Development and Evolution

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Development and Evolution

• Chapter 18

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A great deal of the problem of neo-Darwinian
theory is that it is strictly a theory of genes,
yet the phenomenon that has to be explained in
evolution is that of the transmutation of form.
True, genes may be invented to account for the
selection of any desired form, but the real
solution to the problem lies in that uncharted
realm between genes and morphology.M. W. Ho
and P. T. Saunders. 1979. Beyond neo-Darwinism
an epigenetic approach to evolution. J.
Theoretical Biology 78573-591.
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The morphologists complaint

• The modern synthesis, which has dominated much
  evolutionary thinking since the mid-20th Century,
  is a synthesis of Darwinian verbal argument and
  mathematic population genetics it seeks to
  explain evolutionary change ultimately in terms
  of forces acting to change allele and genotype
  frequencies in populations
• Population genetics thinking does not, and
  cannot, explain much of what is interesting about
  evolution particularly the evolution of
  morphology of multicellular animals

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Morphology is epigenetic

• Morphology results from interaction between many
  gene products and between gene products and the
  environment and is expressed only through
  development ( ontogeny)
• We cant understand the evolution of morphology
  simply by reference to forces that change allele
  and genotype frequencies in populations, or
  simply by understanding how a sequence of DNA
  nucleotides specifies a sequence of amino acids

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Evo-Devo

• Animal body plans
• Formation of limbs in vertebrates and arthropods
• Evolution of the flower

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Homeotic genes and pattern formation

• Homeotic loci are genes that are responsible for
  telling cells where they are spatially in a
  developing 4 -dimensional embryo, for telling
  cells where they are in a developmental sequence,
  and for determining the fates of cells
• In animals, the key homeotic loci are called Hox
  (for homeobox) or HOM genes they are a gene
  family created by gene duplication events
• In plants, the key homeotic genes are the
  MADS-box genes
• Although there are Hox homologues in plants and
  MADS-box homologues in animals, Hox loci and
  MADS-box loci are not homologous to each other

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Hox genes in animals

• Found in all major animal phyla
• Occur in groups (gene duplication events) the
  number of genes in each group and the total
  number of groups varies among phyla
• Perfect correlation between the 3 5 order of
  genes along the chromosome and the anterior to
  posterior location of gene products in the
  embryo. Genes at the 3 end are also expressed
  earlier in development and in higher quantity
  than genes at the 5 end spatial, temporal, and
  quantitative colinearity
• Each locus within the complex contains a highly
  conserved 180 bp sequence, the homeobox, that
  codes for a DNA binding motif Hox gene products
  are regulatory proteins that bind to DNA and
  control the transcription of other genes

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Hox genes in Drosophila

• Two clusters Antennapedia and bithorax
• Mutations in the Antennapedia genes affect the
  anterior of the developing embryo, mutations in
  bithorax genes affect the posterior
• Flies missing one or more Hox gene products
  produce segment-specific appendages such as legs
  or antennae in the wrong place
• Gene products from Hox loci demarcate relative
  positions in the embryo, rather than coding for
  specific structures for example, they specify
  this is thoracic segment 2 rather than make
  wing

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Hox genes in Drosophila(Gerhart and Kirschner
1997) (Fig. 18.1)
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Hox gene mutant phenotypes

• Top normal fly on left antennapedia mutant
  phenotype on right
• Bottom bithorax mutant phenotype

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The phylogenetic position of Hox genes

• Although Hox genes are expressed in a
  segment-specific way in arthropods, they are also
  found in non-segmented animals they are not
  segmentation genes
• Hox genes specify anterior posterior and dorso
  ventral axes in bilateral animals, but
  homologues are present in sponges and jellyfish,
  and plants and fungi
• The original gene duplication event that produced
  the Hox complex may have preceded the evolution
  of multicellularity in animals
• 10 Hox loci probably existed in the common
  ancestor of all bilaterally symmetric animals
  sponges and cnidarians have just 3 4 Hox loci
• There is a rough correlation between the number
  of homeotic loci and complexity of metazoan body
  plans
• Vertebrates have 4 Hox clusters, but other
  deuterostomes have just a single cluster

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Hox genes in various animal phyla (Fig. 18.3)
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Changes in Hox expression arthropod segmentation

• Does variation in Hox gene expression correlate
  with morphological diversity in arthropods?
• All arthropods ( onychophorans) have the same 9
  Hox genes
• Addition of sequences coding for an alanine
  region in the product of Ubx may be responsible
  for the suppression of legs on the abdominal
  segments of insects

