BIOL316 Lectures 20

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BIOL316 Lectures 20 21 DEVELOPMENTAL BIOLOGY OF
INVERTEBRATES
(Gould and Keeton, 1996. Biological Sciences, Vol
1, Norton, New York and London)
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Biol316 Lecture 20 Invertebrate Development Part 1
development

• is the process by which fertilised eggs are
  transformed into functioning multicellular
  organisms
• is driven by cell division, cell migration and
  cellular differentiation
• is controlled by the activation of pre-defined
  cellular programs in response to precise
  positioning signals

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• EVO-DEVO
• the major evolutionary events that gave rise to
  the different invertebrate phyla can be traced to
  the evolution of developmental processes
• therefore, the phylogenetic diversification of
  invertebrates is reflected by the progressive
  diversification of their developmental processes
• Haeckel ontogeny recapitulates phylogeny

Fig 18.2 Ernst Haeckel (1834-1919)
Fig 18.1 Haeckels phylotypes
http//www.angelfire.com/mi/dinosaurs/ontogeny.htm
l
http//www.angelfire.com/mi/dinosaurs/ontogeny.htm
l
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http//en.wikipedia.org/wiki/Ernst_Haeckel
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the triploblast bauplan

• most invertebrates are triploblastic they have
  3 primary germ layers endoderm, ectoderm and
  mesoderm
• current phyla arose from a common ancestor
• that ancestor is depicted by the triploblast
  bauplan, which is roughly equivalent to a stem
  platyhelminth

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• here, I describe key developmental processes
• most often I refer to the 2 best studied
  invertebrate models of development sea urchins
  (Strongylocentrotus purpuratus Heliocidaria
  erythrogramma),which are deuterostomes, and the
  vinegar fly, Drosophila melanogaster - an
  arthropod

Fig 18.3 A molecular phylogeny of the animal
kingdom, http//www.bio.miami.edu/dana/106/106F03_
11.html
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5 key developmental processes

• Fertilization
• Cleavage
• Gastruation/neurulation
• Patterning/body plan
• formation
• 5. Tissue differentiation

lecture 20
lecture 21

• the nature and complexity of these processes is
  dependent on the phylogenetic position of an
  animal

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1. Fertilization in water

• sperm/egg interactions are critical,
  particularly among marine broad spawners, in
  which sperm have to find eggs of their own
  species in the big bad ocean
• Broad spawning interactions
• are regulated by
• synchronised spawning
• based on temperature, moon,
• day length etc
• chemical chemoattraction e.g. L-tryptophan in
  abalone
• species specific molecular interactions at
  sperm/egg surfaces e.g. lysin and VERL in abalone

Fig 18.4 Fertilisation, Gould and Keeton, 1996.
Biological Sciences, Vol 1, Norton, New York and
London.
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1. Fertilization on land
Fig 18.5 Spermatophore transfer in scorpions
http//www.beepworld3.de/members22/scorpionida/22
.htm

• sperm needs to be protected from desiccation
• spermatophore (tough casing)
• also used by aquatic animals (cephalopods,
  decapods)
• transfer into female genital tract
• sperm in seminal fluid
• Some animals deposit spermatophore on substrate
  and females pick it up
• Eg. Scorpions colembola

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1. Fertilization on land

• Sperm transfer
• Direct if intromittant organ connected to testes
• eg. penis (aedeagus)
• Indirect if intromittant organ not connected to
  testes
• eg. spider pedipalps,

Fig 18.6 Spider pedipalps http//images.opentopia
.com/enc/257/256447/20040817_010343_DSC5954.jpg
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2. Cleavage

• rapid mitotic division immediately after
  fertilisation
• division is rapid because G1 and G2 stages of
  the cell cycle are reduced to the point where
  cells do not have a chance to increase in volume
  after division
• therefore, multicell embryos are often no bigger
  than an unfertilised egg

Fig. 18.7 Cleavage in the purple sea urchin, S.
purpuratus (http//worms.zoology.wisc.edu/urchins)

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• cleavage can occur in two orientations

Fig. 18.8a Spiral cleavage in protostomes,
http\MyFile\Courses\Biology\Bio211\notes for
website\04\04 EMBRYOLOGY
cleavage plane rotates on a spiral axis between
divisions so, the cells of the upper layer are
located in the angles between the cells of the
lower layer
Fig. 18.9b Radial cleavage in deuterostomes http\
MyFile\Courses\Biology\Bio211\notes for
website\04\04 EMBRYOLOGY
cleavage plane alternates at 90 angles between
divisions - so the cells of the upper layer are
located directly above the cells of the lower
layer
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• cleavage patterns are affected by the amount of
  yolk in an egg

e.g. sea urchins
Fig. 18.10
e.g. insects
http\MyFile\Courses\Biology\Bio211\notes for
website\04\04 EMBRYOLOGY  
Cleavage in Drosophila
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• the generalised fate of cells is determined
  during cleavage

epidermal (Epi), neuron (Neu), structural (Str),
and death (X)
Fig. 18.11 Cell lineage of the nematode C.
elegans, http//scienceblogs.com/pharyngula/2006/0
3/modeling_metazoan_cell_lineage.php
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Fig 18.12 A cleavage fate map for a polychaete
worm, Arenicola marina, Neilsen, C. 2004. Journal
of Experimental Zoology. 302B 35-68
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• the fate of cells can be either fixed
  (determinant) or plastic (indeterminant)
• deuterostome embryonic stem cells remain
  totipotent long into development, whilst the fate
  of protostome stem cells is fixed very early in
  cleavage and cannot be reversed

