Principles of Plant Genetics and Breeding

414 CHAPTER 22
Select diploid line
Produce tetraploid plants
Develop tetraploid lines
Evaluate tetraploid lines
Triploid development
Evaluate triploid
Colchicine treatment;
e.g., shoot apex treatment
Female
4x = 44
(gray rind)
Figure 22.2 Generalized steps in the breeding of seedless
watermelons.
Breeding seedlessness
Fresh fruits without seeds are more convenient to eat,
because there are no seeds to spit out. Common fresh
fruits in which seedless cultivars exist include watermelon,
grape, orange, and strawberry.
The conventional way of producing seedless fruits
is the use of triploid hybrids. A tetraploid (4x) parent
that is crossed with a diploid (2x) line is exemplified
by seedless watermelon breeding (Figure 22.2). In the
watermelon, 4x = 44 and 2x = 22. The tetraploid is
always the female parent; the reciprocal cross (with the
diploid as male parent) does not produce seed. The
resulting triploid (3x = 33) is female-sterile and hence
the fruit is seedless. Furthermore, because the triploid is
also male-sterile, producers of seedless watermelon must
plant rows of diploid lines as pollinators for stimulation
of fruit formation. In commercial production fields,
growers usually plant a ratio of three triploid rows to
one diploid row. It is important that the rows be marked
to enable harvesters pick fruits from only triploid lines.
The breeding of seedless watermelon will be described
to illustrate the conventional production of seedless
fruits. The process involves a number of steps:
1 Selection of diploid lines for developing
tetraploids. Many producers use tetraploids with
gray-colored rind and diploids with stripped rind.
This way, it is easy to identify and discard selfed
tetraploids (gray rinds) in the progeny.
×
Male
2x = 22
(striped rind)
3x = 33
(striped rind)
(female/male sterile)
2 Tetraploid induction. Chromosome doubling to
produce tetraploids from the diploid line is accomplished
by using colchicine. Other methods are also
available. Colchicine is applied to the shoot apex of
diploid plant seedlings, just as the cotyledons first
emerge from the soil. This causes the chromosomes
at the shoot apex to double resulting in the tetraploid
seeding. The mutagen is applied at a concentration
ranging between 1% and 2%, the lower concentration
for small-seeded cultivars. Similarly, one or two drops
of colchicine are then applied, one drop to smaller
seedlings and two drops to larger seedlings. The success
rate of chromosome doubling is low (about 1%).
Detection of tetraploids is by chromosome count or
other methods such as a count of the number of
chloroplasts in each side of the guard cell (diploids
have 5–6 per side, whereas tetraploids have 10–14
per side). Morphologically, tetraploids tend to have
thicker leaves, shorter stems, and slower growth than
diploids.
3 Development of tetraploid lines. The putative
tetraploids from the mutagenesis (T 1 plants) are evaluated
in the next generation (T 2 ) to authenticate
their polyploidy. Selected plants are selfed or sibbed
repeatedly in isolation to stabilize the new genotypes
and increase fertility for high seed yield to produce
sufficient seed for evaluation of the tetraploid lines.
4 Evaluation of tetraploid lines. The tetraploid lines
in the T 4 generation are evaluated especially for performance
in a hybrid by making selected crosses to
produce triploids. These triploids are evaluated to
identify tetraploids with high potential as parents.
Successful parents produce hybrids with high yield,
good rind color (gray), and that lack empty seed coats.
5 Development and evaluation of triploid lines.
Successful tetraploid parents are used in producing
triploids by hand pollination (expensive) or insect
pollination in isolation blocks. Hand pollination
enables the breeder to exercise control over the pollination
process to ensure a high success rate of desirable
crosses. Pistilate flowers from the female parent
may be tagged. When developing triploids by insect
pollination, the two parents should have different
rind colors for easy identification of hybrids. As indicated
previously, the tetraploid female usually has a
gray rind whereas the diploid is stripped. The triploid
produced by hybridization will be striped.
Breeding for industrial uses
Some crops have multiple uses – food, feed, and industrial
uses. Corn, for example, may be used for flour