Download Summary Variations in chromosome number, also called as

Summary
Variations in chromosome number, also called as
heteroploidy, are of two types, viz. euploidy and
aneuploidy. Euploidy is a condition where one or
more complete sets of chromosomes are involved.
Euploids are classified with respect to the basic
chromosome number of a species. Thus an organism can
be ‘X’ (monoploid or haploid), 2X (diploid), 3X (triploid),
4X (tetraploid) etc. Two main types of euploidy include:
monoploidy or haploidy and polyploidy. Monoploids
have a single chromosome set designated by ‘X’,
whereas haploids have half of the somatic chromosome
number designated by ‘n’. However, the term haploidy is
more commonly used than monoploidy. Haploids
originate spontaneously by parthenogenetic development
of egg, synergids or antipodal cells or can be artificially
produced by distant hybridization and anther / pollen
culture. Meiotic bevahiour of haploids is irregular.
Haploids are important for production of homozygous
diploid lines, production of aneuploids, hybrid sorting,
gene transfer etc. Polyploidy is of two types, viz.
autopolyploidy and allopolyploidy. Autopolyploids
have the same basic set of chromosomes multiplied. e.g.
autotriploids (AAA), autotetraploids (AAAA) etc., where
‘A’ represents a single chromosome set. Autopolyploids
originate spontaneously due to mitotic and meiotic
irregularities leading to chromosome doubling. The most
common method of production of autopolyploids is by
using colchicine – a drug which inhibits spindle
organization resulting in chromosome doubling without
cell division. Meiotic behaviour of autotriploids is
irregular, whereas autotetraploids and autohexaploids
have
comperatively
normal
meiotic
behaviour.
Autopolyploids have an important role in crop
improvement. Triploids are produced in those crop plants
where seed is not the commercial requirement, whereas
tetraploids and hexaploids are highly valuable in various
fruit crops, vegetables, ornamentals, forage crops etc.
Allopolyploids contain genomes from two or more than
two different species, e.g. allotetraploid (AABB),
allohexaploid (AABBCC or AAABBB) etc. Allopolyploids are
frequently produced in nature due to hybridization
between two different species followed by chromosome
doubling of the hybrid. They can also be produced by
hybridization between two or more species followed by
colchicine treatment, leading to chromosome doubling of
the hybrid. Allopolyploids are also called as amphidiploids
due to their normal diploid like meiotic behaviour.
Allopolyploids have tremendous role in the evolution of
crop plants because many of our important crops, such
as, wheat, brassica, tobacco, cotton, triticale, etc. are
allopolyploids.