Transcript Brooker Chapter 23

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Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 23
DEVELOPMENTAL GENETICS
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INTRODUCTION

Multicellular organisms begins their lives as a fertilized egg


As this occurs, cells divide, migrate, and change their
characteristics


They become highly specialized units within a multicellular individual
Developmental genetics studies the genes that orchestrate
the changes that occur during development


From this simple organization they proceed step by step to a much
more complex arrangement
It is currently one of the hottest fields in molecular biology
Here, we will consider several examples in which geneticists
understand how genes govern the developmental process
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23-2
The Early Stages of Embryonic
Development
Multicellular development in plants and animals follows a
body plan or pattern

Pattern refers to the spatial arrangement of different body regions
 At the cellular level, the body pattern is due to the arrangement of
cells and their specialization

The progressive growth of a fertilized egg into an adult
organism involves four types of cellular events:


Cell division


Cell movement

Cell differentiation

Cell death
The coordination of these four events leads to the formation of a
body with a particular pattern
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23-4
Yolk
Zygote goes through a series
of nuclear divisions but NOT
cytoplasmic divisions
Portions of the cell membrane
surround each nucleus
In the middle
On the inside
This stage involves a great
deal of cell migration which
forms the
Figure 23.1 Drosophila development
On the outside
23-5
At the end of embryogenesis



In Drosophila, there are
three larval stages
separated by molts

During molting, the
larva sheds its cuticle
After the third larval stage,
Drosophila proceeds
through a process termed
metamorphosis

Groups of cells called
imaginal disks were
produced earlier in
development

These imaginal disks
grow and differentiate
into the structures
found in the adult fly
The fly then emerges from
its pupal case
In metazoa, the final result of
development is an adult body
organized along three axes
Even before hatching, the embryo
develops the basic body plan that
will be found in the adult organism
Figure 23.1 Drosophila development
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Imaginal discs

Early in development, the segmentation genes, play
a role in the formation of body segments


The expression of segmentation genes in specific regions
of the embryo causes it to become segmented
There are three classes of segmentation genes




Gap genes
Pair-rule genes
Segment-polarity genes
Figure 23.3 shows a few phenotypic effects observed on
Drosophila larvae when a segmentation gene is defective
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23-8
Eight adjacent
segments are
missing from the
larvae
Anterior portion
of each segment
is missing from
the larvae
Defective
gene
Even-numbered
parasegments
are missing
from the larvae
Defective
gene
Figure 23.3
Defective
gene
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The Generation of a Body Pattern
Depends on Positional Information

Position relative to the other cells

Positional information affects cells in various ways:


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
1. A cell may be stimulated to divide into two daughter
cells
2. A cell may be stimulated to differentiate into another cell
type
3. A cell may be stimulated to migrate from one region to
another
4. A cell may be stimulated to die!
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23-10

Morphogens are molecules that convey
positional information and promote
developmental changes
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

A morphogen influences the developmental fate of
a cell
A key feature of morphogens is that they act in
a concentration-dependent manner
They often have a critical threshold
concentration


Above that they exert their effect and restrict a cell
into a particular developmental pathway
Below that, they are ineffective
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23-11
This provides positional
information that establishes the
general polarity of an embryo
The process by
which a cell or group
of cells governs the
developmental fate
of neighboring cells
Is known as
induction
Figure 23.4 Three molecular mechanisms of positional information
23-12


In addition to morphogens, positional information is conveyed by cell
adhesion
Each cell makes its own collection of cell surface receptors


These are known as cell adhesion molecules (CAMs)
CAMs cause the cell to adhere to the extracellular matrix (ECM) and/or
to other cells
CAMs
Figure 23.4 Three molecular mechanisms of positional information
23-13
Stripe pattern of pair-rule gene expression in
Drosophila embryo.
This embryo is stained to show patterns of
expression of the genes even-skipped and fushitarazu;
Caption:
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23-14
The Establishment of
the Body Axes

The first stage in Drosophila embryonic pattern
development is the establishment of the body axes


During oogenesis, certain gene products important in early
development are deposited asymmetrically within the egg
After fertilization, these gene products establish
independent developmental programs


These govern the formation of the body axes of the embryo
These gene products act as key morphogens or receptors
for morphogens

Refer to Figure 23.5
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23-15
Nanos is required for the
formation of the abdomen
Figure 23.5
The establishment of the axes of polarity in the Drosophila embryo
23-16

Let’s now take a closer look at the molecular
mechanism of bicoid

The bicoid gene got its name because a larva defective in
this gene develops with two posterior ends
Normally found only
at the posterior end
Figure 23.6
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23-17

Bicoid exhibits a maternal effect mode of inheritance

Refer to Chapter 7

Consider a female fly that is

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Phenotypically normal (because its mother was heterozygous for
the normal bicoid allele)
But genotypically homozygous for an inactive bicoid allele
(because it inherited the inactive allele from her mother and father)

This fly produces 100% affected offspring even if mated to
a male that is homozygous for the normal bicoid allele

