Development and Evolution

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Development and Evolution
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Just a few helpful terms

• 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)

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Development and Evolution -Today

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

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Not as many as you might think

• New discoveries are providing clues to the answer
  
• Research shows that there are often very few and
  small genetic differences between species that
  exhibit very different adult forms. (example
  humans, apes and chimps)
• similar genetic and cellular mechanisms underlie
  the development of embryos in species whose adult
  forms are very different

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

1. It is now possible to identify small genetic
   changes which are responsible for large
   phenotypic variations
2. A history of these changes is also being pursued
   through phylogenetic analysis

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Homeotic genes

• 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 homeodomain.
• The homeobox is a nucleotide sequence that codes
  for this homeodomain
• The homeobox is very similar in many eukaryotic
  organisms and is about 180 base pairs.

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Homeotic genes

• Homeotic gene products provide positional
  information in a multicellular embryo.
• They are involved in evolutionary 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)

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Specification of a Cells Fate Is a Major
Evolutionary Mechanism for Production of
Different Organismal Forms

• 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.)

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Specification of a Cells Fate Is a Major
Evolutionary Mechanism for Production of
Different Organismal Forms (cont)

• 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

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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

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Hox Genes (cont)

• 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.

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Hox Genes (cont)

• Each locus in the complex has a highly conserved
  region called the homeobox
• These homeobox bases code for a DNA binding motif
• HOM and Hox genes code for regulatory proteins
  that control the transcription of other genes.

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Hox Genes (cont)

• Hox Genes regulate the location of appendages on
  body segments.
• Work in embryo to specify the location where
  appendages should be located
• Also determine when in embryonic development
  these structures will be formed
• Hox genes do not control the actual formation of
  appendages
• Other genes control the formation of the actual
  structure whether a wing, leg, antenna, etc
• However, these other genes are thought to be
  regulated by the regulatory protein products of
  Hox loci

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PBS video segment
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When did the HOM/Hox loci originate?

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

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When did the HOM/Hox loci originate? (cont)

• 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

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How do changes in HOM/Hox gene clusters lead to
phenotypic changes?

• 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

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How do changes in HOM/Hox gene clusters lead to
phenotypic changes?

• 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

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Puzzles arising from the study of Hox genes

• In the lower animals (sponges and cnidarians)
  there is a correlation between the number of HOM
  loci and the complexity of the animals body plan
  
• The most primitive sponges and Cnidarians have
  just 5 loci
• Sea urchins have 10
• mice have 39

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Puzzles arising from the study of Hox genes
(cont.)

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

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Arthropod segmentation and HOM/Hox expression

• 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

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Arthropod segmentation and HOM/Hox expression
(cont)

• Still, all of the arthropod taxa have the same
  complement of 9 Hox genes
• Morphological diversification is due to changes
  in localized expression of genes rather than the
  addition of new HOM loci
• The genes which control morphological
  diversification are activated or suppressed by
  the action of a large number of HOM gene
  products

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Arthropod segmentation and HOM/Hox expression
(cont)

• 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 where Hox genes are expressed
• Fig 19.4 the arthropod groups
• Fig 19.5

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The arthropod limb

• What we know about genes involved in making
  arthropod limbs
• The decision to make a limb is controlled by a
  gene called wingless. Wingless is found in
  anterior part of embryo. No wingless gene no
  limbs at all.
• Also engrailed gene product is found in posterior
  part of embryo. So it is thought that these two
  define the anterior-posterior axis
• the extension of the limb distally (away from the
  body) is dependent on a gene called Distal-less
• The type of limb is controlled by homeotic genes
• There is evidence from studies that changes in
  these genes correlate with significant
  evolutionary events

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The arthropod limb (cont)

• One might be tempted to think that the
  Distal-less gene would be found in all animals
  with appendages, such as worms with legs, but
  would not be found in true worms
• It is true that Distal-less is found in all worms
  with legs
• However, the Distal-less gene is also found in
  all true worms (which have no legs but do have
  setae) and also in tube feet of sea urchins which
  have no true limbs
• Distal-less seems to instruct cells to form an
  outgrowth with proximal-distal polarity and at
  the genetic level every sticky-outey appears to
  be homologous (under the same control mechanism

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The genetics of homology limbs as an example
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The tetrapod limb

• 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

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From whence came the limb?

• 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

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What genetic changes led to the tetrapod limb?

• 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

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What genetic changes led to the tetrapod limb?
(cont)

• 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

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What genetic changes led to the tetrapod limb?
(cont)

• 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)

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What genetic changes led to the tetrapod limb?
(cont)

• 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

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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 or level 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

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HHMI video clips

• Lecture 3 chapter 28 36 min ? 50 min
• Lecture 4 chapter 10 12 min ? 24 min

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TAKE HOME MESSAGE
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Two examples from the fruit fly
http//www.people.virginia.edu/rjh9u/flyemb1.html
PBS DVD antennapedia and eye less genes
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Flowers

• 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.

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Flower development

• 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

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Flower development (cont)

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

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Four Homeotic Floral Genes

• 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
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Four Homeotic Floral Genes

• 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
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Four Homeotic Floral Genes

• 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
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Four Homeotic Floral Genes

• 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

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Four Homeotic Floral Genes

• AP1 without AG or AP3 induces sepals
• AP1 with AP3 induces petals
• AP3 with AG induces stamens
• AG without AP1 or AP3 induces carpels

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ABC Model of flower devlopment
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cont.

• 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

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Clarifying the roles of LFY and its target genes

• 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

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DEEP HOMOLOGY

• 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.

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The End
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Onychophorans
Uniramians
Chelicerates
Trilobites
Crustaceans
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Figure 19.8 a pg 737
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carpels, stamens, stamens, carpels
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sepals, sepals, carpels, carpels
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sepals, petals, petals, sepals
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Triple mutant, lacks expression of AP2, AP1 and
AG loci
Normal
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Figure 19.3 pg 732