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Lactase persistence
The Cell Biology of Lactase
Persistence
Biology of the Digestive Tract
Enterocytes (the cells
that line the inside of
digestive tract) are
responsible for breaking
down and absorbing
nutrients from the food
in the stomach and small
intestine.
First Food: Mother’s Milk
The enterocytes of all infant mammals exhibit high
levels of lactase during infancy, when milk is the main
source of nutrition.
Photo credit: Jim French, Flickr
Lactase Unlocks an Energy Source
Lactose is a disaccharide sugar found in milk.
Lactase breaks down lactose into
two monosaccharides, glucose and galactose.
These simple sugars can be absorbed by cells in the
small intestine and used as a source of energy.
Lactose
Lactase
(enzyme)
Galactose
Glucose
Location of Lactase
This is the “brush
border” of an
enterocyte, which is
the side of the
enterocyte that
comes in contact with
the contents of the
small intestine.
Lactase is stained
brown.
Lactase Breaks Down Lactose
INTESTINE CONTENTS
ENTEROCYTE
Lactose
Lactase
Glucose
Galactose
BLOODSTREAM
Lactase Regulation
Almost all known mammals experience a decrease
in lactase biosynthesis in the years after weaning.
The regulation of lactase biosynthesis after weaning
is the main factor that separates Lactase Persistent
from Lactase Non-Persistent individuals.
Lactase Regulation
Why does this happen?
Why can’t adult mammals digest milk?
Lactase Regulation
The decrease in lactase production after weaning is likely a
matter of energy conservation at a cellular level:
1. It takes energy to produce any enzyme, including lactase,
the enzyme needed to digest milk.
2. Typically, mammals do not consume milk once they have
stopped nursing.
3. Without milk consumption, energy spent producing
lactase would be energy wasted at the cellular level.
Therefore, over time, the more energetically favorable option
has been selected for: a decrease in lactase production after
weaning.
Lactase not Produced in Adults
If undigested lactose passes into the large intestine, it will
trigger the symptoms of Lactose Intolerance.
1. The increased sugar concentration in the large
intestine creates an osmotic gradient that draws
water into the gut. This causes cramping and
diarrhea.
2. Bacteria in the large intestine digest the lactose as
food, creating gaseous by-products like methane,
carbon dioxide, and hydrogen. This leads to gas
build-up and flatulence.
Lactose Interolerance
For around 10% of Americans,
10% of Africa's Tutsi tribe,
50% of Spanish and French people,
and 99% of Chinese,
a tall cold glass of milk means
an upset stomach and other unpleasant
digestive side effects.
Lactase Persistence
Some humans do produce lactase after
weaning, and are therefore able to continue to
consume milk and other dairy products into
adulthood.
Discussion
1. How do you think this is possible?
2. What changed in order for these adult
humans to be able to digest milk?
Discussion
1. How do you think this is possible?
This is possible through Lactase Persistence, or the continued production
of lactase at high levels throughout adulthood.
2. What changed in order for these adult
humans to be able to digest milk?
The down-regulation of lactase biosynthesis that normally occurs must
have been prevented or counteracted.
The Neolithic Revolution
The Neolithic revolution describes a period of time,
between 12,000 and 6,000 years ago,
during which humans around the world began
transitioning from a hunter-gatherer lifestyle
to a farming-herding lifestyle.
Pastoralism
Pastoralism, the cultural practice of milking livestock
(such as goats, sheep, cows, and camels), was another
innovation of the Neolithic Revolution. It was adopted in
various cultures between 12,000 and 7,000 years ago.
Milking Livestock and Drinking that Milk
The Biocultural Coevolution
theory proposes that
pastoralism and lactase
persistence coevolved.
This means that they arose
around the same time*, and
both changes were reinforced
by each other.
An Apparent Paradox?
Anthropological evidence places the advent of pastoralism at
10,500 to 6,500 years ago.
However, genetic research says the lactase persistence trait
did not become widespread in Europe until 7,000 to 5,000
years ago.
This suggests that for several thousand years some humans
were milking sheep, goats, cows, or camels despite being
unable to digest milk.
What was going on?
Cheese
It is likely that Neolithic humans fermented milk into
cheese, which greatly reduced the lactose content of
the dairy product, making it more accessible.
Ancient pottery remnants
like this, found in
Northern Europe, were
likely sieves used to strain
and ferment milk into
cheese
Lactose Tolerant Mutation
• The lactose tolerance mutation arose
randomly (as all mutations do), but once it
arose, it had a distinct advantage in these
populations.
• Natural selection would have favored
individuals carrying the lactose tolerance
mutation, spreading it through ancient
European populations that depended on
dairying.
How Did Lactase Persistence Spread?
Lactase persistence is thought to have arisen and
spread through two types of natural selection:
positive selection
(selection for advantageous traits)
and
negative selection
(selection against disadvantageous traits).
How Did Lactase Persistence Spread?
