parent species, calledunreduced gametes. If two unreduced gametes fertilize one another,
the resultant hybrid would have the complete genome of each parental species. In this case,
meiosis can function normally, and the hybrid plant may represent a new species with a
unique chromosome number. Species that contain multiple genomes or multiple sets of
chromosomes beyond the diploid level are calledpolyploids. Again, among the myriad
of organismal types, plants are champions at polyploid production—and indeed many
plant species are polyploids.
Polyploidy may arise in two ways: by the doubling of a homologous set of chromosomes
(autopolyploidy) or by combining two complete sets of chromosomes from genetically
different parent plants (allopolyploidy). An autotetraploid contains four sets of homologous
chromosomes, and pairing between the four homologous chromosomes is often irregular,
with chromatids showing random segregation during gamete formation. In anallotetra-
ploid, on the other hand, the parental chromosomes in each of the two sets of homologous
chromosomes tend to pair with each other as they would in the parental plants, thus contri-
buting to the stability and fertility of such plants. Several natural allopolyploids are known,
and several have been created in the plant breeding field.
Hybridization is an important process that has occurred in the development of many of
our agricultural crops. Many polyploid crop plants have been produced by either the com-
bination of unreduced gametes or the doubling of the chromosomes after hybridization of
haploid gametes. Canola (Brassica napus), which is used for vegetable cooking oil, is
composed of the complete genomes of two different species (B. rapa, genome AA and
B. oleracea, genome CC); similar polyploid origins have been confirmed for two other
BrassicacropsB. junceaandB. carinata(Fig. 2.13). Bread wheat,Triticum aestivum,
was produced from the hybridization between three different species. In this case, each pro-
genitor species donated their complete diploid genome (AA, BB, DD genomes, respect-
ively) to making a species with three complete sets of chromosomes and a very large
“new” wheat genome (AABBDD).
Polyploidization is undoubtedly a frequent mode of diversification and speciation in
plants. More recent studies indicate that most plants have undergone one or more episodes
of polyploidization (i.e., increase in the whole DNA complement beyond the diploid level)
Figure 2.13.Triangle of U (1935) shows the relationships between several diploid and polyploid
crop species within theBrassicagenus.
40 MENDELIAN GENETICS AND PLANT REPRODUCTION