POLYPLOIDY
1.3 Genome: (b) iii Polyploidy
From the Arrangements
(iii) Polyploidy
Errors during the separation of chromosomes during cell division can result in
cells with whole genome duplications. Importance of polyploidy in evolution
and human food crops.
Teacher’s notes
1.
To set the scene with some background information the following link
gives a super animation of meiosis:
http://www.cellsalive.com/meiosis.htm
You should stress that it is the failure of the spindle fibres, complete
non-disjunction, which gives rise to a doubling of the chromosome
complement of the gamete cells. For example, a plant which undergoes
complete non-disjunction will form diploid gametes. If these self fertilise a tetraploid plant will result.
Polyploidy organisms have more than two sets of chromosomes: some
sources estimate that over 70% of flowering plants are polyploids.
2.
You can download three worksheets in the form of a PowerPoint
presentation from the following link:
http://polyploidy.org/index.php/File:Polyploidy_worksheet.ppt
There are excellent exercises linking ploidy levels with yield in maize
and (simple) ways of determining the ploidy status. The file format is
PowerPoint but the exercises can be printed.
3.
There is a relatively easy comprehension exercise (Polyploidy – Instant
evolution, parts 1 and 2) on the following pages, each with a passage of
text and some questions that explore the nature of polyploidy.
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There is also a short extension activity at the end of this section. The
internet is the most obvious place to look. This could be set as a
homework activity and one or two students could present their findings
to the rest of the class.
Note: Some of the old Higher Biology past paper questions deal with
polyploidy, especially the necessity of having even sets of homologous
chromosomes to produce fertile offspring.
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Student activities
Polyploidy
Learning objectives
By completing this exercise you will know what is meant by polyploidy ,
understand that polyploidy is more common in plants than animals and know
some of the reasons why this is so.
You will know that polyploidy gives rise to new variation, and n ew species,
within a single generation and that this is due to errors during the separation
of chromosome sets during the formation of sex cells.
Most of our crop plants are polyploid.
Instant evolution, part 1
(From an article by Mary Aegerter: Instant Evolution,
http://www.wsu.edu/NIS/Universe/instant.html )
Evolution has been studied most thoroughly for species we humans consider
‘normal’, those with two copies of each of their chromosomes. It’s what we
have. ‘Normal’ evolution usually takes a long time – generations – during
which a group of individuals of one species becomes separated and
genetically distinct from those they left. If they become distinct enough, they
become a new species.
All of the plant species that have evolved more than once have done so via a
mechanism called ‘polyploidy’. A polyploid species is one that has more than
two copies of each of its chromosomes. More than half of all land plants
species are polyploid, including wheat, corn and cotton. Few animal species
are, one notable exception being salmon .
Evolution via polyploidy is different. ‘It’s instantaneous,’ says Soltis. It takes
just one generation, and it doesn’t require that the new species be spatially
separated from the old. The mechanism appears to involve an error in a
plant’s production of its pollen or egg which results in its containing too
many copies of each chromosome. When that pollen or egg unites with its
opposite, the offspring is a plant with extra chromosomes.
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‘It appears to be the rule rather than the exception that polyploid species
evolve more than one time,’ says Soltis. Essentially all polyploids that have
been tested have been found to have done so.
Polyploidy can happen between individuals of two distinct plant species, a
process termed allopolyploidy. Or it can happen between two individuals of
the same species, which is termed autopolyploidy. The latter process was
considered rare and maladaptive for many years since it was thought
autopolyploidy would result in a high proportion of sterile eggs or pollen.
Conventional methods of following plant inheritance were based on
distinguishing physical characteristics. This approach was both difficult and
time consuming and produced little evidence to support autopolyploidy. But
Soltis and collaborator Professor Pam Soltis have found that molecular
methods such as enzyme and DNA analysis show that autopolyploidy has
been more common than previously thought.
The Soltises are interested in how polyploid species evolve and in why
polyploidy is an important evolutionary mechanism for plants. ‘The more we
learn about polyploidy, the better we will be able to manipulate it to create
more vigorous or larger crop plants,’ says Doug Soltis.
Read the passage and answer the following questions.
1.
How long does ‘normal’ evolution normally take?
2.
What has to happen to a group of individuals before evolution
‘normally’ takes place?
3.
What is meant by a polyploid species?
4.
Roughly, what proportion of land plants are thought to be polyploid?
5.
Explain what is meant by ‘evolution via land plants is different. It ’s
instantaneous.’
6.
In plants which tissues give rise to polyploid cells?
7.
What is meant by ‘polyploid species evolve more than one time’?
8.
What do you think is meant by ‘conventional methods of following
plant inheritance’?
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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POLYPLOIDY
9.
What sort of analysis has shown that polyploidy is more common than
previously thought?
10.
In terms of natural selection what do autoploids have compar ed to their
parent plants?
Instant evolution, part 2
(From an article by Mary Aegerter: Instant evolution,
http://www.wsu.edu/NIS/Universe/instant.html )
Polyploid animals are far less common than polyploid plants. Professor of
Zoology Gary Thorgaard says, however, that polyploidy may have been
important in the evolution of lower vertebrates. Fish, frogs, salamanders,
lizards and chickens all have some polyploid species.
As to why it’s not more common in animals, Thorgaard says there are a
variety of ideas as to why. ‘Animals usually have sex chromosomes while
plants rarely do, and these must be present in the proper ratio for a fertilized
egg to develop,’ he says. Sometimes more genes are expressed from one of
the parental sex chromosomes. Again, the proper ratio of chromosomes from
each sex would be crucial. In support of these ideas is the observation that
polyploid animal species tend to have sex chromosomes that are more similar
than those of animals that do not.
It may also be true that basic differences in how plants and animals grow are
involved. Plants have an open growth pattern – they just keep growing, says
Doug Soltis. Animals have more deterministic growth patterns. They have a
multitude of complex systems, skeletal and nervous for example, that must be
precisely integrated. Extra gene products from extra chromosomes could
easily upset the balance needed for this integration. ‘The errors in egg and
sperm production that would result in polyploidy probably also occur in
animals, but these cells don’t survive,’ he says.
Read the passage and answer the following questions .
1.
What does Professor Gary Thorgaard mean by ‘lower vertebrates’?
2.
What do animals have that have plants rar ely have and what does this
have to do with the development of a zygote?
3.
Explain what is it about the sex chromosomes of polyploid animals that
makes them different from non-polyploid animals.
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4.
Explain why the growth differences between plants and animals may
have a bearing on the success of polyploid mutants .
5.
Polyploid mutation events in animals are thought to be just as common
so why are there not as many?
Extension activity
Find out the levels of ploidy in banana, potato, swede, oilseed rape (canola),
wheat and strawberry.
Einkorn, spelt, durum and modern wheat ( Triticum aestivum) are all types of
wheat. What distinguishing features do they have and how does this relate to
their ploidy levels?
How many animals can you find which show po lyploidy?
What mammals can you find which are polyploid?
Present your results in a table.
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UNIT 1, PART (III) GENOME (H, BIOLOGY)
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