Presentation on theme: "The Case of Seed Taste Evolution in Pea Plants slide version 3.0"— Presentation transcript:
The Case of Seed Taste Evolution in Pea Plants slide version 3.0
About this Case: 1.These slides were created by the Evo-Ed Project : 2.Funding for the Evo-Ed Project is provided by the National Science Foundation and by Lyman Briggs College, Michigan State University. 3.These slides are provided as a teaching resource. You are encouraged to modify them to meet your specific teaching and learning needs. 4.Please adhere to the copyright conditions specified on the following slide. 5.There is a reference slide at the end of the presentation that lists the sources for the images we have used in this presentation. 6.If you would be willing to be in involved in our research study examining how the use of these case studies impacts learning, please contact us at
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Introduction These slides are provided as a teaching resource for the Pea Seed Taste case as described on ed.com. 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.”
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 WrinkledRound
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
Round Starchy Peas Round Starch is comprised of 75% amylopectin (branched starch) and 25% amylose (unbranched starch).
Wrinkled Sweet Wrinkled Peas No amylopectin production means that less total starch is produced.
Wrinkled When excess amylose is present, excess Glucose 1-P is transformed into sucrose. Sweet Wrinkled Peas
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! Wrinkled (r allele of sbe1 gene) Round (R allele of sbe1 gene) G3P (Intermediates) Amylose Amylopectin G3P (Intermediates) Amylose non-functioning starch branching enzyme Sucrose 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?
Unbranched starch in the seeds leads to seeds with high water content. They are sweet, but when the seeds dry, they wrinkle. How does the altered SBE1 protein lead to wrinkled seeds? It gives rise to unbranched starch only (amylose). What does the “r” allele do?
The R allele and the r Allele: 800 bp fragment of DNA inserted R allele genetic code r allele genetic code
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:
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 non- functional 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 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 - 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: