Principles of Plant Genetics and Breeding

216 CHAPTER 13
Table 13.3 Naming of polyploids.
Genome formula General name Specific name
n A Haploid
(monoploid)
2n AA Diploid
3n AAA Triploid Autotriploid
AAB Triploid Allotriploid
4n AAAA Tetraploid Autotetraploid
AABB Tetraploid Allotetraploid
6n AAAAAA Hexaploid Autohexaploid
AABBDD Hexaploid Allohexaploid
different genomes. It should be pointed out that autoploidy
and alloploidy are extreme forms of polyploidy.
Intermediates occur between them on a continuum of
genomic relationships. C. L. Stebbins called the intermediates
segmental alloploids. Polyploids are named
such that the prefix to the standard suffix (ploid) refers
to the basic chromosome set (Table 13.3). For example
“triploid” refers to a cell with three genomes (3x) while
“hexaploid” refers to a cell with six genomes (6x).
General effects of polyploidy of plants
In terms of general morphology, an autoploid would
resemble the original parent whereas an alloploid
would tend to exhibit a phenotype that is intermediate
between its parental species. Autoploidy increases cell
size, especially in meristematic tissues. Autoploids usually
have thicker, broader, and shorter leaves. Other
plant organs may increase in size compared to their
corresponding parts in diploids, an effect called gigas
features. The gigas luxuriance contributes more to
moisture content of the plant parts than to biomass. The
plants tend to be determinate in growth.
The growth rate of polyploids is less than that of
diploids. This may be due to their lower auxin content
than that of their diploid counterparts, as found for
tetraploids. Polyploids tend to flower later and over a
longer period of time. In grasses, autoploidy tends to
reduce branching or tillering.
Polyploidy also affects the chemical composition of
plant parts. For example, vitamin A activity in tetraploid
corn is about 40% more than in diploid species.
Similarly, the vitamin C content of vegetables and fruits
has been known to increase following chromosome
doubling. The nicotine content of tetraploid tobacco
is about 18–33% higher than in diploid species.
Autoploids, generally, have fertility problems, and have
poor pollen production. In some cases, reduction in
fertility as compared to their diploid counterparts may
be as high as 80–95%. This reduction in fertility is
attributed to genetic imbalance following chromosome
doubling that leads to disharmonies in development
(e.g., abnormal pollen sac, failure of fertilization). Some
changes in ecological requirements such as photoperiod
and heat requirements have been reported in some
species following chromosome doubling.
Origin of polyploids
The breeding strategies employed in the breeding of
polyploids are determined primarily on their origin. J. R.
Harlan and J. M. de Wet (1975) concluded from an
extensive review of the literature that nearly all polyploids
arise by the path of unreduced gametes. They pointed
out that the most common factor leading to polyploidy
is the fusion of 2n and n gametes to form a triploid,
followed by either backcrossing or selfing to produce a
tetraploid. Further, they observed that the occurrence
of unreduced gametes is variable and pervasive in the
plant kingdom.
The unreduced (2n) gametes arise by one of two
mechanisms – first division restitution (FDR) or
second division restitution (SDR) – during meiosis
(Figure 13.2). Each mechanism has a different genetic
consequence. In FDR, the 2n gametes result from
parallel spindle formation after the normal first division
of meiosis. The cleavage furrows occur across the plane
of the parallel spindles, producing dyads and 2 × 2n
pollen. The genetic consequence of the mechanism is
that most of the heterozygosity of the diploid hybrid is
conserved in the 2n gametes. In the SDR mechanism,
the first meiotic division is followed by cytokinesis, but
the second division is absent. This results in a dyad with
2 × 2n gametes. However, in terms of genetic consequence,
SDR results in significantly reduced heterozygosity
in the 2n gametes. Researchers such as T. Bingham
have proposed, in breeding potatoes, the fusion of two
FDR 2n gametes to harness the heterosis that results.
This heterosis can be fixed; the elite lines produced will
then be clonally propagated. Seed-propagated species
(e.g., alfalfa) cannot benefit from this strategy.
Autoploidy
As previously defined, autoploids comprise duplicates
of the same genome. Autoploids are useful in making
alloploids and wide crosses.