Transcript 25.5 - Laurel County Schools

Concept 25.5: Major changes in body form can result
from changes in the sequences and regulation of
developmental genes
How can we understand life’s diversity?
1. Fossils – evidence of past biodiversity
2. Continental drift, mass extinction, adaptive
radiation – environmental changes influence
biodiversity
3. Genetic Change – changes in DNA
sequences and regulation modify
bodies/cells
Evolutionary Effects of Development Genes
• Developmental genes guide the formation of
the body from embryo to adult
• Even a small change in rate, timing, and spatial
pattern can produce major morphological
differences between species
• Let’s look at some examples!
Changes in Rate and Timing
• Heterochrony evolutionary
change in the rate
or timing of
developmental
events
• Ex. human and
chimpanzee skull
differences due to
small changes in
relative growth
rates
Fig. 25-19b
Chimpanzee fetus
Chimpanzee adult
Human fetus
Human adult
(b) Comparison of chimpanzee and human skull growth
Changes in Rate and Timing
• Different parts of
our bodies grow
at different rates
Changes in Rate and Timing
• Paedomorphosis –
retention of juvenile traits
features in the adult
• This adult salamander
retains the gills of the
larval form – adults
usually have lungs
• Development of reproductive organs accelerates
compared to other organs
Changes in Spatial Pattern – placement and
organization
• Homeotic genes – control placement and
spatial organization of body parts
• Ex. Where do legs develop, where does the
head form, how are the parts of a flower
arranged
• They are master switch genes which
activate/regulate other genes needed for
formation of body structures
• Hox genes provide positional information in
animal embryos
“Scarce as hens teeth”
Changes in Spatial Pattern – placement and
organization
• The transition from invertebrate to vertebrate
may have been influences by alterations of Hox
genes
• In particular duplication of hox genes may have
played an important role
Fig. 25-21
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Vertebrates (with jaws)
with four Hox clusters
The Evolution of Development
• Changes in developmental genes can result in
new morphological forms
• This may answer the puzzle of the Cambrian
Explosion
• WE JUST DISCUSSED GENE DUPLICATION
– BE SURE TO LOOK AT Figure 25.22
• NEXT CHANGES IN GENE REGULATION
• First we need to see how you think about how
genes are expressed in cells!
OR
ANOTHER ANALOGY
• EVERYDAY – NO MATTER WHAT THE
WEATHER – I PUT ON SOCKS AND
UNDERWEAR BUT………………..
• But weather conditions influence how I select
the rest of my clothing
Changes in Gene Regulation
• Changes in the form of organisms are often by
changes in the regulation of developmental
genes instead of changes in their sequence
• For example three-spine sticklebacks in lakes
have fewer spines than their marine relatives
• The gene sequence remains the same, but the
regulation of gene expression is different in the
two groups of fish
Stickleback Fish and a Gene Called Pitx 1
• Why do marine stickleback have spines on their
lower surface while freshwater stickleback have
none (or few)?
• Hypothesis A: Developmental gene Pitx 1 had
changed (nucleotide sequence changed)
– Test – compare DNA for Pitx 1 in both kinds of
fish
• Hypothesis B: Regulation of the gene Pitx 1 had
changed
– Test – monitor expression of Pitx 1 in
developing embryo
Fig. 25-23
RESULTS
Test of Hypothesis A:
Differences in the coding
sequence of the Pitx1 gene?
Result:
No
Test of Hypothesis B:
Differences in the regulation
of expression of Pitx1 ?
Result:
Yes
Marine stickleback embryo
Close-up
of mouth
Close-up of ventral surface
The 283 amino acids of the Pitx1 protein
are identical.
Pitx1 is expressed in the ventral spine
and mouth regions of developing marine
sticklebacks but only in the mouth region
of developing lake stickbacks.
Lake stickleback embryo
Fig. 25-23a
Marine stickleback embryo
Close-up
of mouth
Close-up of ventral surface
Lake stickleback embryo
Regulatory Genes
Pitx 1 gene
Pitx 1 protein
CONCLUSION
• LOSS OF SPINES DUE TO CHANGE IN
REGULATION OF GENE NOT THE
NUCLEOTIDE SEQUENCE OF THE GENE
ITSELF