Fig 18.13 Fate determination in protostome and
deuterostomes, http\MyFile\Courses\Biology\Bio211
\notes for website\04\04 EMBRYOLOGY ,
deuterostome
protostome
 
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• the end product of cleavage is a spherical
  blastula with a fluid filled interior (blastocoel)

Fig 18.14a. Sea urchin (S. purpuratus) blastula
(http//worms.zoology.wisc.edu/urchins/SUgast_intr
o.html)
Fig 18.14b. Drosophila blastula
(http//flybase.net/images/lk/Embryogenesis/Gastru
lation/Gastrulation-Lateral/GLV-1-lbl.jpeg)
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3. Gastrulation

• the first series of cellular migrations in an
  embryo
• forms the gut and primary germ layers
• provides embryos with an anterior/posterior axis
• is the first stage of patterning or body plan
  formation

Fig 18.15 Gastrulation in S. purpuratus
(http//worms.zoology.wisc.edu/urchins/SUgast_move
ments1.html)
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• gastrulation results from the migration of cells
  from the surface of the blastula into the
  blastocoel
• there are a number of ways in which this
  migration can occur, determined largely by the
  residual yolkiness of the embryo

Fig 18.16 Three different methods of
gastrulation, http//worms.zoology.wisc.edu/urchin
s/SUgast_intro.html
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• sea urchins have little yolk and so use
  invagination
• invagination is preceded by the detachment of
  primary mesenchyme cells (PMCs) into the
  blastocoel these cells will form part of the
  mesoderm

PMCs
Fig. 18.17 The beginnings of invagination in S.
purpuratus, http//worms.zoology.wisc.edu/urchins/
SUgast_primary.html
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• after PMC migration, a group of cells at the
  vegetal pole of the blastula begin to bend inward
  forming the archenteron (presumptive gut) and the
  blastopore (entrance to archenteron
• bending is caused by the contraction of actin
  microfilaments at the apical surface of the cells

B
actin microfilaments
A
C
http//worms.zoology.wisc.edu/urchins/su_veg_plate
_SEM_72dpi
D
http//worms.zoology.wisc.edu/urchins/SUgast_ingre
ssion.html
Fig 18.18 Invagination
http//worms.zoology.wisc.edu/urchins/su_veg_plate
_phall_72dpi
Fig 13.5. Gould and Keeton, 1996. Biological
Sciences, Vol 1, Norton, New York and London.
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• the archenteron continues to push through the
  blastocoel until it reached the opposite side of
  the blastula where it fuses with the wall of the
  blastula
• this forms a flow through gut and leaves the
  embryo with 3 primary germ layers of cells

mesoderm
http//worms.zoology.wisc.edu/urchins/SUgast_ingre
ssion.html
http//worms.zoology.wisc.edu/urchins/SUgast_ingre
ssion.html
ectoderm
Fig 18.19 The archenteron
Fig 18.20 The three primary germ layers
endoderm
gut
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• the order in which the mouth and anus are formed
  during gastrulation differs between protostomes
  and deuterostomes, hence their names

Protostome mouth first Deuterostome mouth
second
Fig 18.21 Mouth and anus formation in protostomes
vs deuterostomes, http\MyFile\Courses\Biology\Bio
211\notes for website\04\04 EMBRYOLOGY
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Gastrulation
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Biol316 Lecture 21 Invertebrate Development Part 2
4. Patterning

• processes that begin with gastrulation establish
  the basic body plan, or pattern, for the final
  individual
• this pattern provides the embryo with a 3
  dimensional image of itself that is used to guide
  the final development of organs, building limbs,
  wings, eyes etc (i.e. tissue differentiation)
• most often, the body plan is based around the
  anterior/posterior, dorso-ventral axis formed by
  the gut and nervous system
• these axes allow the body to be broken up into
  distinct domains, each of which goes on to
  develop relatively independently

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morphogens and segmentation in Drosophila

• like many invertebrates (all arthropods,
  annelids, urochordates), in Drosophila the body
  plan is based on segmentation
• in Drosophila each segment (6 head, 3 thorax and
  8 abdomen) develops specific functions in the
  adult
• during development those segments are
  established by the release of morphogens
  proteins that establish concentration gradients
  throughout the embryo to provide cells with
  precise 3-dimensional positioning information

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• segmentation begins with the deposition of mRNAs
  for the morphogens bicoid and nanos into the egg
  before fertilisation.
• these mRNAs are translated into their proteins
  after fertilisation to produce
• an anterior to posterior concentration gradient
  of the morphogen, bicoid
• a posterior to anterior concentration gradient of
  the morphogen, nanos

