Polyploidy. Up until now we’ve been talking about
plants with two sets of chromosomes—that is, whose somatic cells are diploid. However
some plants have more than two sets of chromosomes, a condition called polyploidy.
Polyploid plants often have bigger flowers and fruit, more intensely colored flowers,
bigger or thicker leaves, or bigger tubers. Polyploidy is quite a common condition; in
fact it’s thought that almost half of the angiosperms (flowering plants) are
polyploids.How does this characteristic come about?
Triploids. Sometimes "errors"
occur during cell division. If the chromosomes fail to segregate properly to the daughter
cells, the result can be a diploid gamete (with two sets of chromosomes rather than the
usual one). If this gamete unites with a normal, haploid gamete, then the resulting zygote
will be triploid—that is, it will have three sets of chromosomes. Though this
condition can sometimes result in plants with larger flowers or other desirable
characteristics, triploid plants are usually sterile. Because the chromosomes occur in
threes, rather than in pairs, the chromosomes cannot segregate evenly to the daughter
cells. This often results in infertile pollen and reduced sterility in the female plant.
Despite their sterility, triploids are still
horticulturally important. Not only are triploid flowers often larger and more colorful
than their diploid counterparts, since they don’t produce mature seeds the flowers
often last longer as well, and require little or no "dead-heading." One
interesting triploid is the familiar banana. Diploid bananas do exist, but they have hard
seeds. However in triploid bananas, the seeds abort, leaving only the tiny remnants you
see. So don’t bother looking for banana seeds—at least not if you want to eat
the fruit! The plant must be multiplied from cuttings. (We’ll be talking about
cuttings and other forms of asexual propagation next week.)
Tetraploids. Plants with four sets of
chromosomes are called tetraploids; this is the most common type of polyploidy. Tetraploid
plants are often larger than their diploid counterparts, but they are usually less uniform
in appearance. One of the more common examples are tetraploid snapdragons, which have
significantly larger blooms than the diploids. Tetraploidy arises from the union of two
diploid gametes. Because the chromosomes occur in sets of four, they can be evenly divided
to daughter cells during cell division; therefore, tetraploid plants are usually fertile.
Plant breeders can create tetraploids by treating diploid seeds or young plant tissues
with a substance called colchicine, a poisonous alkaloid.
Mutants. Long before people knew of the
concepts of chromosomes, genes, or mutations, any time a plant differed from the norm in
some way, it was simply called a sport. For example, a plant might be unusually
tall, or short; or the flowers or fruit might be different in size, color, etc.
If this type of variation is not the result of unusual
growing conditions, soil type, temperature, or other environmental conditions, nor a
reaction to infection by a microbe, then it is most likely caused by a mutation. Remember
that a mutation is a genetic "accident" that results from an error in chromosome
replication. Though you still see the terms sport (if the variation occurs on the entire
plant) and bud sport (if the variation occurs only on a single branch), a plant that
differs from the norm because of a genetic accident is more properly called a mutant.
Some new flower colors are the result of bud sports, and
many florists’ flowers have been propagated from mutants, including some varieties of
carnations, dahlias, chrysanthemum, and roses. Some familiar fruits, such as the ‘Red
Delicious’ apple, were originally bud sports.
Bud mutation is also a common cause of the variegation you
see in some plants’ foliage. A white or yellow area on the leaf indicates that a
mutation has affected that portion’s ability to produce chlorophyll. The familiar
houseplant Pothos (devil’s ivy) often has some stems that are entirely green
and some that have portions of variegation. Plants in which only sections of a stem are
variegated are called chimeras. If you want to root a cutting to grow a new,
variegated plant, you’ll need to root a section of stem that exhibits the
variegation. A fully green section will produce a non-variegated plant.
