2. Developmental Plasticity
• The Environment plays as much a role in development
as our genes
• Phenotypic plasticity is when an organism can react to
the environment by changing its state, form, or activity
• Developmental plasticity is when this occurs in the
larval stage
• a good example of this is metamorphosis- the
same genes are present in the adult and larvae,
but you find different manifestations in
phenotype
3. Types of Phenotypic Plasticity
• The are two main types
• Reaction Norm- the genome has a continuum of phenotypes,
and the environment will determine which will form
• Think of muscle growth when one works out.
• Polyphenism- This is an on/off or an either/or situation. It is
not on a continuum. There are various types such as:
• Diet-induced
• Methylation
• Predator induced
4. Diet-Induced Polyphenism
• The caterpillar Nemoria arizonaria will look like a twig or an oak flower depending on
what it eats.
• The flowers are seasonal, so it doesn’t want to look like a flower when there are no
other flowers out
• This depends on eating young leaves
• If older leaves are eaten, the caterpillar will look, instead, like a twig
• Honey bees feed a royal jelly containing royalactin to bees destined to become queen.
• Royalactin stimulated egg production in the queen
5.
6. continued
• Dung beetle larvae eat food left by the mother
• The quality and the quantity of the food left in a particular hole determines the amount
of juvenile hormone made in the last molt
• This determine the presence of horns or not
• Recall that methyl groups can be added to segments of DNA. In general, it will repress
transcription of the gene
• This will cause changes in the phenotype
• Some supplements (like folic acid) are methyl donors
7. Predator-Induced
Polyphenisms
• When water fleas detect chemicals from predatory fly larvae, they alter
their development
• They will make spiked heads and tails which make them difficult to
swallow
8. Predator-Induced Polyphenisms
• When embryos of the Costa
Rican Red-Eyed tree frog sense
vibrations from a snake crawling
on a leaf, the eggs prematurely
open and the larvae fall to the
water below
9. Temperature as an Environmental Agent
• In some species, the decision on the development of testes or ovaries
depend on temperature
• For example, for crocodiles, extreme temperatures will produce females
whereas moderate temperatures produce males
• In the Malawian butterfly Bicyclus anynana, there is a dry season morph in
which the butterfly camouflages in dead leaves on the forest floor
• The wet season morph has eyespots on their wings and tend to fly more
• This decision is made in the pupal stage
10.
11. Larval Settlement
One type of cue is a biofilm. These are often sheeting of
bacteria
This may be why barnacles form on the hull of
boats for example
Some marine larvae can suspend development until they
experience a particular environmental cue. This is called
larval settlement
12. Stress as an Agent
• The Spadefoot Toad will change their development
according to an environmental stress
• This depends on the water levels in the ponds
• Tadpoles will go through metamorphosis at different
times based on the available water in the pond
• When the water levels in the pond drop, the tadpoles
sense it and develop a wider mouth and stronger jaws
• They become carnivorous (cannibalistic) and morph
very quickly
13.
14. Developmental Symbiosis
A symbiotic relationship is
where an organism comes to
depend on another
• Usually, the host the larger
of the two
• The symbiont is the smaller
of the two
There are two types:
• In parasitism, the symbiont
benefits at a cost to the host
• In mutualism, both the
symbiont and host benefit
from the relationship
15. Endosymbiosis
• In Endosymbiosis, the symbiont will live in (or partly in)
the host. There are two typical examples of this in plants:
• Nitrogen-Fixing bacteria live in the roots of many
plants (you can find particular high amounts in
legumes
• These bacteria ‘fix’ nitrogen in the air by
reducing its triple covalent bond to a single
bond compound so the plant can use it.
• The bacteria, in turn take sugar from the plant
16. Endosymbiosis
• Mycorrhizae fungi are associated with most plant roots
(although there are different species of fungi
associated with different plants
• These fungi increase the area in which the plant
can obtain water at the roots
• In turn, the fungi consume sugar from the plant.
17. Mechanisms of
Developmental Symbiosis
Vertical Transmission is the
transfer of symbionts from
one generation to the next
• Some symbiosis is obligate
mutualism- meaning that
they can not survive
without each other.
• An example of this can
be found in lichens (a
symbiotic relationship
between fungi and algae)
Horizontal Transmission is
when one becomes infected
by other members of the
species
• Humans acquire some
symbionts as they travel
through the birth canal.
Others can be transmitted
through contact with the
skin during breast feeding
18. Developmental
Symbiosis in
the Mammal
Intestine
In mice, bacteria help with cell differentiation. The gut
bacteria up-regulate transcription of many genes
Studies in mice show that without intestinal bacteria,
villi (the absorptive part of the intestine) will not be
vascularized and there will be no brush border enzymes
Gut bacteria is also important for mammalian gastric-
associated lymphatic tissue
Bacteria may be involved in things like brain formation
and reproduction.
19. Developmental
Symbiosis in the
Mammal
Intestine
Mammals have co-evolved with bacteria to the
point that our body phenotypes do not develop
without them
There are more bacteria in humans than we
have cells in our body
There are around 400 species of gut bacteria in
us
We are only beginning to understand how they
interact with us