In other words, the genotype of the mother determines the
phenotype of the offspring

This occurs because the bicoid gene product is provided to the
occyte via the nurse cells
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23-18

In the ovaries of female flies, the nurse cells are localized asymmetrically
towards the anterior end of the oocyte

Thus, maternally encoded gene products enter one side of the oocyte


This side will eventually become the anterior side of the embryo
The bicoid gene is actively transcribed in the nurse cells

Bicoid mRNA enters the anterior end of the oocyte and is trapped there
Figure 23.7
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23-19

Figure 23.7b shows an in situ hybridization
experiment of bicoid mRNA


Bicoid mRNA is highly concentrated near the anterior end
Figure 23.7c shows an immunostaining experiment
of Bicoid protein

When the bicoid mRNA is translated, a gradient of Bicoid
protein is established

The Bicoid protein functions as a transcription factor
It influences gene expression based on its concentration



It stimulates a gene called hunchback in the anterior part of the
embryo
But not in the posterior part, where its concentration is low
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23-20
Fig. 23.7b (TE Art)
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In situ hybridization of bicoid mRNA
Fig. 23.7c (TE Art)
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Immunostaining of Bicoid protein
The Establishment of
Segmentation

The next developmental process after axes
formation, is the organization of the embryo into
segments

The segmentation pattern is shown in Figure 23.8

This pattern of positional information will be maintained or
“remembered” throughout the rest of development
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23-21

Note that the segments and parasegments are out of register

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The anterior part of a segment coincides with the posterior region of a parasegment
The posterior part of a segment coincides with the anterior region of a parasegment

The pattern of gene expression that occurs in the anterior region of one
parasegment and the posterior region of an adjacent parasegment

Results in the formation of the corresponding segment
Figure 23.8
A comparison of segments and parasegments in the Drosophila embryo
23-22

The genes that play a role in the formation of
body segments are called segmentation genes



Gap genes
Pair-rule genes
Segment-polarity genes
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23-23

1. Maternal effect gene products, such as bicoid mRNA are
deposited asymmetrically in the oocyte


These will form a gradient that will later influence the formation of axes
2. After fertilization, maternal effect gene products activate
zygotic genes

The first set to be activated is the gap genes

3. The gap genes and maternal effect genes then activate the
pair-rule genes

4. The pair-rule genes then activate the segment polarity
genes
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
Later in development, the anterior end of one parasegment and the
posterior end of another parasegment will develop into a segment
Each segment will have particular morphological characteristics
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23-24
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23-25
Activation of a gap gene
•Bicoid activates Hunchback
•Bicoid activates Krüppel;
•Hunchback activates Krüppel at low
concentrations and represses Krüppel at high
concentrations
•Knirps represses Krüppel
Activation of a pair-rule gene
•The even-skipped gene contains seven discrete
enhancers- each are sensitive to different concentrations
of maternal and gap morphogens
Figure 23-9
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Figure 23-10
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Homeotic Genes and
Segment Phenotype

The term cell fate describes the ultimate
morphological features of a cell or group of cells


In Drosophila, the cells in each body segment have their
fate determined very early in embryological development
The term homeotic refers to mutant alleles in which
one body part is replaced by another

It was coined by the English zoologist William Bateson
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23-26

Drosophila contains two clusters of homeotic genes
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The antennapedia complex contains five genes
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Antennapedia complex
Bithorax complex
Both complexes are located on chromosome 3, but a large segment of
DNA separates them (Figure 23.10)
lab (labial)
pb (proboscipedia)
dfd (deformed)
scr (sex combs reduced)
antp (antennapedia)
The bithorax complex contains three genes



ubx (ultrabithorax)
abdA (abdominal A)
abdB (abdominal B)
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23-27
The order of gene expression,
from anterior to posterior,
parallels the order of genes on
the chromosome
The expression pattern of
four genes is shown
Figure 23.10 Expression pattern of homeotic genes in Drosophila
23-28
Figure 23-17
Copyright © 2006 Pearson Prentice Hall, Inc.

The role of homeotic genes has been revealed by
mutations that alter their function

For example, Figure 23.11 shows the antennapedia
mutation in Drosophila

This is a gain-of-function mutation in the antp gene

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
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It causes it to be expressed in an additional place in the embryo
In this case, it is also expressed in the anterior segment that normally
gives rise to the antennae
The abnormal expression of the antp gene in this region causes the
antennae to be converted into legs!
Investigators have also studied many loss-of-function
alleles in homeotic genes

When a particular homeotic gene is defective, its function is
replaced by the gene that acts in the adjacent anterior region
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23-29
Fig. 23.11b
Gain of function
Fig. 23.2
loss of function


The homeotic genes are regulated by segmentation
genes
Homeotic genes encode transcription factors

The coding sequence of homeotic genes contains a 180 bp
consensus sequence, termed the homeobox


