epigenetic control – control of development that occurs through the products of genes other than the gene(s) which actually are responsible for formation of the structure itself.

heterochrony – change due to change in timing heterotopy – change in the position at which a

character is expressed.(spatial) (exp. on thorax rather than abdomen)

heterometry – a change in quantity or degree of gene expression. (exp. two pairs of wings instead of one)

heterotypy- phenotypic change from one type to another (exp. walking legs to swimming legs)

How many gene changes are needed to account for the diversity of forms seen in the animal and plant kingdoms?

New discoveries are providing clues to the answer

1. Research shows that there are often very few and small genetic differencessmall genetic differences between species that exhibit veryvery different adult different adult forms.forms. (example humans, apes and chimps)

2.2. similar genetic and cellular mechanismssimilar genetic and cellular mechanisms underlie the development of embryos in species whose adult forms are very adult forms are very differentdifferent

3. It is now possible to identify small genetic changes which are responsible for large phenotypic variations

4. A history of these changes is also being pursued through phylogenetic analysis

A class of genes that code for proteins that bind to DNA and regulate the expression of a wide range of other genes.

The actual binding capability resides in a particular location of the regulatory protein called the homeodomainhomeodomain.

The homeoboxhomeobox is a nucleotide sequence that codes for this homeodomain

The homeobox is very similar in many eukaryotic organisms and is about 180 base pairs.

Homeotic gene products provide positional information in a multicellular embryo.

They are involved in evolutionaryevolutionary change when....

New features of multicellular organisms arise due to manipulation of pre-existing cell types. For example when… the same cells arrive at new locations

(heterotopy) OR the same cells are expressed at different

times of development (heterochrony)

Multicellular organisms need a system for arranging cells in 3 dimensional space to assure … proper organization of symmetry,

segmentation, and body cavities that cleavage and gastrulation occur

correctly correct differentiation of tissues (such

as nervous, muscle, gut etc.)

Cells need to be identified based on their location in relation to … other cells time of expression

Genes that carry information for this control are called homeotic loci

In animals these loci are called HOM loci in invertebrates Hox loci in the vertebrates

Study of these genes is a new and hot area of research

Genes collectively referred to as Hox genes

Found in all major animal phyla organized in gene complexes i.e. they are

found in close proximity to each other on the chromosome

Appear to be the result of duplication events

each taxa surveyed shows unique patterns of duplication or loss in these loci

Hox genes have temporal and spatial colinearity which is unique to HOM and Hox genes Have perfect correlation between the order of

genes along the chromosome and the anterior-posterior location of their gene products in the embryo. ▪ Genes at the 3’ end are expressed in the head

region and genes at the 5’ end in the posterior part of the embryo

Also 3’ genes are expressed earlier in development than those located toward the 5’ end

Finally the 3’ end produces greater quantity of product.

Called spatial, temporal and quantitative colinearity. This is unique to Hox sequences.

Hox Genes regulate the location of appendages on body segments.

Work in embryo to specify the locationlocation where appendages should be located

Also determine whenwhen in embryonic development these structures will be formed

Hox genes do notdo not control the actual formation of appendages

Other genes control the formation of the actual structure whether a wing, leg, antenna, etc HoweverHowever, these other genes are thought to be

regulated by the regulatory protein products of Hox loci

These genes are found in not only segmented animals but also in… plants fungi roundworms other non-segmented animals sponges!

Origin may have accompanied the development of multicellularity

Predates the development of a differentiated body axis

Therefore these genes also influence processes other than the specification of anterior to posterior cell fates

Use of phylogenetic mapping has helped determine which genes in the Hox complex have been gained or lost at key branching points in the tree

Each major clade in the bilateral animals is characterized by a particular suite of Hox genes

Examples addition of the locus called

Abdominal-B is associated with the evolution of bilateral animals

duplication of the entire Hox complex several times leads to the mouse and other vertebrates

In the lower animals (sponges and cnidarians) there is a correlation between the number of HOM loci and the complexity of the animal’s body plan The most primitive sponges and

Cnidarians have just 5 loci Sea urchins have 10 mice have 39

This would support the idea that increase and elaboration of the number of Hox  genes helped make the Cambrian “explosion” possible

However, in the Bilateria, this trend does not hold up and there is no real correlation between morphological complexity and numbernumber of Hox loci

Most of the post-Cambrian diversification is due to changes in the timing or spatial locationtiming or spatial location of Hox gene expression rather than in the number of the loci present

Initial diversification of the Arthropods occurred in the Cambrian, with even more diversification occurring much later.

