The Case of
Seed Taste Evolution in Pea
Plants
slide version 3.0
http://www.evo-ed.com
About this Case:
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Introduction
These slides are provided as a teaching resource for
the Pea Seed Taste case as described on www.evoed.com. A fuller description of the case can be
found on the website.
Teaching notes can be found in the notes section
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The Natural History of
Round and Wrinkled Peas
Fact Sheet: Pisum sativum
• Common names: Field peas.
• Native to: Near East  Iraq,
Turkey, Israel, Palestine, etc.
• Habitat: Agricultural and
steppe fields.
• A key crop of the Neolithic
Agricultural Revolution, oldest
recorded samples date to
10,000 BCE.
Traits Selected during Domestication
• Non-dehiscent pods.
– Seeds stay protected
within pods longer.
• Smooth seed coat.
– As opposed to a rough
seed coat, smooth coats
are harder to remove
from the seed. This gives
them more protection
when stored.
A Dehiscent Pod (note: not a Pisum spp.)
Traits Selected during Domestication
• Larger seed size.
– Higher volume of
food.
• Wrinkled seed shape.
– Wrinkled peas are
sweeter than round
peas.
The Cell Biology of
Round and Wrinkled Peas
Two Traits
• This case examines the evolution of wrinkled pea seed shape.
• Wrinkled peas have 2 traits that differentiate them from
round peas.
(1) Seed Shape
Round
Wrinkled
Two Traits
(2) Seed Taste
What do seed shape and seed taste
have in common?
- Wrinkled peas are wrinkled because they have
a higher water content than round peas.
When they dry, round peas retain their shape
while wrinkled peas do not due to water loss.
Question:
- Why would wrinkled (sweet) peas have a
higher water content than round (starchy)
peas?
Inside the Pea Cell: Sugar and Starch
Inside the Pea Cell: Sugar and Starch
Inside the Pea Cell: Sugar and Starch
Inside the Pea Cell: Sugar and Starch
Starch-branching
enzyme
Inside the Pea Cell: Sugar and Starch
Round Starchy Peas
Round
Sweet Wrinkled Peas
Wrinkled
Sweet Wrinkled Peas
When excess amylose is present, excess
Glucose 1-P is transformed into sucrose.
Wrinkled
Revisit: What do seed shape and seed
taste have in common?
- Wrinkled peas are wrinkled because they have
a higher water content than round peas.
When they dry, round peas retain their shape
while wrinkled peas do not due to water loss.
Question:
- Why would wrinkled (sweet) peas have a
higher water content than round (starchy)
peas?
From Mendel to Molecules
Famous Experiments
• Gregor Mendel – the
father of modern
genetics.
• Inheritance of traits in
pea plants (P. sativum).
• Flower color, seed shape,
pod shape, pod color,
flower position, stem
length, embryo color.
Traits: Blending or Inheritance
• Prior to Mendel, many
thought that the traits
of offspring must be
a blended version of
parentaltraits.
• Mendel showed that
this is not the case and offspring traits are
determined by the combination of heritable
units passed on by parents.
Inheritance of Pea Plant Traits
• In pea plants, traits are
often controlled by one set
of alleles. These alleles can
combine in a homozygous
(dominant or recessive) or
heterozygous fashion
depending on parental
alleles.
• Examples:
– The Round vs. Wrinkled pea
trait is controlled by the alleles
R and r
– The Yellow vs. Green pea trait is
controlled by the alleles Y and
y.
Trait-Based Genetics
• Mendel suggested that
inheritance of traits
followed specific rules.
• The Law of Segregation.
– Each trait is linked to a pair
of alleles. Each parent
passes on one of these two
alleles to their offspring.
• The Law of Independent
Assortment.
– The inheritance of one trait
is independent from the
inheritance of another trait.
From Traits to Genes
• Amazingly, Mendel had no
knowledge of DNA, genes or
chromosomes.