Fig 20.1 Primary morphogens in Drosophila, Gould
and Keeton, 1996, Biological Sciences, Norton,
New York and London.
nanos
bicoid
concentration of morphogen
posterior
anterior
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• bicoid and nanos have opposite effects on a gene
  called hunchback
• bicoid is a transcription factor that activates
  hunchback
• nanos inhibits the translation hunchback mRNAs
• these effects combine to produce a high levels
  of hunchback protein at the anterior of the
  embryo and low levels toward the posterior

Fig. 20.2 The effect of bicoid and nanos on
hunchback expression, red arrow activation,
blue bar inhibition, file///C/WORK/UNDERGRAD/B
IOL20316/Segmentation.html
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•                                                  
  
• the combined concentration gradients of bicoid,
  nanos and hunchback sequentially activate or
  inhibit other morphogen genes in increasingly
  sharply-defined regions of the embryo
• the final complex language of morphogens
  precisely defines the 3 dimensional structure of
  an embryo allowing cells to determine precisely
  which segment, and which area of that segment,
  they are in

embryo stained for eve expression
Fig 20.3 The gene, eve, is expressed in 7
segments in the abdomen. The morphogens bicoid
(bcd) and hunchback (hb) stimulate the
transcription of eve, whilst giant (gt) and
Krüppel (Kr) inhibit eve expression,
http/www.WORK/UNDERGRAD/BIOL20316/Segmentation.h
tml
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5. Tissue differentiation - homeotic genes

• the genes that are ultimately activated by the
  positioning signals provided by morphogens are a
  group of about 20 segment-specific homeotic
  genes
• all of these homeotic genes share homeobox
  domains, which allow them to act as transcription
  factors

Fig 20.4 Amino acid sequence for typical homeotic
gene, Antennapedia, showing its homeobox domains
in orange, http//www.WORK/UNDERGRAD/BIOL20316/Ho
meoboxGenes.html
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• most homeotic genes are organised in two tight
  chromosomal clusters, the Antennapedia and
  Bithorax clusters
• the segment in which they are expressed depends
  on the order that they appear in the cluster

Fig 20.5. Sites of expression for members of the
Antennapedia cluster of homeotic genes,
http//www/WORK/UNDERGRAD/BIOL20316/Segmentation.
html
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• homeotic genes act as selectors they
  activate the construction of segment-specific
  traits like wings and legs on the thorax,
  antennae and eyes on the head etc
• for example, antennapedia (Antp) activates the
  construction of legs on the thoracic segments
• Antp is expressed in the thorax, but turned off
  (repressed) in the head

Fig 20.6 Flies with the antennapedia mutation
express Antp in head segments, Gould and Keeton,
1996, Biological Scienes, Norton, New York and
London.
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Larvae and metamorphosis

• in many invertebrates, development is further
  complicated by the presence of intermediate
  larval stages between egg and adult
• larval stages have significant benefits, like
  dispersal in species that have sessile adults
• but they require one additional step in
  development metamorphosis into the final adult
  form

B
Fig 20.7 Invertebrate larvae A. insect. B.
echinoderm pluteus (Gould and Keeton, 1996,
Biological Scienes, Norton, NewYork and London
http//www.microscopy-uk.org.uk/mag/indexmag.html?
http//www.microscopy-uk.org.uk/mag/artjul00/urchi
n1.html)
A
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• the complexity of metamorphosis depends on the
  relationship between larvae and adults

ametabolous
hemimetabolous
holometabolous
Fig 20.8 3 different generalised life history
strategies in insects, http//www.ndsu.nodak.edu/e
ntomology/topics/growth.htm
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• in some cases, metamorphosis can be as simple as
  adding new, almost identical segments to an
  elongating body

Fig 20.9 Metamorphosis of a polychaete
trochophore larva, Neilsen, C. 2004. Journal of
Experimental Zoology. 302B 35-68
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• however, holometabolous metamorphosis requires
  elaborate systems to construct adults
• in insects like Drosophila, selector genes that
  control adults secondary characteristics form
  imaginal discs in larvae as repositories of adult
  characteristics

Fig 20.10 The construction imaginal discs in
Drosophila larvae is controlled by selector
(homeotic) genes, Gould and Keeton, 1996,
Biological Sciences, Norton, New York London.
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• important things that Im NOT going to talk
  about
• the formation of body cavities (coelom and
  pseudocoelom)
• neurolation (formation of the nervous system)
• development of buds in asexually reproducing
  species
• development of zooids in colonial organisms

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links
Development in general http//www.personal.psu.edu
/faculty/w/x/wxm15/Online/Zoology20Unit/zoology_l
inks.htm Sea urchin development http//worms.zool
ogy.wisc.edu/urchins/SUIntro.html http//www.stanf
ord.edu/group/Urchin/ani-plus.htm Drosophila
development http//flybase.net/allied-data/lk/inte
ractive-fly/aimain/1aahome.htm Homeoboxes
http//users.rcn.com/jkimball.ma.ultranet/Biology
Pages/H/HomeoboxGenes.html Metamorphosis http//
www.ndsu.nodak.edu/entomology/topics/growth.htm