1. The Case of Seed Taste Evolution in Pea Plants slide version 3.0

2. About this Case:
These slides were created by the Evo-Ed Project:
Funding for the Evo-Ed Project is provided by the National Science Foundation and by Lyman Briggs College, Michigan State University.
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4. Introduction
These slides are provided as a teaching resource for the Pea Seed Taste case as described on A fuller description of the case can be found on the website. Teaching notes can be found in the notes section beneath each slide when viewing the slides in “Normal View” in PowerPoint. To select this option in PowerPoint, go to the main menu, choose “View” and then “Normal.”

5. The Natural History of Round and Wrinkled Peas
** Notes on the slides in this section:
The following few slides are included as an overview of the natural history of the Pea Taste Case.
For more information visit:

6. 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.

7. 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.
There are four traits that are associated with domestication in P. sativum. Non-dehiscent pods are important so that pods do not open before they are harvested (the image shows a dehiscent pod). Many scholarly articles cite smooth seed coats on ancient remains of P. sativum as evidence of prehistoric domestication, but few articles give reasons as to why it is an important trait (e.g: Ljustina M. and Mikic A. (2010) A brief review on the early distribution of pea (Pisum sativum L.) in Europe. Field and Vegatable Crops Research 47: ) We infer from the characteristics of rough and smooth seed coat in other species that a smooth seed coat is likely a desirable trait for storing seeds for long periods of time because of the protection the smooth coat offers. Rough seed coats tend to come off the seed more easily and thus may not have been beneficial for storage.
A Dehiscent Pod (note: not a Pisum spp.)

8. Traits Selected during Domestication
Larger seed size.
Higher volume of food.
Wrinkled seed shape.
Wrinkled peas are sweeter than round peas.
Note: Slide 1.2 and slide 1.3 talk about different seed traits. Specifically, a smooth seed coat is as opposed to a rough seed coat; a wrinkled seed shape is as opposed to a round seed shape.

9. The Cell Biology of Round and Wrinkled Peas
** Notes on the slides in this section:
The following few slides are included as an overview of the cell biology of the Pea Taste Case.
For more information visit:
Questions are provided for discussion or as a basis for interactive engagement.

10. Two Traits
This case examines the evolution of wrinkled pea seed shape.
Wrinkled peas have 2 traits that differentiate them from round peas.
Seed Shape
Round
Wrinkled

11. Two Traits
(2) Seed Taste

12. 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?
This is an introductory activity is to hook students into the material in this module. If they have come up with an answer, they will be curious whether or not their ideas are correct. By not giving an answer, at this stage, you are generating interest among students for the content in this module.

13. Inside the Pea Cell: Sugar and Starch
This is an introductory slide to the production of sugar within the pea cell as a product of photosynthesis. There are many forms of “sugar” and with this slide you may not want to go into further detail – the upcoming slides provide a better context.

14. Inside the Pea Cell: Sugar and Starch
This slide begins a series of three slides that show a complex enzyme-mediated multi-step pathway that leads to starch production in plants. The purpose of these slides is not to have students memorize names of intermediate products or enzymes. Students should understand that (a) starch has two forms (amylopectin and amylose), both of which are end products of this sequence, (b) starch production is enzyme-mediated, and (c) starch production begins at photosynthesis and it is a multi-step pathway. If you have an advanced class or are focusing on biochemical pathways then it may be appropriate to generate activities based on these slides.

15. Inside the Pea Cell: Sugar and Starch

16. Inside the Pea Cell: Sugar and Starch

17. The G3P produced by the Calvin Cycle can form long polymers (starch)
The G3P produced by the Calvin Cycle can form long polymers (starch). When these glucose polymers are simple chains, they are known as Amylose (simple starch). 17

18. Starch-branching enzyme
When the Starch Branching Enzyme is functional, is facilitates the conversion of simple starch (Amylose) into complex, branched starch (Amylopectin). 18

20. When the starch branching enzyme is not functional, some of the sugar products get converted into sucrose.

21. The biosynthetic pathway that results in sucrose is not fully shown; it is indicated by the series of arrows leading to sucrose.

22. Inside the Pea Cell: Sugar and Starch
This is a simplified version of slides of the biosynthetic pathway that leads to Amylose, Amylopectin and Sucrose production.

