From DNA Mutations to Selectable Phenotypes to Genetically Diverged Populations

Jim SmithMichigan State UniversityLyman Briggs CollegeDepartment of Entomologyjimsmith@msu.edu

Teaching Evolutionary Principles by Integrating Biology’s Sub-disciplines

Goals for today’s session

• Why were Mendel’s peas wrinkled?

• Cell biology of natural selection on a beach.

• People are evolving, too!wcco.com

On the Origin of Species

Darwin’s Logic

Variation under Domestication

We can easily see how people have artificially selected “favored races” of pigeons, sheep, roses and dahlias.

Natural Selection

One can easily also envision how this same “selection” process can occur naturally, in the wild.

The patterns of variation we observe in nature support this idea.

Downe House from the Sand Walk

We’ll Follow Similar Logic Today

Mendel’s Peas

Artificial Selection

Wrinkled Peas are sweeter, and therefore people will tend to select them and propagate them.

Hopi’s Mice

Natural Selection

Mice with white coats on white sand beaches are less visible to prey.

Thus, they are less subject to predation and selectively survive.

goodlifefarm.com

Hoekstra et al. 2006. Science 313: 101–107.

Basic Principles of Mendelian Genetics

Alleles at a locus

Genotype

Phenotype

R R

R

Mother

Rr

Female gametes

R r

Father

Rr

Mal

e g

ame

tes R

r

RrRR

Rr rr

Resulting genotypes: 1/4 RR : 1/2 Rr : 1/4 rr

Resulting phenotypes: 3/4 : 1/4Freeman 1e Fig. 10.7

R = Dominant allele for seed shape (round)r = Recessive allele for seed shape (wrinkled)

= Round-seeded phenotype= Wrinkled-seeded phenotype

Mendelian Genetics

Why are wrinkled peas wrinkled?(use the word, “protein” in your answer!)

R

Hint: What does “R” do?

Freeman 1e Fig. 10.2

Integrating Across Sub-disciplines

The Central Dogma of Molecular Biology

Connect to Mendelian Genetics

Connect to Evolution

What does the “R” gene do?

R

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.

Freeman 1e Fig. 10.2

Modified from Campbell 4e Fig. 5.6

Molecular Basis ofWrinkled vs. Round Peas

R allele Functional SBE Protein

800 bp fragment of DNA inserted

R protein

R protein

r proteinr protein

r allele Non-Functional SBE

Courtesy of Dr. April Cognato, Mich. St. Univ., Dept. Zoology

R

3550 nucleotides!

What does the “R” symbol represent?

What does the “R” gene do?

R

The “R” gene DNA encodes a starch branching enzyme protein (SBEI)!

A polypeptide of approx. 960 amino acids

Gregor Mendel

Observed 5474 round seeds (dominant) and 1850 wrinkled seeds (recessive)

• 5474/1850 = 2.96:1

Do a 2-test

• Is this a 3:1?

Basic Principles of Evolution

Phenotypic Variation

Environment w/ Selection

Evolved Population

Connect to Mendelian Genetics

Connect to Cell and Molecular Biology

Bridging Mendel and Darwin

Observed 5474 round seeds (dominant) and 1850 wrinkled seeds (recessive)

• 5474/1850 = 2.96:1

How would the wrinkled allele (r) go to fixation in a population?

r

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A Population of Peas

How do new species evolve?

From:http://evolution.berkeley.edu/evolibrary/article/_0/history_20

Merging Genetics, Molecular Biology, and Evolution

Phenotypic Variation

Environment w/ Selection

Evolved Population

Alleles at a locus

Genotype

Phenotype

DNA

RNA

Protein

R

Peas Connect Parallel Paths!!

Beach Mice - The Mc1r locus and MC1R ProteinHoekstra et al. 2006

Mice living on exposed white sand beaches along the Gulf Coast have different Mc1r allele frequencies

The mutant Mc1r gene has a single C T substitution, leading to the substitution of Cysteine for Arginine at position 65 in the MC1R protein

MC1R proteins (polypeptides) with the R65C amino acid substitution bind -MSH less well and produce less cAMP

From: Barsh 1996 Trends in Genetics

R65C MC1R Proteins Function Differently in Melanocytes than their Counterparts

Steiner et al. 2009. Mol Biol Evol 26: 35-45.

