Glossary Plant Breeding

Alloploidy. A situation wherein the individual contains genetically different sets of
chromosomes, for example, from two or more species. If two distinct species are
hybridised, and the chromosome complement of the resulting hybrid is doubled, the
result is the merging of the two species into a new amphidiploid species. This is
exemplified by N. digluta (2n=72), which can be synthesized artificially by doubling
the chromosome complement of interspecific hybrid between N. tabacum (2n=48)
and N. glutinosa (2n=24). Among various types of polyploidy, alloploidy has been the
most important in the ancestry of cultivated species. It provides a mechanism for
direct speciation. A large portion of the cultivated species, possibly as many as half
(and 70% of the grasses), is alloploids. Induced alloploidy has found an even smaller
place in practical breeding than autoploidy breeding. Principles that govern the
success remain the same in both auto- and alloploidy breeding (see polyploidy).
However, induced alloploidy can be used to (a) establish phylogeny of a polyploid
species, (b) produce new plant genotypes, (c) facilitate transfer of genes from related
species, and (d) facilitate transfer or substitution of individual chromosomes or pairs
of chromosomes.
Allopolyploid. A polyploid that contains sets of dissimilar genomes having been derived
from two or more distinct taxonomic species. As the genomes are different, the
polyploid is also called hybrid polyploid. As it combines complete chromosome sets
from two or more species, it is sometimes called amphidiploid. In general, alloploids
combine in more or less blending fashion the characteristics of the species from
which they are derived. They have generally increased vigour and have a high degree
of fertility and stability compared to their diploid parents. The high degree of fertility
and stability of the naturally occurring alloploids is because of regular bivalent
formation at meiosis. Since different genomes are highly divergent, thus pairing
occurs only between homologous chromosomes belonging to identical genomes. Thus
for practical purpose, there are only two chromosomes of each kind in the alloploids.
There are, however, many exceptions to this generalization. For example, Ph gene
present in the long arm of 5B chromosome ensures bivalent formation in bread wheat.
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