Principles of Plant Genetics and Breeding

(a)
(b)
A B
A B
a b
a b
A B
A B
a b
a b
A B
Natural autoploids of commercial importance
Autoploidy is not known to have profoundly impacted
the evolution of species. Having increased sets of chromosomes
does not necessarily increase performance.
Autoploids of commercial value include banana, a
triploid, which is seedless (diploid bananas have hard
seeds not desirable in production for food). Other
important autoploids are tetraploid crops such as
alfalfa, peanut, potato, and coffee. Spontaneous autoploids
are very important in the horticultural industry
where the gigas feature has produced superior varieties
of flowering ornamentals of narcissus, tulip, hyacinth,
gladiolus, and dahlia among others. Autoploid red
clovers and ryegrasses with lusher and larger leaves,
A
a
b
B
a b
POLYPLOIDY IN PLANT BREEDING 217
Causal event: parallel spindle
(abnormal) orientation
2n gametic ratio 1 AaBB
2 AaBb
1 Aabb
A B
A
a
B
a b
b
Figure 13.2 The origin of polyploidy by (a) first division restitution (FDR) and (b) second division restitution (SDR).
Dyads occur in telophase II. FDR is caused by the presence of parallel or fused spindles, while SDR is caused by the
presence of a cell plate before anaphase II. 2n pollen tend to be bigger in size than 1n pollen.
A
B
A b
Causal event: premature
cytokinesis before 2nd meiosis
2n gametic ratio 1 AABb
2 aaBb
2x
2x
2x
A B
A b
a B
a b
2x
a B
a b
Sister
chromatids
Sister
chromatids
Non-sister
chromatids
Non-sister
chromatids
taking advantage of the gigas feature of polyploidy,
have been bred for commercial use as palatable and
digestible livestock forage. It should be mentioned that
there is no overwhelming evidence to suggest that
autotetraploids are productively superior to their diploid
counterparts.
Cytology of autoploids
Autoploids contain more than two homologous chromosomes.
Consequently, instead of forming bivalents
during meiosis as in diploids, there are also multivalents
(Figure 13.3). For example, autoploids have mostly
trivalents but some bivalents and univalents are also present.
Tetraploids have quadrivalents or bivalents as well