• The traits (alleles) that
Mendel described
correspond to specific genes
within the pea plant
genome.
We can make connections that Mendel
could not
We now know that each allele for a given
characteristic resides on one of a homologous
pair of chromosomes. Each allele is a slightly
different form of a gene coding for the same
characteristic. The process of meiosis tells us
how the inheritance of these alleles correlates
to Mendel’s Laws of Segregation and
Independent Assortment.
First: A review of meiosis
Metaphase I of
Meiosis
Each chromosome is
duplicated earlier in
the process of
meiosis. At
Metaphase I, the two
versions of a given
chromosome pair
“find” each other.
These chromosome
pairs separate.
Subsequently, two new
cells form, each having
one member of a
chromosome pair. Then,
the two duplicates of a
chromosome separate
to produce four cells,
each with a single
chromosome from the
original pair.
Law of segregation: A look at the pair of
chromosomes having the information for pea
taste
• The white bead
represents the R allele
and the pink bead
represents the r allele.
These two alleles
separate or segregate
during the process of
meiosis. One allele for
pea taste is contributed
to a given offspring.
Law of Assortment: independent
inheritance of chromosomes
• Each member of a pair of
chromosomes randomly
align on either side of the
“equator” of the cell at
Metaphase I. We’ve
shown two here, A and B.
What combinations of
alleles for three
characteristics each
different chromosomes
are there?
Molecular Biology: Central Dogma
• The “R” allele of
the sbe1 gene
codes for a
protein.
• This protein is the
SBE1 enzyme that
converts amylose
to amylopectin.
Producing the Wrinkled Peas
Wrinkled peas aren’t just wrinkled…
they taste good too!
Round
Wrinkled
(R allele of sbe1 gene)
(r allele of sbe1 gene)
G3P

(Intermediates)

Amylose
functioning starch branching enzyme 
Amylopectin
G3P  Sucrose

(Intermediates)

Amylose
non-functioning starch branching enzyme
The Molecular Genetics of
Round and Wrinkled Peas
Genetics
• The section of DNA on a chromosome that
codes for a protein is called an gene.
The Starch Branching Enzyme
• The loss of function in the SBE1 protein is
responsible for the wrinkled/sweet
phenotype.
Question:
How does this protein differ in round/starchy vs.
wrinkled/sweet peas?
The Starch Branching Enzyme
• The loss of function in the SBE1 protein is
responsible for the wrinkled/sweet
phenotype.
Question:
How does this protein differ in round/starchy vs.
wrinkled/sweet peas?
Answer:
It has a different amino acid sequence, coded by
a different genetic code!
The Central Dogma of Molecular Biology
The sbe1 Gene
The sbe1 gene has two alleles:
What does the “R”
allele represent?
3500 Nucleotides
The R Allele
These nucleotides code for a polypeptide
protein of about 960 amino acids in length:
What does the “R” allele do?
How does the SBEI protein lead to round seeds?
 It gives rise to highly branched starch (amylopectin).
Highly branched starch in the seeds leads to seeds with low water content.
When the seeds dry, they stay round.
The original 3550 nucleotides
plus an additional 800 nucleotides
What does the “r”
allele represent?
What does the “r” allele do?
How does the altered SBE1 protein lead to wrinkled seeds?
 It gives rise to unbranched starch only (amylose).
Unbranched starch in the seeds leads to seeds with high water content.
They are sweet, but when the seeds dry, they wrinkle.
The R allele and the r Allele:
R allele genetic code
r allele genetic code
800 bp fragment of DNA inserted
The Population Genetics of
Round and Wrinkled Peas
Artificial Selection of Sweet Peas
• The population genetics of wrinkled peas are
driven by artificial selection.
• Wrinkled peas are sweeter and in ancient
times farmers tended to
select them for breeding.
Online Farming Simulator
• Visit the online farming simulator at:
http://www.evo-ed.com/Pages/Peas/Farming/Farming.html
Artificial Selection:
• Consider two fields with pea plants.
