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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 78:573-591.
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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 can’t 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 MADSbox 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
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genes
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 bud
AER = 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)
a)
b)
Uniramous
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 Distalless 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 1980’s
• 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
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