23. Round Starchy Peas Round
This shows the resultant product as it pertains to peas. Round peas have both amylose and amylopectin.
Starch is comprised of 75% amylopectin (branched starch) and 25% amylose (unbranched starch).
Round

24. Sweet Wrinkled Peas Wrinkled
In wrinkled peas, the starch branching enzyme happens to be non-functional. This means that amylopectin cannot be produced, resulting in less total starch.
No amylopectin production means that less total starch is produced.
Wrinkled

25. Sweet Wrinkled Peas Wrinkled
In wrinkled peas, excess Glucose 1-P is converted to UDP-Glucose that combines with Fructose 6-P and then, through a series of intermediates, becomes sucrose. Wrinkled peas therefore have a higher concentration of sugar and a lower concentration of starch (through loss of amylopectin).
When excess amylose is present, excess Glucose 1-P is transformed into sucrose.
Wrinkled

26. 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?
This activity, paired with the question at the beginning of the cell biology section can help you gauge your students understanding of the biochemical nature of sweet and starchy peas. It also encourages students to think in an analytical way and to discuss scientific ideas with their peers.

27. From Mendel to Molecules
** Notes on the slides in this section:
The following few slides are included as an overview of the Mendel to Molecules section of the Pea Taste Case.
For more information visit:
We have found that students are often very good at giving Mendelian explanations for phenotypic changes, but they are rarely able to compliment these explanations with genetic details.

28. 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.
This slide is presented as a refresher to Gregor Mendel. Mendel examined seven characteristics associated with P. sativum pea plants. For more general background information on Mendel you can visit:

29. 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.
Prior to Gregor Mendel’s theories, inheritance was not well understood. Some believed that there was considerable blending involved of parental traits to produce offspring traits. The image on this slide is a dramatization of this – a plant with blue flowers and a plant with red flowers will blend to make offspring with purple flowers. The lower panel assigns allele designations to flower color. Mendel did not articulate “allele”, but rather he believed that there were heritable units that came from parents that determined offspring traits. The word allele was popularized starting in the 20th century, first used by British geneticist William Bateson. It is a short form of the word allelomorph that comes from the Greek language (άλλος μορφή) and can be translated as “other form/shape”.

30. 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.
Coincidentally, each traits that Mendel worked with happened to be controlled by a single gene. This is true of the round/wrinkled trait that is dependent on the R vs. r allele. If a pea plant has an R allele at the sbe1 gene locus, it will produce a functioning SBE1 protein (enzyme). If a pea plant has an r allele at the sbe1 gene locus, it will produce a non-functioning SBE1 protein (enzyme). More details of this will follow in the Genetics module.

31. 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.

32. 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.

33. 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.

34. First: A review of meiosis

35. 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.

36. 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.

37. 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?

38. 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.
Review – Central Dogma. This material comes up again in the next module. The information for a given trait is present on a segment of DNA called a “gene”. This gene is transcribed to RNA that is then translated into a polypeptide string that, by definition, is called a protein. The “round allele”, R, is present on the sbe1 gene. It is transcribed to RNA and is then translated to the SBE1 protein. This protein acts as an enzyme that catalyzes the transformation of amylose to amylopectin.

39. Producing the Wrinkled Peas
Wrinkled peas aren’t just wrinkled… they taste good too!
Round
(R allele of sbe1 gene)
Wrinkled
(r allele of sbe1 gene)
G3P
 (Intermediates)  Amylose  Amylopectin
G3P
 (Intermediates)  Amylose
 Sucrose
functioning starch branching enzyme
non-functioning starch branching enzyme

40. The Molecular Genetics of Round and Wrinkled Peas
** Notes on the slides in this section:
The following few slides are included as an overview of the genetics of the Pea Taste case.
For more information visit:
Questions are provided for student discussion.

41. Genetics
The section of DNA on a chromosome that codes for a protein is called an gene.

42. 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?
Discussion question.

43. 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!

44. The Central Dogma of Molecular Biology

45. The sbe1 gene has two alleles:

46. What does the “R” allele represent?
3500 Nucleotides
The R allele of the sbe1 gene is comprised of 3550 nucleotides that code for the SBE1 protein (enzyme).

47. The R Allele
These nucleotides code for a polypeptide protein of about 960 amino acids in length:
The 3550 nucleotides code for a polypeptide protein that is 960 amino acids in length.

