Hi, I'd like to talk about limb
development in drosophila with
a particular emphasis on how drosophila
genetics can be used to understand
hedgehog signaling and pattern formation.
The embryonic development of higher
organisms is precisely controlled.
Body plans are impressively
they're stored to a large
extent in the genome.
if you look at identical twins,
which inherit the same set of chromosomes,
you can witness how many details
are written down in these plans and
how carefully these plans are executed.
But how does the development of
a multicellular organism occur, such that
animals that receive the identical
genome also end up looking the same like
Dolly the first cloned mammal that
strikingly resembled her mother?
Pattern formation in animals is
controlled by cell to cell signaling,
cells communicate with each other during
development by extracellular signals.
Certain cells, for example,
release signals that serve other cells to
determine their position within a tissue.
The information specified
by such signals has been
referred to as positional information.
The concept of positional information is
around already for an entire century.
It is based mainly on
such as transplantation experiments only
relatively late in the 20th century.
Also, genetic experiments supported
the concept of positional information.
Two classical experiments
many others are illustrated on this slide.
An embryology experiment by Saunders
revealed in the 1960s that there is
a source of positional information in
the posterior region of the developing
chick limb bud.
These cells referred to as ZPA,
zone of polarizing activity
are able to specify a precise
anteroposterior digit pattern,
if transplanted to an ectopic
site in a host embryo.
A representative example of a genetic
approach is illustrated below.
This drosophila embryo from
the Nusslein-Volhard Wieschaus screen has
lost a great deal of positional
information in the patterning process that
normally leads to the specification
of precisely spaced and
patterned denticle belts
on the ventral side.
The loss of a single gene
here is responsible for
the collapse of this patterning process.
This gene has been termed hedgehog due
to the hedgehog like appearance of these
Nobody would have guessed at
the time that these two activities
conferring positional information
in vertebrate limbs above and
in an insect embryo below represent one
and the same secreted signaling molecule.