Negative Selection
Positive Selection
(disadvantages of Lactose Intolerance)
(advantages of Lactase Persistence)
• Individually: Non-LP individuals
missed out on a potentially
important source of nutrition and
hydration
• Experiencing painful, dehydrating
symptoms upon consuming milk
(see slide 15), which could be
deadly.
• Culturally: Without pastoralism,
herders must slaughter their
livestock to gain dietary protein
from their meat.
• Individually: Milk supplies protein,
fat, sugar, and vitamins, and is
dependable despite cold weather
and/or bad crops
• Neolithic women who could digest
milk were estimated to produce
32% more offspring.
• Culturally: Milk is a more efficient
protein source: it does not require
killing livestock, yet the milk from
one cow nearly equals the caloric
value of the meat from a whole
cow.
The Natural History of
Color Vision and Colorblindness
What is colorblindness?
Reduced ability to interpret light as color.
1 in 12 males are colorblind.
< 1 in 100 females are colorblind.
Color Vision in Monkeys
Species: Grey Cheeked Mangabey
Lives: Africa
Vision: Trichromatic vision (i.e. like
most humans)
Species: White Headed Capuchin
Lives: Central and South America
Vision: Dichromatic vision (i.e.
“colorblind”.
Color Vision in Monkeys
Species: Japanese Macaque
Lives: Asia
Vision: Trichromatic vision
Species: Black Squirrel Monkey
Lives: Central and South America
Vision: Dichromatic vision
Color Vision in Monkeys
Species: Guinea Baboon
Lives: Africa
Vision: Trichromatic vision
Species: White Faced Saki
Lives: Central and South America
Vision: Dichromatic vision
Color Vision in Monkeys
Species: Roloway Monkey
Lives: Africa
Vision: Trichromatic vision
Species: Pied Tamarin
Lives: Central and South America
Vision: Dichromatic vision
What have you noticed?
Is there a relationship between color
vision and continent of origin among
monkey species?
Monkeys of the World
Question
- Why do most New World Monkeys have
dichromatic vision while most Old World
Monkeys have trichromatic vision?
The Ecology of
Color Vision in Monkeys
Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
One of the above leaves has black fungus on it.
Can you tell which one?
Ecology: Why Trichromatic Vision?
TRICHROMATIC VISION
Leaf with black fungus
Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
In the above picture, the red leaves are not palatable
but the green leaves are nutritious.
Which leaves are which?
Ecology: Why Trichromatic Vision?
DICHROMATIC VISION
TRICHROMATIC VISION
With trichromatic vision, distinguishing between
important colors becomes possible.
Food Selection –
The Driver of Trichromacy Evolution?
Food Selection
• Research has linked
color vision with the
ability to select ripe
food when foraging.
Food Selection – Summary
Research
suggests that
trichromatic
vision is more
likely to be
selected for
when food is
distinguished
from non-food
by color.
Research
suggests that
dichromatic
vision is more
likely to be
selected for
when food is
distinguished
from non-food
by shape.
The Cell Biology of
Color Vision in Monkeys
How Does Color Vision Work?
Cell Biology
Three types of Cone Cell
• Different kinds of opsin proteins embedded in
the membrane of cone cells.
• Central Dogma of Molecular Biology:
DNA  RNA  Protein
Genes code for…. proteins.
Chromatic Vision: Cone Cells
Cone cells in the retina of the eye allow light of
different wavelengths to be interpreted as color in
the brain.
The Brain
Light Waves
Color
The Cone cell
The Role of Opsins
There are three types of opsins:
Short Wave Sensitive (SWS)
Medium Wave Sensitive (MWS)
Long Wave Sensitive (LWS)
An individual possessing only
SWS and MWS opsins will have
dichromatic vision.
An individual possessing SWS,
MWS and LWS opsins will have
trichromatic vision.
The Genetics of
Color Vision in Monkeys
The Genetics of Color Vision
• The section of DNA on a chromosome that
codes for an opsin protein is called an opsin
gene.
Location of Opsin Genes
The gene coding for
the SWS opsin
protein is located on
chromosome #7.
The gene coding for
the MWS and LWS
opsins are located on
the X-chromosome.
Evolution of LWS Opsin Gene
The LWS gene arose
through gene
duplication and gene
mutation of the MWS
gene on the Xchromosome.
Gene Duplication
Unequal Crossing Over
(Meiosis, Prophase 1)
Phylogenetics – Exploring Relationships Among Species
Wooly Monkey
Spider Monkey
Sakis
Marmoset
Owl Monkey
Squirrel Monkey
Capuchin
Colobus
Langur
Mona
Mangabey
Baboon
Rhesus
Gibbon
Orangutan
Gorilla
Chimpanzee
Human
Continents Split
50 Million Years Ago
Color Vision Evolves!
Gene Duplication and Mutation
Primates In New/Old World
55 Million Years Ago
Rise of Primates
75 Million Years Ago
What next?
• Research indicates that some human
females have tetra-chromatic vision.
Describe how this can be possible from a
cell biology and genetic perspective.