Gene Expression and Development II

Final Exam

• Sunday, May 27, 8:30-11:30 a.m.

• Here – SMC A110

• Please do course evaluations!

Important Readings for Gene Expression and Development

• Campbell chapter 18.4

• Campbell chapter 21.6

• Matt Ridley, Genome, chapter 12 ‘Self-Assembly’

Changes in gene regulation and development as Tootsie Rolls evolved to be Tootsie Pops

Tootsie Larvae

Tootsie Adult

New Tootsie Species

Hierarchy of Gene Expression in Fly Development

Hierarchy of Gene Expression in Fly Development

• Maternal co-ordinate genes- Anterior or posterior end of egg is missing. bicoid mutants lack anterior head structures, nanos mutants have no abdomen.

• Gap genes- There are large "gaps" in the embryo- many adjacent segments are missing. Krüppel mutants are missing T1-A5, hunchback mutants are missing head segments and knirps mutants lack posterior segments.

• Pair-rule genes- Have only half the number of parasegments- they are actually missing every other parasegment. fushi-tarazu mutants lack odd-numbered parasegments and even-skipped mutants lack even-numbered parasegments.

• Segment polarity genes- Within each segment either the anterior or posterior half is duplicated. gooseberry and engrailed mutants have the posterior part of each segment replaced by the anterior part of the adjacent segment.

Genetic Analysis of Early Development

• Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus won a Nobel Prize in 1995 for decoding pattern formation in Drosophila

• In the 1940s Lewis discovered the homeotic genes, which control pattern formation in late embryo, larva, and adult stages

Homeotic Genes

• Lewis was able to demonstrate that bizarre developmental mutations could be mapped on to the Drosophila chromosome map, providing the first concrete evidence that genes somehow direct the developmental process

Figure 18.20

Wild type Mutant

Eye

AntennaLeg

Widespread Conservation of Developmental Genes Among Animals

• Molecular analysis of the homeotic genes in Drosophila has shown that they all include a 180 BP sequence called a homeobox

• An identical or very similar nucleotide sequence has been discovered in the homeotic genes of both vertebrates and invertebrates

• Homeobox genes code for a domain that allows a protein to bind to DNA and to function as a transcription regulator

• Homeotic genes in animals are called Hox genes

Widespread Conservation of Developmental Genes Among Animals

• One of the most fascinating aspects of homeotic genes, is that they are arranged in sequence from anterior to posterior on the chromosome, in the same order as the parts of the body they influence

Figure 21.18

Adultfruit fly

Fruit fly embryo(10 hours)

Fly chromosome

Mousechromosomes

Mouse embryo(12 days)

Adult mouse

Figure 21.18a

Adultfruit fly

Fruit fly embryo(10 hours)

Fly chromosome

Figure 21.18b

Mousechromosomes

Mouse embryo(12 days)

Adult mouse

Conservation of homeotic genes

• Conservation of homeotic genes allows for the existence of a phenomenon known as genetic rescue

• In genetic rescue, a homeotic gene is knocked out in a Drosophila lineage

• Then the equivalent human gene is inserted into those flies, and offspring will develop into a normal fly

• This is a key finding because it shows the regulatory effect of homeotic genes is universal across distantly related animals

Conservation of homeobox genes

• Related homeobox sequences have been found in regulatory genes of yeasts, plants, and even prokaryotes

• In addition to homeotic genes, many other developmental genes are highly conserved from species to species

Conservation of homeobox genes

• Sometimes small changes in regulatory sequences of certain genes lead to major changes in body form

• For example, variation in Hox gene expression controls variation in leg-bearing segments of crustaceans and insects

• In other cases, genes with conserved sequences play different roles in different species

Figure 21.19

Thorax AbdomenGenitalsegments

Thorax Abdomen

Fig. 18-17a

ThoraxHead Abdomen

0.5 mm

Dorsal

Ventral

RightPosterior

LeftAnteriorBODY

AXES

(a) Adult

Genetic Regulation of Dorsal and Ventral

• In Drosophila, two genes are key in determining dorsal and ventral – decapentaplegic leads to cells becoming part of the back or dorsal part of body; short gastrulation leads to cells becoming part of the ventral body

• Vertebrates have two very similar genes – BMP4 reads like decapentaplegic but it codes for cells on the ventral side; chordin reads like short gastrulation but it codes for cells on dorsal side

• Essentially arthropods and vertebrates are upside down versions of each other

Dorsal nervous system - vertebrates

Ventral Nervous System – Arthropods, Worms (shown in blue)

Comparison of Animal and Plant Development

• In both plants and animals, development relies on a cascade of transcriptional regulators turning genes on or off in a finely tuned series

• Molecular evidence supports the separate evolution of developmental programs in plants and animals

• Mads-box genes in plants are the regulatory equivalent of Hox genes in animals

• Animals have mads-box genes and plants have hox genes, but they function very differently in the two groups

Basic Flower Structure

Mads-box genes in flowering plants