Principles of Plant Genetics and Breeding

218 CHAPTER 13
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2
3
(a)
(b)
Figure 13.3 Cytology of polyploids: (a) triploidy and (b) autotetraploidy. Bivalents and quadrivalents usually produce
functional gametes, while univalents and trivalents produce sterile gametes.
as some trivalents and univalents. These meiotic abnormalities
are implicated in sterility to some extent, more
so in triploids. The microspores and megaspores with x
or 2x genomes are usually viable.
The amount and nature of chromosome pairing
directly impacts the breeding behavior of autoploids.
Autoploids are induced artificially by chromosome doubling
using colchicine. Doubling a hybrid between two
diploid cultivars would produce a tetraploid in which
there may be a tendency for the doubled set of chromosomes
from one parent to pair independently of the
doubled set of chromosomes of the other parent. This
propensity is called preferential or selective pairing, a
phenomenon with genetic consequences. If preferential
pairing is complete, there would be no new genetic
recombination and hence the progeny would look
like the doubled F 1 . Furthermore, the bivalent pairing
would contribute to sterility originating from meiotic
disorders, while preserving heterosis indefinitely, should
there be any produced by the original cross. The concept
of preferential pairing is applied in the modern
breeding of polyploids whereby alloploids are stabilized
Trivalent
Trivalent
Bivalent
Univalent
Bivalents
Quadrivalents
Univalent
Possible functional
gametes
Non-functional
gametes
and made reliable as diploids, a process called
diploidization.
Genetics of autoploids
The ploidy level may also be defined as the number of
different alleles that an individual can possess for a single
locus on a chromosome. A diploid can have two alleles
per locus, whereas an autotetraploid can have four different
alleles. The genetics of autoploids is complicated
by multiallelism and multivalent association of chromosomes
during meiosis. Consider the segregation of
alleles of a single locus (A, a). In a diploid species,
there would be three possible genotypes AA, Aa, and
aa. However, in an autotetraploid there would be five
genotypes ranging from nulliplex (aaaa) to quadruplex
(AAAA) (Table 13.4). The proportion of dominant
(A) to recessive (a) genes is different in two of the
five genotypes (AAAA and Aaaa) in autotetraploids
from that which obtains in diploids. The number of
phenotypes observed depends on the dominance relationship
of A and a. If allele A is completely dominant