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Hox expression and arthropod segmentation (Knoll
and Carroll 1999) (Fig. 18.5)
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The origin of the tetrapod limb

• Phylogenetic and morphological analyses support
  the hypothesis that the tetrapod limb is derived
  from the fins of lobe-finned fish
• The first tetrapods (amphibians) appear in the
  late Devonian, about 365 mya

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Lobe-finned fish and the tetrapod limb (Figs.
18.6 and 18.7)

• Eusthenopteron, a lobe-finned fish from the
  Devonian (409-354 mya)

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The developing tetrapod limb budAER apical
ectodermal ridge (Fig. 18.8)
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The development of the tetrapod limb -1

• The tip of a growing limb bud is the apical
  ectodermal ridge (AER) cells in the AER secrete
  a substance that keeps the underlying cells in a
  growing and undifferentiated state (the progress
  zone) this determines the long axis of the limb
• The zone of polarizing activity (ZPA) is formed
  by a group of cells at the base of the limb bud
  these cells secrete a substance that forms a
  gradient in the surrounding tissue and gives
  cells in the limb bud positional information

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The development of the tetrapod limb -2

• The substance secreted by cells in the AER is the
  product of the gene fibroblast growth factor 2
  (FGF-2) this determines the proximal - distal
  axis of the limb
• The substance secreted by cells in the ZPA is the
  product of a gene called sonic hedgehog (shh)
  this determines the anterior - posterior axis of
  the limb
• Expression of a gene called Wnt7a is responsible
  for determining the dorso - ventral axis
  (wingless int-1)
• Hox genes are also expressed in the tetrapod limb
  and may tell cells where they are along the
  length of the limb

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The development of the tetrapod limb -3

• One implication of this line of research is that
  evolution of limb morphology in tetrapods may
  result from changes in the timing or level of
  expression of the pattern forming genes Fgf-2,
  shh, Wnt, or the Hox genes
• Evolution of the hand and foot (not present in
  lobe-finned fish) may be due in part to turning
  expression of shh and Hox genes back on in the
  late limb bud of tetrapods

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Arthropod limbs (Brusca and Brusca 2002) (Fig.
18.12)

1. Uniramous
2. Biramous

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Genetic control of limb formation in arthropods

• The decision whether to make a limb depends on a
  gene called wingless (wg)
• Wingless is expressed in the anterior of limb
  primordia and another gene, engrailed (en), is
  expressed in the posterior these two genes
  appear to determine the anterior - posterior axis
  of the limb
• The decision to extend the limb distally appears
  to be due to the expression of the gene
  Distal-less (Dll) this is the first gene
  activated specifically in limb primordia
• The decision on which type of limb will develop
  is controlled by Hox genes
• Variation in the timing and location of
  expression of Distal-less appear to affect the
  branching pattern of arthropod limbs

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Deep Homology

• Distal-less has been found to play a role in limb
  formation in all bilaterians examined to date
  arthropods, vertebrates (cells of the AER),
  onychophorans, annelids (parapodia), echinoderms
  (tube feet)
• Furthermore, it is also known that similar genes
  in mice and fruit flies are involved in the
  formation of eyes, hearts, nerve cords, and
  segmentation

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MADS-box homeotic genes and development of flowers

• Specify which floral organs appear where
• Each locus encodes a DNA binding protein domain
  (MADS box) that is analogous to the DNA binding
  domain encoded by Hox genes
• Mutations in specific MADS-box genes are
  associated with abnormal floral morphology

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Parts of a flower (Fig. 18.15)
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The ABCs of flower development mutations(Coen
1999) (Fig. 18.16)
APETALA1 mutation
APETALA3 mutation
AGAMOUS mutation
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A conceptual model of flower formation by
homeotic genes (Parcy et al. 1998) (Fig. 18.18)
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Genes and development summary

• The evo-devo research program of the last 20
  years has done much to answer the criticisms of
  the modern synthesis that were made by
  developmental biologists and morphologists in the
  early 1980s
• We are now beginning to understand the genes and
  gene interactions that are responsible for the
  development and evolution of complex body plans
  and morphology in animals, and floral structures
  in plants
• Macroevolutionary change in morphology can be
  understood in terms of changes in a set of genes
  common to all animals (or plants) deep homology
  and that are affected by microevolutionary
  processes selection, drift, mutation, gene
  duplication