Diversification of Arthropods relies heavily on differentiation of body segments from posterior to anterior

This differentiation is due to Hox gene expression

Still, all of the arthropod taxa have the same complement of 9 Hox genes

Morphological diversification is due to due to changes in localized expression of genes changes in localized expression of genes rather than the addition of new rather than the addition of new HOMHOM loci loci

The genes which control morphological diversification are activated or suppressed by the action of a large number of HOM gene products

In turn the Hox genes influence the expression of a large number of other genes and developmental processes

Much of the evolutionary diversification of the arthropods probably occurred as a result of changes in wherewhere Hox genes are expressed

Fig 19.4 the arthropod groups Fig 19.5

Tetrapod limbs are a variation on a theme

Found on birds, mammals, reptiles, amphibians

First developed to allow mobility on land and then have undergone enormous variation

sister group of tetrapods is the Panderichthyidae lobe-finned large predators in shallow

fresh water habitats There are structural homologies

between the groups Fig 17.7 Member of the

Panderichthyidae is the recently discovered Tiktaalik roseae

All tetrapods show the same basic features of limb development A bud forms from mesodermal cells Fig 19.8a At the tip of the bud is the AER (apical

ectodermal ridge)▪ AER secretes molecules that keep cells in an

undifferentiated state, this area is called the progress zone

▪ The progress zone grows outward and defines the long axis of limb development

▪ Fig 19.8 b and c

At the base of the bud is a group of cells, ZPA (zone of polarizing activity). molecules secreted from the ZPA diffuse

into the surrounding tissue and establish a gradient that supplies positional information to cells in the structure

the concentration of the molecule is critical and controls the timing and the 3-dimensional spacing of cell types

Spatial dimensions are defined in relation to the main body orientation From anterior to posterior (thumb to little

finger)

dorsal to ventral (back of hand to palm)

proximal to distal (arm to fingertips)

The concentrations of the molecules coming from the AER and ZPA coordinate the system and tell the cells where they are in 4 dimensional space ( time plus 3 dimensional space)

The molecules that control the 3 spatial coordinates are known (shh, Wnt7a, and Fgf-2 ) the ones that control the timing (temporal element) are not yet known

Hox genes are responsible for telling the cells where they are along the limb and control development of limb parts

1. Homologous genes and developmental pathways underlie the structural homology of tetrapod limbs

2. Adaptive changes could be due to changes in the timing of timing of or level oflevel of expression of pattern-forming genes (shh, Fgf-2 or Wnt7a) or the Hox complex

3. In fact it has been demonstrated that tetrapods have gained hands and feet as a result of a change in timing and location of homeotic gene expression

Lecture 3 chapter 28 36 min 50 min

Lecture 4 chapter 10 12 min 24 min

Made the transition to land in the Silurian there have been four major radiations Rhyniophyta –first land plants Ferns –first vascular tissue for conducting

water Early seed plants without flowers – seeds,

pollen and spores freed them from their need for water during reproduction

angiosperms – floral diversity and association with insects allows wider distribution.

Figure 19.15 parts of the flower Flower morphology is dependent on

homeotic genes specify which organs appear in which

locations have DNA-binding regions called MADS

analogous to homeobox of the Hox loci

Have identified three types of mutants based on various homeotic mutations Class A Class A Fig 19.16 a Class B Class B Fig 19.16 b Class C Class C Fig 19.16 cFig 19.16 c

Combinations of mutations lead to the replacement of floral organs with leaf-like structures Fig 19.17

Act very much like homeotic mutants in animals where one limb type can be replaced by another or limb differentiation can be prevented altogether

AP1 ( APETALA1) – expressed early in floral development controls formation of outer two whorls, sepals and petals. (Class A)

Failure of this gene leads to no sepals or petals and a class A mutant

AP3 (APETALA3) – expressed later and involved in middle whorls of the flower bud. (Class B)

Failure of this gene leads to lack of stamens and petals and a Class B mutant

AG (AGAMOUS) – expressed late and involves the center of the flower bud. (Class C)

Failure of the AG gene leads to loss of stamens and carpels and a class C mutant

LFY (LEAFY) is a master control gene on which the other 3 are dependent

The LFY protein activates AP1, AP3 and AG expression in the appropriate cells of the four whorls Fig 19.19b

AP1 without AG or AP3 induces sepals

AP1 with AP3 induces petals AP3 with AG induces stamens AG without AP1 or AP3 induces

carpels

Control of Flower formation could evolve with a small number of changes in timing or location of gene expression which could result in large morphological changes in the flower

Coevolution with insects may lead to the rapid selection of some of these changes

LFY and its target genes ( AP1, AP3, AG) are not the genes which form the flower but seem instead to indicate the location and timing of floral parts

In fact LFY, AG and MADS-boxes have been identified in non-flowering plants such as pines and ferns

In these other plants the genes involve the formation of reproductive structures but not flowers

Like the HOM/Hox genes in animals, the MADS-box genes of plants may have evolved for some other function and then been co-opted later for the control of flower formation

Though Arthropod limbs and tetrapod limbs are not normally considered to be homologous, at a much deeper level in the developmental process controlled by genes, they do share a common control mechanism, the homeotic loci and the developmental genes such as Distal-less.

CheliceratesChelicerates

UniramiansUniramiansOnychophoransOnychophorans

TrilobitesTrilobites

CrustaceansCrustaceans

Figure 19.8 a pg 737

carpels, stamens, stamens, carpels

sepals, sepals, carpels, carpels

sepals, petals, petals, sepals

Normal

Triple mutant, lacks expression of AP2, AP1 and AG loci

Figure 19.3 pg 732