• Some plants produce wrinkled peas, others produce
round peas (50% wrinkled peas, 50% round peas).
Artificial Selection:
• Question: assuming you were a farmer and could weed
out wrinkled peas or round peas, would it be easier to
create a permanent monoculture of round peas or a
permanent monoculture of wrinkled peas?
Artificial Selection:
• From a mixed field, what is easier to facilitate
through artificial selection?
(a) Monoculture of
(dominant) round peas.
(b) Monoculture of
(recessive) wrinkled
peas.
(c) Both scenarios have
equal difficulty.
Darwin and Selection
• Darwin used the example
of artificial crop selection
as an example to support
his theory of natural
selection.
Discuss:
• Which is more powerful:
a) The force of Natural Selection
b) The force of Artificial Selection
Discuss:
• Which is more likely to result in the fixation of
a new trait in a population?
a) The force of Natural Selection
b) The force of Artificial Selection
Discuss:
• Which is more likely to result in the evolution
of a new species?
a) The force of Natural Selection
b) The force of Artificial Selection
Summary and Application Round and Wrinkled Peas
Pea Plants: Natural History
• The domestication of
pea plants occurred in
ancient civilization.
• One of the key traits
that was selected for
was a wrinkled seed
shape.
Pea Plants: Cell Biology
• Wrinkled seeds are sweet, round peas are
starchy.
• There is a complex multi-step pathway that goes
from triose-phosphate (sugar) to amylose and
amylopectin(starches).
• The SBE1 enzyme catalyzes the pathway from
amylose to amylopectin.
• In sweet peas the SBE1
enzyme is nonfunctional and extra
sugar is produced.
Pea Plants: Mendel to Molecules
• Gregor Mendel identified
heritable units as the
mechanism for traits passing
from parents to offspring in
pea plants.
• These heritable units, called
alleles, are versions of
specific genes that code for
proteins – in this case the
SBE1 protein (enzyme).
Pea Plants: Genetics
• The difference between
the R and r alleles is a
800bp insertion in the r
allele that makes the
SBE1 protein (enzyme)
non-functional.
Pea Plants: Population Genetics
• Artificial selection was
one of the examples
used by Darwin when
he was forming and
explaining his theory
of natural selection.
• Artificial selection can
fixate traits and form
new species – similar
to natural selection
but using a different
selective agent.
References
•
•
The sources for the images we used in this presentation are listed below. If an image is not
listed it is believed to be Public Domain.
Did we use one of your pictures and not give you proper credit? If so, please let us know:
evoed@msu.edu.
Section, Credit
Natural History:
Book, Evo-Ed / Valerie Henry
Pea plant, Public Domain
Dehiscent pod, mdf
Yellow and Green Peas Evo-Ed / Valerie Henry
Cell Biology:a
Pea zooming, Evo-Ed
Wrinkled and Round Peas Evo Ed / Valerie Henry
Other content, Evo-Ed
Amylopectin, Campbell 4e Fig. 5.6
Mendel:
Mendel thought, Evo-Ed / Valerie Henry
Mendel, Public Domain
Flowers, Evo-Ed / Valerie Henry
Plants, Evo-Ed / Valerie Henry
Chromosomes, Evo-Ed / Valerie Henry
Genetics:
Flower plants, Evo-Ed / Valerie Henry
Chromosome to Gene Evo-Ed / Valerie Henry
Amylopectin and Amylose, Campbell 4e Fig. 5.6
Population Genetics:
Field of peas, Evo-Ed / Valerie Henry
Hand of peas, Evo-Ed / Valerie Henry
Selecting peas, Evo-Ed / Valerie Henry
Pedigree, Evo-Ed; Corn, PLOS http://biology.plosjournals.org/perlserv/?request=slideshow&type=figure&doi=10.1371/journal.pbio.0000008&id=39332
http://www.evo-ed.com