Genes and Body plans http www pbs orgwgbhevolutionlibrary034reall03404

Genes and Body plans http: //www. pbs. org/wgbh/evolution/library/03/4/real/l_034_04. html

Normal fly Extra set of wings! Ubx (Ultrabithorax) gene mutant Antennae are transformed into legs! Antp (Antennapedia) gene mutant Carroll S. B. et al. From DNA to Diversity (2001) Blackwell Science

What caused those body parts to be in the WRONG places? What causes body parts to grow in the CORRECT places? Remember that EVERY CELL of your body contains ALL your GENES

http: //www. pbs. org/wgbh/evolution/library/03 /4/l_034_04. html • Evolution: Library: Genetic Tool Kit

Homeotic and Homeobox Genes • Control how an organism’s body develops as it grows from a zygote into a complete organism. • They determine the body plan including the polarity (front and back part) and positioning of organs. • Homeotic genes define a region or position in the embryo and code for transcription factors that regulate the activity of other genes. • Genes in this group contain a conserved stretch of DNA called the homeobox and are nearly identical in all species.

• The homeobox is only a portion of each gene • e. g. if the words below were homeotic genes, the capital letters would represent the homeobox: • toge. THEr / THEoretical / ga. THEring / bo. THEr • The homeobox is a 180 -basepair sequence of DNA that has been found in many regulatory genes.

• A particular set of genes, the Hox genes, are responsible for assigning the head to tail body pattern in very early embryos. • Hox genes are a subset of homeotic genes that contain a homeobox • Homeobox genes code for the production of polypeptides about 60 aa long (the homeodomain) and act as transcription factors. • These factors bind to a particular region of DNA and cause it to be transcribed. • So, a single homeobox gene can switch on a whole collection of other genes.

On your worksheet • Choose 3 colours and assign a colour each to a, g and i • Locate the regions labelled a, g and i in the diagrams of the Drosophila Homeobox gene, the embryo and the adult • Use your colour key to colour in these sections in the Drosophila Homeobox gene, the embryo and the adult What do you notice?

The pattern of Homeobox expression Colinearity between the order of the homeobox genes and the expression pattern Adult Eight homeobox genes regulate the identity of regions within the adult and embryo. The order of the genes determines the body plan Embryo Carroll S. B. et al. From DNA to Diversity (2001) Blackwell Science

• All animals have homeobox genes that are similar (homologous) and highly conserved.

Evidence that there was colinearity in homeobox genes between species Evolution: Library: Genetic Tool Kit http: //www. pbs. org/wgb h/evolution/library/03/4/l _034_04. html

Hox Genes

Hox genes are a specific family of homeobox genes that regulate other genes They are a group of related genes that control the body plan of an embryo along the anteriorposterior (head-tail) axis. They are highly conserved Every Hox gene is a homeobox gene, but not every homeobox gene is a Hox gene

• This conservation of genes implies that their activity is fundamental to the development of an animal. • Homeosis is a mutation of a homeobox gene that causes transformation of one area of the body into another area. • A mutation in a homeobox gene is so disastrous that the organism usually doesn’t survive.

Drosophila (fruit fly) • Body plan of typical insect – head thorax and abdomen. • Thorax is made up of 3 segments (T 1, T 2, T 3). • A pair of legs grows from each segment. • A pair of wings grows from T 2. • A pair of halteres (used for balance when flying) grows from T 3.

• A homeobox gene called Antp is usually turned on in the legs where it causes legs to develop. • It is turned off in the head. • In some mutant flies the Antp gene is switched on in the head producing legs instead of antennae. Mutants Wild-type

Mutant fly with 2 wings Normal adult fly • Drosophila has a homeobox gene called Ubx – this stops wings forming in T 3. Bithorax mutant • If the fly has a mutation in both copies of Ubx, wings grow in T 3 instead of halteres.

Hox clusters • Hox genes are arranged in clusters. • their order on the chromosome is the same as the order in which they appear along the body.

• What this means is that if you look at the chromosome that contains the Hox genes they are all lined up in order on the strand of DNA. • This shown in the drosophila fruit fly below:

• Hox genes are universal in animals. • Vertebrates have Hox genes, and they have the same properties: each gene contains a homeobox, and they are organized on the chromosome in the order of their expression from front to back

Larger Hox gene number More complicated body pattern Nematodes (round worms) = 1 Hox cluster Drosophila = 2 Vertebrates = 4

Drosophila and human Hox genes Mammals • In mammals Hox genes are found In 4 clusters Hox A: Chr. 16 Hox B: Chr. 11 Hox C: Chr 15 Hox D: Chr. 2

Evolution of Hox gene clusters Hypothetical common ancestor Amphioxus

Haeckel's 1874 version of vertebrate embryonic development. The top row shows an early stage common to all groups, the second row shows a middle stage of development, and the bottom row shows a late stage embryo.

The Role of retinoic acid • Retinoic acid is derived from vitamin A • It is an activator of HOX genes in vertebrates • It activates the genes in the correct sequence running from head to tail • It is called a morphogen because it regulates the pattern of tissue development • If it is in xs it can however start to switch on the genes in wrong order resulting in major birth defects • Pregnant women should be very careful about their vitamin A intake

Homework • Make notes and diagrams on the development of drosophila and its genetic control p 114