Principles of Plant Genetics and Breeding

3 Creation of new alloploids. Wide crosses often
produce sterile hybrids. The genome of such hybrids
can be doubled to create a new fertile alloploid
species (a polyploid with the genomes of different
species), such as triticale.
4 Scientific studies. Cytogenetic studies following a
wide cross may be used to understand the phylogenic
relationships between the parents of a cross.
5 Curiosity and aesthetic value. Wide crosses may
produce unique products of ornamental value, which
can be useful to the horticultural industry. Sometimes
just being curious is a good enough reason to try new
things.
Selected success with wide crosses
Developing commercial cultivars with genes introduced
from the wild can be an expensive and long process (see
prebreeding in Chapter 6). Some linkages with wild
genes need to be broken. In tomato, it took 12 years
to break the linkage between nematode resistance and
undesirable fruit characteristics. Nonetheless, some
significant successes have been accomplished through
wide crosses.
Natural wide crosses
Natural wide crosses have been determined by scientists
to be the origin of numerous modern-day plants of economic
importance. Ornamentals such as irises, cannas,
dahlias, roses, and violets, are among the list of such
species. In tree crops, apples, cherries, and grapes are
believed to have originated as natural wide crosses, and
so are field crops such as wheat, tobacco, and cotton, as
well as Irish and sweet potatoes. Most natural wide cross
products of economic value to modern society are used
as ornamentals and are usually propagated vegetatively.
This led G. L. Stebbins to observe that wide crosses may
be more valuable in vegetatively propagated species than
seed-propagated species.
Synthetic (artificial) wide crosses
Apart from natural occurrences, plant breeders over the
years have introgressed desirable genes into adapted cultivars
from sources as close as wild progenitors to distant
ones such as different genera. Practical applications of
wide crosses may be grouped into three categories.
1 Gene transfer between species with the same
chromosome number. Wide crosses between two
tomato species, Lycopersicon pimpinellifolium × L.
SEXUAL HYBRIDIZATION AND WIDE CROSSES IN PLANT BREEDING 173
esculentum, have been conducted to transfer resistance
genes to diseases such as leaf mold and
Fusarium wilt. Gene transfers in which both parents
have identical chromosome numbers is often without
complications beyond minor ones (e.g., about 10%
reduction in pollen fertility). It is estimated that
nearly all commercially produced tomatoes anywhere
in the world carry resistance to Fusarium that derived
from a wild source.
2 Gene transfer between species with a different
number of chromosomes. Common wheat is a polypoid
(an allohexaploid) with a genomic formula of
AABBDD. It has 21 pairs of chromosomes. There is
diploid wheat, einkorn (Triticum monococcum), with
seven pairs of chromosomes and a genomic formula
of AA. There are several tetraploid wheats (AABB)
such as emmer wheat (T. dicoccum). Transfer of genes
from species of lower ploidy to common wheat is possible
(but not always the reverse). Stem rust resistance
is one such gene transfer that was successful.
3 Gene transfer between two genera. Common
wheat comprises three genomes of which one (DD) is
from the genus Aegilops. Consequently, gene transfers
have been conducted between Triticum and Aegilops
(e.g., for genes that confer resistance to leaf rust).
Overcoming the challenges of reproductive
barriers
The reproductive barriers previously discussed confront
plant breeders who attempt gene transfer between distant
genotypes via hybridization. The primary challenge
of wide crosses is obtaining fertile F 1 hybrids, because
of the mechanisms that promote, especially, gametic
incompatibility. As previously indicated, this mechanism
acts to prevent: (i) the pollen from reaching the stigma
of the other species; (ii) germination of the pollen and
inhibition of growth of the pollen tube down the style,
or the union of the male gamete and the egg if the
pollen tube reaches the ovary; and (iii) the development
of the zygote into a seed and the seed into a mature
plant. Gametic incompatibility ends when fertilization
occurs. However, thereafter, there are additional obstacles
to overcome. Gametic incompatibility and hybrid
breakdown are considered to be barriers to hybridization
that are outside the control of the breeder.
Several techniques have been developed to increase
the chance of recovering viable seed and plants from a
wide cross. These techniques are based on the nature of
the barrier. All techniques are not applicable to all
species.