aarbatim at reading.ac.uk
Tue Jun 2 12:59:49 EST 1998
I apologise if this is too long an answer... My PhD dealt with a crop
protection problem on bananas! I have tried to make it is easy to read.
A long time ago, our bananas were wild giant herbs with two sets of
chromosomes (also called diploid). Wild banana plants today still produce
fruit which are like seed pods, with a little fleshy material being
produced around the pea-sized seeds. They spread both by seeds and
vegetatively (through suckers, which are clones of the mother plant).
Over the 2500 years (at least) that bananas are known to have been grown
by man, the plants have been selected for a number of things. Primarily,
it would have been for more fleshy material around the seeds and generally
a smaller seed size - it's what made a banana become a fruit/vegetable.
But the plants would have been selected for plant vigour, for flavour, for
yield, and probably for a number of other things too. This selection was
not carried out by taking the seed from a good plant after it had died
down, it was made by taking a sucker from the plant producing the best
fruit and transplanting it to a new field.
Suppose, as happens in nature, there was a mutation when a new sucker was
created. Normally, the resulting plant would be unable to breed. At
fertilisation, the differences between the two copies of chromosomes (one
normal, one mutated) usually mean that it is not possible for a seed to
develop or germinate properly. Result: end of the mutation.
Another problem might be if a diploid (=2 sets of chromosomes) cell ready
to divide accidentally failed to split after multiplying its DNA. This
would result in the formation of a tetraploid cell (4 sets of
chromosomes), and maybe even of a whole tetraploid plant. This is not as
much of a problem for plants as it is for animals. In fact, the resulting
tetraploid plants often grow stronger than diploid ones. Of course, there
is the problem of breeding, as their sex cells carry two sets of
chromosomes and need another two sets to be able to split in even equal
halves: unless there are plenty more tetraploids around, these plants
would die out as well.
Now think back of the human farmer. A tetraploid, fast growing plant is a
blessing: you get far more growth and fruit from such a plant. So you will
save and propagate this well. Somewhere else, someone finds that the fruit
from this particular diploid tastes just right, and will keep it and
multiply its suckers.
Someone, some day must have crossed them. While it would usually fail, it
would seem that one of the seeds produced was able to cope with having
three sets of chromosomes. It had a problem when it came to breeding, as
it could not split it cells into two equal halves. Its seeds aborted their
development: even after fertilisation, there wasn't the right DNA to split
and make an embryo. Result: a fleshy, tasty seed pod in which no seeds
develop. While this would be a failure in nature, the farmers loved it and
propagated it by moving suckers from field to field. Our modern seedless
banana plant was born.
I can remember tiny soft seeds in the bananas I had in my childhood during
the late 1970s. A bit like the seeds in cucumbers. These days, the variety
of triploid Cavendish bananas we get in our supermarkets don't even have a
mark where the seeds might have been.
And doesn't that make them nice?
Tim Baker (aarbatim at reading.ac.uk) * ____ *
Crop Protection Research Unit, * / \ *
Department of Agriculture, * | o o | Who else remembers *
Earley Gate, * | | PAC MAN? *
Reading, RG6 6AT, UK. * |/\/\/\| *
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