Transcript Lecture 29 (4-15-11)

Evo-Devo: Development in an Evolutionary Context
Dll expression
En/Inv expression
Control of eyespot development on a developing butterfly wing
Gene regulation and
evolution
Activation of transcription factors
Signal transduction: an extracellular signal is changed to an intracellular functional
change
Upper beak bud
One gene that affects beak size
and shape
Probe used to identify mRNA from
bone morphogenic protein-4 gene
Regulated expression
Things to consider
• Many genes (and proteins) are homologous across
evolutionarily diverse groups.
• Therefore, many evolutionary changes are based on
• controlling the expression of homologous genes
• Temporal control
– Expression at different times
– Expression ffor different lengths of time
• Spatial control
– Expression in different places (tissues).
• Examples: plants in Solanaceae; Darwin finch beak
development
Homeotic Genes and Animal Body Plans
• Bilaterally symmetrical animals develop in four
dimensions.
• 3 spatial + temporal
• Each cell has to have
• 1. location information: where it is relative to other
cells
• 2. time: what is presently taking place in the
developmental sequence.
• Homeotic genes (Hox genes): (1) transcription factors
determine which (2) structural genes are activated to
produce (3) particular structures.
Location information
from what gene
products are present
Cells or tissues located along the major body axes use positional
information during development
(After Strickberger.)
Hox genes in Drosophila (body segmentation)
Occur in clusters (gene duplication)
Provides positional information
Colinearity:
1. Time of expression
2. A-P axis
3. Quantity of
transcription factors
Where genes
are expressed
Morphogens =
signal polypeptides
Hox Gene Cluster
• Each Hox gene contains a highly conserved 180 bp sequence –
the homeobox.
Codes for a DNA binding segment (aa sequence) in the
transcription factor.
Activation of structural genes produce structures
appropriate for that location.
Mutations in Hox genes result in inappropriate structures
for that location.
Mutations in Hox genes
bx, pbx, and abx
1 pair of wings normally develop on body
segment T2
Hox mutations change identity of
T3 cells to T2 cells.
Appendages appropriate for a T2
Location are produced.
Ancestors of dipteran flies
had 4 wings.
Mutation of Hox gene antp
Location information changes
Rather than head segment location
Cells respond as if they are in a thoracic
segment
Diversification of animals (e.g. Cambrian “explosion”
and beyond  Hox gene diversification
• Hox homologues: in everything from sponges to humans to
fungi and plants (MADS-box genes).
• Therefore, Homeobox genes predate the origin of animals.
• Example of diversification: Arthropoda
 Over 1 million sp. described; millions more waiting for
recognition and description
 Exoskeleton; segmented body (H–T–A) and segmented legs
 Paired appendages on body segments; open circulatory
system
Crustaceans
Myriapods
Hexapods
Chilicerates
An onychophoran (velvet worm)
Closest living relative of arthropods
1 pr. unjointed legs on each of the
similar body segments