This has been found in all homeotic boxes and in other genes
affecting pattern development, such as bicoid
The protein domain encoded by the homeobox is called a
homeodomain


The arrangement of a-helices promotes the binding of the protein
to the major groove in DNA
In addition, homeotic protein also contain a transcriptional
activation domain
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23-31
The DNA-binding sites
are found within genetic
regulatory elements (i.e.,
enhancers)
Figure 23.12
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23-32

The transcription factors encoded by homeotic genes
activate the next set of genes, the realizator genes


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These genes produce the morphological characteristics of
each segment
Realizator genes encode

1. Transcription factors

2. Proteins involved in cell-to-cell signaling pathways
Future research will shed more light on how the
realizator genes control morphological changes
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23-33
23.2 VERTEBRATE
DEVELOPMENT

Historically, development was extensively studied in
amphibians and birds


Eggs are rather large and easy to manipulate
Vertebrate species that have been studied include




Chicken
Frog (Xenopus laevis)
Small aquarium zebrafish (Brachydanio rerio)
Among mammals, the most extensive genetic analyses
have been performed in the mouse
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23-45
Researchers Have Identified
Homeotic Genes in Vertebrates

Vertebrates typically have long generation times
and produce relatively few offspring


Therefore, it is not practical to screen large numbers of
embryos or offspring in search of developmental mutants
Rather, cloned Drosophila genes are used as
probes to identify homologous vertebrate genes


Using this method, researchers have found complexes of
homeotic genes in many vertebrate species
In the mouse, the groups of adjacent homeotic genes are
called Hox complexes
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23-46
This suggests that there is
a “universal body plan”
for animal development
Orthologous genes
Chromosome 6
Chromosome 11
Chromosome 15
Chromosome 2

Thirteen different types of homeotic genes are found in the mouse

However, none of the four Hox complexes contains all 13
Figure 23.15
A comparison of homeotic genes in Drosophila and the mouse
23-47

The arrangement of Hox genes along the mouse
chromosome reflects their pattern of expression from the
anterior to the posterior end
Figure 23.16

This phenomenon is seen in more detail in Figure 23.16b

The expression pattern for a group of HoxB genes is shown
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23-48
Figure 23-18
Copyright © 2006 Pearson Prentice Hall, Inc.

In mice, few natural mutations affect development
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This makes it tough to understand the role genes play in
the development of the mouse and other vertebrates
To circumvent this problem, geneticists are using
reverse genetics
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1. The Hox genes are first cloned using Drosophila genes
as probes
2. A mutant version of a Hox gene is created in vitro
3. The mutant allele is the re-introduced into a mouse
4. A gene knockout is generated when the function of the
wild type gene is eliminated

This allows the geneticist to determine how the mutant allele affects
the phenotype of the mouse
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23-49

In recent years, a reverse genetics approach has
been used to understand the role of the Hox genes

Overall, the indication is that they play a key role in
patterning the antero-posterior axis in vertebrates

Nevertheless, additional research is necessary to clarify
the individual roles of each of the 38 Hox genes
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23-50
Cell Differentiation

Cell determination


Cell differentiation
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A cell is destined (predetermined) to become a particular
cell type
A cell’s morphology and function have changed, usually
permanently, into a highly specialized cell type
At the molecular level, the profound difference
between cell types arises from gene regulation

Though different cells contain the same set of genes, they
regulate the expression of their genes in different ways
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23-51

In Drosophila a hierarchy of gene regulation is
responsible for establishing the body pattern
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Researchers have identified specific genes that
cause cells to differentiate into particular cell types


A similar type of hierarchy is though to underlie cell
differentiation
These genes trigger undifferentiated cells to differentiate
into their proper cell fates
In 1987, Harold Weintraub and his colleagues
identified a gene, which they called MyoD

MyoD plays a key role in skeletal muscle cell differentiation
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23-52

4 genes that initiate muscle development:
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All four genes encode transcription factors that
contain a

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
MyoD, Myogenin, Myf5, and Mrf4
basic domain and a helix-loop-helix domain
(bHLH)
Binds DNA and
activates skeletalmuscle specific genes

Necessary for dimer formation
between transcription factor proteins
The four genes are called myogenic bHLH proteins


They are found in all vertebrates and even some
invertebrates (Drosophila and C. elegans)
They are activated during skeletal muscle cell development
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23-53

Molecularly, two key features enable myogenic
bHLH proteins to promote muscle cell
differentiation

1. The basic domain binds specifically to a
muscle-cell- specific enhancer sequence


This is adjacent to genes that are only expressed in
muscle cells
2. Their activity is regulated by dimerization

Heterodimers may be activating or inhibitory
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23-54

At later stages, the levels
of the Id protein fall


Inhibitor of
differentiation
The Id protein is produced
during early stages of
development

Figure 23.17
Myogenic bHLH can now
combine with the E
proteins to induce muscle
differentiation
It prevents myogenic
bHLH from promoting
muscle differentiation too
soon
23-55
Fruit fly with extra eyes
Figure 23-3
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