48. What does the “R” allele do?
How does the SBEI protein lead to round seeds?
 It gives rise to highly branched starch (amylopectin).
Functionally speaking, when the R allele is present the SBE1 enzyme is coded for and amylose can be catalyzed into amylopectin. The high starch allows the pea seeds to keep their round shape when they are dried.
Highly branched starch in the seeds leads to seeds with low water content. When the seeds dry, they stay round.

49. What does the “r” allele represent?
plus an additional 800 nucleotides
The original 3550 nucleotides
The r allele is identical to the R allele, except that it has an extra 800 nucleotides inserted into the middle of the gene sequence. The polypeptide string (i.e. the protein) that results form this sequence is not able to function in the same way that the normal SBE1 protein functions – amylose cannot be converted to amylopectin

50. What does the “r” allele do?
How does the altered SBE1 protein lead to wrinkled seeds?
 It gives rise to unbranched starch only (amylose).
The result of this r allele – excess amylose results in excess Glucose 1-P. This is converted to UDP-Glucose that combines with Fructose 6-P and produces (through many intermediates) sucrose. Additional sucrose makes the peas sweeter. The additional sucrose content leads to higher water content. When these sweet peas are dried the water content is lose and the peas look wrinkled.
Unbranched starch in the seeds leads to seeds with high water content. They are sweet, but when the seeds dry, they wrinkle.

51. The R allele and the r Allele:
R allele genetic code
r allele genetic code
800 bp fragment of DNA inserted

52. The Population Genetics of Round and Wrinkled Peas
** Notes on the slides in this section:
The following few slides are included as an overview of the population genetics of the Pea Taste case.
For more information visit:
Questions are provided for student discussion.

53. 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.
In artificial selection, the force of selection if the human farmer who selects the desirable traits. This is a process that has been occurring since the beginning of recorded human history (see Natural History section).

54. Online Farming Simulator
Visit the online farming simulator at:
The farming simulator is paired with a worksheet (available online) that helps walk students through the game. Learning objectives and a general overview are available online.

55. Artificial Selection:
This begins a series of three slides that has the students work through a problem of artificial selection. This culminates in a clicker question on slide 5.4. Since the recessive trait must be homozygous recessive and the dominant trait may be either homozygous or heterozygous, it will be much easier to select for a recessive trait. Once you weed out all of the round peas, there will be no more R alleles in the population. Conversely, if all of the wrinkled peas are weeded out, the r allele will still be present in the population as part of a heterozygous pair.
Consider two fields with pea plants.
Some plants produce wrinkled peas, others produce round peas (50% wrinkled peas, 50% round peas).

56. 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?

57. Artificial Selection:
From a mixed field, what is easier to facilitate through artificial selection?
Monoculture of (dominant) round peas.
Monoculture of (recessive) wrinkled peas.
Both scenarios have equal difficulty.

58. Darwin and Selection
Darwin used the example of artificial crop selection as an example to support his theory of natural selection.
It is worth mentioning that artificial selection of crops was one of the examples that Darwin used to support his theory of evolution. The picture in this slide shows ancestral corn compared to modern corn. Selection for bigger ears over several centuries has led to what we recognize as modern corn. It may be worthwhile to have students speculate how similar (genetically) the three corn ears (shown) are. In all likelihood, it may be that these species are nearly identical with the exception of different genetic sequences for a few key genes that control traits like seed size, cob size, cob taste, etc.

59. Discuss: Which is more powerful: The force of Natural Selection
The force of Artificial Selection
This begins a group of three slides that can be used as clicker questions or as discussion questions. The answers are not always straightforward and may be conditional to specific examples that the students may bring up. However, generally speaking, the force of natural selection and the force of artificial selection are equivocal – differing only in the agent that carries out the selection (i.e. environmental/community related forces vs. human selection force).

60. Discuss:
Which is more likely to result in the fixation of a new trait in a population?
The force of Natural Selection
The force of Artificial Selection

61. Discuss:
Which is more likely to result in the evolution of a new species?
The force of Natural Selection
The force of Artificial Selection

62. Summary and Application - Round and Wrinkled Peas
**The following are review slides.

63. 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.

64. 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 non-functional and extra sugar is produced.

65. 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).

66. 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.

67. 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.

68. 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:
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 -