What do the ‘Delicious’ apples, seedless oranges,
and bananas in your fruit bowl have in common? They were all created by asexual
propagation, or cloning.The news story about Dolly,
the first official sheep clone, brought the word clone into general usage, often
with doomsday connotations and great hubbub about the ethical implications. What exactly
do the words cloning and clone mean, anyway?
Cloning
is the duplication of an organism by asexual means, involving mitotic, as opposed to
meiotic, cell division. The resulting offspring are genetically identical to the parent,
and therefore exhibit its unique characteristics. A population of genetically identical
offspring is called a clone; in common usage the term also refers to an individual created
by cloning.
Why is cloning important? As we saw with the violet plant,
asexual propagation, and the resulting production of clones, is a "backup" plan
for many plants. Though sexual reproduction is critical because it increases genetic
diversity, asexual propagation is a means by which a plant can reproduce many exact copies
of itself without relying on fickle pollinators or the availability of compatible pollen.
A population of genetically identical organisms isn’t
a particularly favorable outcome in nature. As we said in an earlier discussion, genetic
diversity is important because it allows plants to adapt to changing environments.
However, for horticulturists, a population of genetically identical plants sometimes is
the goal. Why?
First of all, uniformity is a more important attribute to
horticulturists than it is in nature. Suppose you were a nurseryman
producing chrysanthemums in anticipation of the autumn demand. By propagating mums
asexually, you would be able to carefully choose the type, then plant exactly the number
of yellows, reds, and purples you wanted, based on your experience with your customers.
Now imagine propagating by seed. Even if mums weren’t hybrids—and most of them
are—propagation by seed is chancy and the results are unpredictable.
Cloning is also important in the propagation of many
fruits. Most fruit and nut species have complex ancestries, and their seeds don’t
grow true to type. (Remember the F2 generation, when recessive traits not
expressed in the parents show up in the offspring.) Many familiar fruits and nuts are
propagated asexually, so the offspring will retain the favorable characteristics of the
original. The ‘Bartlett’ pear is a good example. It originated from a seedling
in an English orchard around 1770, and has been propagated asexually ever since. The
‘Delicious’ apple is another example of a fruit that has been maintained by
cloning—in this case, for about 100 years.
The likelihood that an apple tree grown from seed will
have a genetic blueprint identical to the ‘Bartlett’ pear or
‘Delicious’ apple—and therefore all their desirable characteristics—is
extremely low.
Finally, while reproduction by seed—sexual
reproduction—is favorable in nature, from the horticulturist’s (and
consumer’s) point of view, the absence of seeds is often a desirable trait. Think of
seedless oranges and grapefruits, seedless grapes, bananas, and pineapples—since
these plants don’t produce seeds, they must be propagated asexually.
Now that we’ve seen why asexual propagation is
important, let’s look at some of the ways plants propagate asexually in nature, as
well as at some horticultural techniques. But first we’ll introduce a few new terms.
