Polyploidy Flashcards
What is polyploidy?
Whole genome doubling.
Produces organisms with multiple sets of chromosomes.
Can arise spontaneously or through non-disjunction at meiosis.
Can be artificially induced with drugs (e.g. colchicine) which inhibit cell division.
Can be limited to certain tissues – endopolyploidy.
Much more common in plants than animals.
Polyploids have multiple sets of chromosomes.
What are autopolyploids?
Autopolyploids derive from a single species.
Chromosomes are identical.
What are allopolyploids?
Allopolyploids derive from two or more closely related species.
Relatives are close enough genetically so that one is able to fertilise the other. Progeny has both sets of chromosomes.
Why is polyploidy important?
Major force in plant evolution, especially in ferns and flowering plants.
It directly effects the physical properties of plants (e.g. size, life cycle).
Many important crop plants are polyploids.
Most plants retain the vestiges of ancient polyploidization events.
How did many flowering plants arise?
Roughly 35% of flowering plants arose through polyploidization.
How does polyploidy affect the plants?
Increased size of some cells and some organs- if the nucleus has to hold more DNA. Cell doesn’t need to increase, only the nucleus does, but often the cell does too.
Changes in shape and texture of organs.
Differing ability to colonize new habitats. Colonisation changes can lead to speciation due to potentially different growth properties and niche requirements.
Reduction in fertility and seed production.
How are polyploids recognised?
Count chromosomes using cytogenetics –
If a species has pairs of closely related chromosomes it is
probably polyploid.
Increased organ size (e.g. stomatal cells)- stomatal cells have doubled in size.
Can also check to see if chromosomes fall into pairs- if they do, you have a polyploid.
How has polyploidy affected Brassica napus?
Brassica napus is a tetraploid that shows ‘hybrid vigour’ over its two diploid progenitors.
Can cross two small plants and get a much larger plant- this is hybrid vigour.
What kind of tetraploidy is usually found in Brassica plants?
Allotetraploidy.
Hybrids tend to be plants that we grow as crops.
How can you distinguish between auto and allpolyploidy?
Allopolyploids (amphidiploids) are variable. Multivalent pairing when closely related. If 2 chromosomes derive from wildly separate species (millions of years old)- they won’t pair. If species are only separated by eg. 1 million years- they behave like a tetraploid.
Autopolyploids typically have multivalent pairing. Chromosomes are more or less identical. All four chromosome pairs line up together.
What can in situ hybridisation be used to identify?
Allopolyploidy.
Probes from parental species.
Red and green chromosomes- come from different species. Identified by transposon families and their abundance in the species- difference in colours.
What can cause novel variation in polyploids?
Increased variation for dosage-regulated genes.
Altered regulatory interaction. Interactions or lack of could end up turning genes on or off.
Genetic changes.
Epigenetic changes- modifications to chromatin are possible due to polyploidisation.
What happens to auto and allopolyploids after polyploid formation?
Autopolyploids:
Genomes recombine freely between all copies – no chance for individual chromosome evolution.
Gene expression-
Dosage effect (linear relationship between gene expression and number of gene copies).
Allopolyploids:
Genomic changes.
Diploidization- Genome structural evolution.
Since chromosomes from different species may not pair, each pair of the same can change independently.
How does diploidisation happen?
‘Shortly’ after a genome duplication event, genes begin to be deleted at a rapid rate.
This happens more or less randomly between the duplicated chromosomes (though there is some evidence for selective loss).
After about 50 million years there’s only a few % of the duplicated genes left (and they are of course diverging from each other). Divergence, since there are only a few genes that are shared now.
Polyploids don’t need all of the extra copies.
What is the story of cotton?
Parental genomes from different continents:
Maternal A-genome from Africa.
Paternal D-genome from the New World.
Polyploidization ~1.5 Mya.
Cotton is a tetraploid, and is the origin of 5 Allopolyploid species.
Three progenitors: New world- G. raimondii. Africa- G. arboreum, G. herbaceum.
Five descendants: G. darwinii, G. tomentosum, G. mustelinum, G. hirsutum, G. barbadense.
Four domesticated species: From decendents- G. hirsutum, G. barbadense. From Africa- G. arboreum, G. herbaceum.
