid Cotton. Brassica napus, B. juncea , B. carinata .


Contd…… As a rule, apomixis is more prevalent in the polyploid sp. e.g. seven wild sp. of Pennisetum are apomictic and all are polyploid. Tripsacum dactyloides , an apomictic relative of Maize, is also polyploid. Apomixis is present also in Elymus rectisetus , a polyploid relative of Wheat.



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