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Evo-Devo: Evolutionary Development
• DNA
• Regulatory genes: code for signal proteins and
transcription factor proteins
– SP: target particular groups of cells for gene expression
• Regulatory sequences: binding sites of transcription
factors (enhancers, promoters).
• Structural genes: Activated by binding of TF & RS
• Responsible for producing phenotypic characters.
Dll expression
En/Inv expression
Eyespots on a butterfly wing
Homeotic Genes and Animal Body Plans
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Multicellular animals develop in four dimensions.
3 spatial + time
Each cell has to have
1. information: where it is relative to other cells
2. where it is in the developmental sequence.
Information provided by Homeotic genes (Hox genes)
Cells along these major body axes assume positional information during
development
(After Strickberger.)
Hox genes in Drosophila (body segmentation)
Positional information
Colinearity:
1. Expressed first
2. Anterior to posterior
3. Greater quantity of
transcription factors
Gene location in hox cluster
Colinear Hox gene expression
• Hox genes provide information on location.
– 1. The transcription factor from the first Hox gene
• required to express downstream Hox genes.
– 2. The effectiveness in initiating sequential gene
expression declines with distance.
Paralogous
and evolutionarily
conserved
• Each Hox gene contains a highly conserved 180 bp sequence – the
homeobox.
Codes for a DNA binding segment (aa sequence) in the transcription
factor.
The transcription factors activate structural genes.
Structural genes produce structures appropriate for that location.
Mutations in Hox genes result in inappropriate structures for that
location.
Hox gene products activate genes responsible for making a particular
structure.
Mutations in Hox genes
Mutation of Hox gene antp
bx, pbx, and abx
Wings normally appear on T2
Hox mutations change identity of
T3 cells to T2 cells.
An extra pair of wings is produced.
Ancestors of dipteran flies
had 4 wings.
Identity of a head segment
changed to that of a thoracic
segment
Hox gene diversification  diversification of animals
• Hox paraloges (homologs): in everything from
SpongeBob Squarepants to humans to fungi and
plants (MADS-box genes).
• Therefore, Homeobox genes predate the origin of
animals.
Representative arthropods: What is the basis of their diversity?
1 million sp. described; maybe 50 million still to be named.
Exoskeleton; segmented body (H –T – A) and segmented legs
Paired appendages on body segments; open circulatory system
Crustaceans
Hexapods
Myriapods
Chilicerates
An onychophoran (velvet worm)
Closest living relative of arthropods
1 pr. unjointed legs on each of the
similar body segments
Evolutionary diversification of arthropods partly based on sites of Hox gene expression
Hox cluster of 9 loci
for all arthropods
abdA always expressed on
ventral side of segment
Evolutionary change in where
a Hox gene is expressed
Ubx and abdA not expressed
in posterior segments
Mutation:
legless abdominal segments
Homeotic genes and Flower formation
C. 300,000 sps. of Angiosperms
Four concentric whorls of modified leaves
Normal order: sepals, petals, stamens, carpels
MADS-box mutants (Hox genes)