Clicker Q1. The ultimate reason for the lighter coat colors of the mice on the white sand Gulf Coast beaches was:

A. predation by hawks and other carnivores on beach mice that do not carry the mutant allele.

B. a substitution of Cys for Arg at position 65 of the MC1R protein.

C. a substitution of T for C at position 193 of the Mc1r gene nucleotide sequence.

D. the failure of melanocytes to lay down melanin pigment in the coat hairs of the lighter colored beach mice.

E. the poorer binding affinity for -MSH and the lower amount of cAMP produced by individuals with the R65C MC1R protein.

Clicker Q2. The C T substitution at position 193 of the Mc1r gene:

A. arose by a mutation in the beach mouse populations in response to a need for protection from predation.

B. leads to the failure of melanocytes to make an MC1R protein.

C. arose by a mutation, which increased in frequency because it was selectively advantageous in the beach mouse populations.

D. had no effect on the beach mouse populations.

E. produced an alternate allele that was detrimental to mice on the white sand beaches.

Humans Are Evolving, Humans Are Evolving, Too!!Too!!

Organismal Biology Honors Organismal Biology Honors PresentationPresentation

[2 Lyman Briggs students][2 Lyman Briggs students]

Jaw Muscle-ApeJaw Muscle-Ape Archaic Ape, Humanoids- Archaic Ape, Humanoids-

heavy jaw (myofascial) heavy jaw (myofascial) muscles muscles

Muscle attached to Muscle attached to sagittal crest on top of sagittal crest on top of headhead

Continued to modern apesContinued to modern apes– Allows for hard chewingAllows for hard chewing– Important for raw vegetable Important for raw vegetable

dietdiet Stedman H, et al. 2004. Myosin gene mutation correlated with anatomical changes in the human lineage. Nature 428: 415-418.

Currie P. 2004. Nature 428: 373-374.

Jaw Muscle-HumansJaw Muscle-Humans

Smaller jaw muscles Smaller jaw muscles then apesthen apes– Decrease in plant Decrease in plant

consumptionconsumption– More reliance on hands More reliance on hands

Decrease in muscles Decrease in muscles eliminated sagittal eliminated sagittal crestcrest

Currie P. 2004. Nature 428: 373-374.

How did this happen?How did this happen? Myosin Heavy Chain (MYH)Myosin Heavy Chain (MYH)

– Critical proteins in sarcomereCritical proteins in sarcomere– Myosin binds to actin and uses ATP to pull itselfMyosin binds to actin and uses ATP to pull itself

Give contractile force to skeletal muscleGive contractile force to skeletal muscle

– Different types for contraction in different Different types for contraction in different musclesmuscles

– Loss of MYH gene cause large loss of muscle Loss of MYH gene cause large loss of muscle

MYH: Humans vs. ChimpsMYH: Humans vs. Chimps MYH 16 in both Human & ChimpsMYH 16 in both Human & Chimps

– Temporalis and Masseter MusclesTemporalis and Masseter Muscles Human gene mutated Human gene mutated

– Accumulate less MYH 16 proteinAccumulate less MYH 16 protein

Seven non-human species compared to six geographically dispersed human populations

Bibliography and ResourcesWrinkled Peas

Guilfoile P. 1997. Wrinkled Peas & White-Eyed Fruit Flies: The Molecular Basis of Two Classical Genetic Traits. The American Biology Teacher 59: 92-95.

Bhattacharyya MK, et al. 1990. The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell 60: 115-122.

Beach Mice

Hoekstra HE, Hirschmann RJ, Bundey RA, Insel PA, Crossland JP. 2006. A single amino acid mutation contributes to adaptive beach mouse color pattern. Science 313: 101–107.

Human Jaws

Stedman H, et al. 2004. Myosin gene mutation correlated with anatomical changes in the human lineage. Nature 428: 415-418.

Slides from today’s session:http://www.msu.edu/user/jimsmith/nabt_2010.pdf