through different routes, one is intraspecific genome duplication (autopolyploidy).
Chromosome doubling in plants is related to a failure of cell division following
mitotic doubling. It may occur in the zygote, young embryo, or meristem of a plant,
and will ultimately lead to the production of polyploid tissues and the generation of
minority polyploids (Soltis et al. 2003 ; Ramsey and Schemske 1998 ). The other
route is the union of two unreduced gametes, or of reduced and unreduced gametes.
The latter is a major mechanism of polyploidization in plants (Otto and Whitton
2000 ).
The merging of genomes of distinct species through hybridization and subse-
quent genome duplication (allopolyploidy), potentially has important ecological
and evolutionary consequences for the fate of introduced plant species
(Hull-Sanders et al. 2009 ; Treier et al. 2009 ). Allopolyploidy has been a prominent
mode of speciation and a recurrent process during plant evolution and has con-
tributed greatly to the large number of duplicated genes in plant genomes (Blanc
and Wolfe 2004 ; Otto and Whitton 2000 ).
Grant’s estimate was based on the assumption that the ancestral number of
chromosomes in angiosperms was 7–9 and that anyflowering plant with n 14
chromosomes had undergone polyploidization at some point during angiosperm
evolution (Grant 1981 ).
Successful manipulation of polyploid plant breeding programs to facilitate the
production of superior varieties are used in many plant species. Ploidy manipulation
is considered as a valuable tool in genetic improvement of many plants (Madon
et al. 2005 ). Polyploidy often generates variants that may possess useful charac-
teristic and by doubling the gene products, polyploidy also provide a wider
germplasm base for breeding studies (Thao et al. 2003 ). Polyploids mostly are
resistance to biotic and abiotic stresses through chromosome doubling. Also our
results onCannabisshowed structural variations in tissue organization. The pro-
duction of triploids is an alternative approach for achieving genetic sterility and this
strategy has been successfully employed in a number of plant species for the
production of seedless fruits or sterile phenotypes (Ortiz and Vuylsteke 1995 ;
Bhojwani 2004 ). In triploids, chromosome pairing during meiosis is done incor-
rectly and aneuploid gametes are produced that are infertile. An approach to pro-
duce triploid plants is to cross diploid and tetraploid plants. Environmental factors
such as cold and heat stimulation, and radiation, acting on diploid fertilized eggs
can promote chromosome doubling; when acting on meiosis of diploids, they can
inhibit the efflux of the polar body, leading to the formation of polyploids (Song
et al. 2012 ).
Polyploid plants compared to diploid varieties often show a new phenotype and
in the range of their diploid progenitors traits such as increased resistance to
drought, apomixis, insect resistance, increased biomass and changes in the quality
and concentration of the active compound are higher than diploid plants.
Polyploid plants, for example, tetraploids, can be produced by the chemical
treatment (colchicine, oryzalin, etc.) of diploid plants. Moreover, other ploidy levels
can be obtained by crossing different ploidy levels. Colchicine is a poisonous
compound extracted from the roots of certain colchicum species. It inhibits
368 H. Mansouri and M. Bagheri