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.

Eyespots on a butterfly wing

En/Inv expressionDll expression

Homeotic Genes and Animal Body Plans

• 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

Gene location in hox cluster

Colinearity:1. Expressed first2. Anterior to posterior3. Greater quantity oftranscription factors

• 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.

Colinear Hox gene expression

Paralogousand evolutionarilyconserved

• 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 genesbx, pbx, and abx

Wings normally appear on T2Hox mutations change identity ofT3 cells to T2 cells.An extra pair of wings is produced.Ancestors of dipteran flieshad 4 wings.

Mutation of Hox gene antp

Identity of a head segmentchanged to that of a thoracicsegment

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 legsPaired appendages on body segments; open circulatory system

An onychophoran (velvet worm)Closest living relative of arthropods1 pr. unjointed legs on each of thesimilar body segments

Hexapods

Chilicerates

Crustaceans

Myriapods

Hox cluster of 9 locifor all arthropods

Evolutionary diversification of arthropods partly based on sites of Hox gene expression

abdA always expressed onventral side of segment

Ubx and abdA not expressedin posterior segmentsEvolutionary change in where

a Hox gene is expressed Mutation:legless abdominal segments

Homeotic genes and Flower formationC. 300,000 sps. of Angiosperms

Four concentric whorls of modified leavesNormal order: sepals, petals, stamens, carpels

MADS-box mutants (Hox genes)