1. Methods to improve crops
• Enhancing the vitamin A content of rice through
genetic manipulation
– Green parts of rice plants contain beta-carotene – vital precursor
of vitamin A – No beta-carotene in grains
– Vitamin A essential for operation of body’s immune system.
Deficiency causes increased risk of infection, night-blindness and
total blindness.
– Genetically-modified strain of rice has been produced which
stores significant levels of beta-carotene in the grains (golden
rice) – contains genes from daffodil and a bacterium
– General method used: incorporate genes into a bacterial plasmid
and use bacteria (Agrobacterium tumefaciens) to carry genes
into the plant cells
– Method used for rice: genes are delivered directly into the cells
using small μm-sized tungsten or gold bullets coated with DNA.
The bullets are fired from a device that works similar to a
shotgun (gene gun)
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2. White rice and golden rice
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3. Gene gun (Courtesy of Bio-Rad
Laboratories)
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5. Producing polyploids in wheat through
hybridisation
• Modern bread wheat (Triticum aestivum) is a hexaploid
plant (6n) – result of hybridisation of several wild
species of grasses (such sterile hybrids can only
reproduce asexually)
• Doubling of chromosome number appears to have
occurred twice during the evolution of modern wheat –
resulting in the formation of fertile polyploids from
previously sterile hybrids
• Formation of polyploids important in plants – animal
polyploids are often not viable
• Now it’s possible to induce the formation of polyploids
by preventing spindle formation, using chemicals (e.g.
colchicine)
• Polyploids are generally more hardy and higher yielding
than their parent species
• Ancestors of wheat are small, not very robust and
produce small ears of small seeds
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6. Probable
ancestral Chromosome
Ploidy level
Plant haploid number
number
domestic oat 7 42 6n
peanut 10 40 4n
sugar cane 10 80 8n
banana 11 22, 33 2n, 3n
white potato 12 48 4n
tobacco 12 48 4n
cotton 13 52 4n
apple 17 34, 51 2n, 3n
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8. Evolution of modern bread wheat
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9. Producing vigorous, uniform maize
through inbreeding and hybridisation
• Through selection, inbreeding and hybridisation, growers
have been able to produce varieties of maize that grow
vigorously (therefore high yielding) and uniformly under
the prevailing conditions (soil type, prevailing
temperatures, rainfall, etc)
• Assuming conditions remain similar year after year,
farmers can continue to grow the same variety and
expect to obtain a similar crop
• Desirable characteristics:
– High yielding
– Disease resistant
– Good quality in terms of market desirability
– Vigorous growth under the prevailing conditions
– Plants all grow to a similar height (easy harvesting)
– Crops are all ready to harvest at the same time
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10. • Plants that show desirable characteristics would be
selected and self pollinated – pollen transferred to
stigma – muslin bag placed around flower (ensure pollen
from other plants does not reach stigma) – homozygous
plants for desired characteristics will be produced if
repeated for many generations
• Inbreeding depression: loss of vigour and fertility, as
well as a reduction in size and yield due to inbreeding
• Inbred maize has very little variation (every plant having
the same allele of every gene)
• However, if 2 inbred lines are crossed, it will produce a
hybrid that has a greater yield and is more vigorous than
either of the parental lines (hybrid vigour)
• This hybrid heterozygous for most genes, so deleterious
recessive alleles are hidden – at the same time, it
inherits the lack of variability from its parents
• Such single cross hybridisation has been used for
selective breeding since the early 1960’s to double the
yield and to breed uniform, high yielding maize
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11. Inbreeding depression
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12. • Double cross hybridisation was used as a selective
breeding tool to increase maize yields between 1920 and
1960 from 1.5 to 4 tonnes per hectare
• In order to carry out the inbreeding or to carry out a
cross to form a hybrid, pollen from a specific male
parent must be used to fertilise a specific female parent
– anthers are removed from some flowers which will
form the female parent – pollen transferred from anthers
of male parents flowers to stigmas of flowers without
anthers – muslin bags are placed around fertilised
flowers
• Selection for measurable characteristics i.e. yield
• Selection for disease or pest resistance
• Seeds are grown and plants showing desirable
characteristics are bred again – can be repeated for
many generations
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