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Hi, my name is François Karch.
I'm a faculty member at the University of Geneva in Switzerland.
In this presentation, I will introduce you to
the world of Homeotic genes also known as Hox gene.
I will focus on the Homeotic gene of the Bithorax complex in Drosophila.
Hox genes play a fundamental role in the body axis of all living animals.
They specify the structures that form along the anterior posterior axis.
Hox gene has been conserved through evolution from
Drosophila where they had been discovered to humans.
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Homeotic gene has been discovered in Drosophila thanks to
the amenability of this organism for genetic studies.
This slide shows you a wild-type female.
We observe easily the segment that forms the body of the fly.
There is the head and the thorax.
The thorax is formed of three thoracic segments,
T1, T2, and T3.
From these three segments develop three pair of legs,
which are the characteristics of all insects.
There is then the abdomen,
which is composed of eight abdominal segments that are not all visible in the adult fly.
Will mention also here,
the pair of wings which developed from
the second thoracic segment T2 and a pair of haltere ,
that developed from the third thoracic segment T3.
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On the left is again our wild-type fly,
on the right is a fly carrying a mutation in
the regulatory sequences of a single gene named ultra bithorax or Ubx.
This fly has two pair of wings and no halteres.
What is going on?
Well, it turns out that this fly has a copy of the second thoracic segment T2,
that developed at the place of T3.
In other words, we say that in this mutant T3 is transforming to T2.
This is a homeotic mutation.
The term homeosis has been introduced by Bateson in 1894 to describe
a phenotypic variation in which something is changed into the likeness of something else.
To observe such a drastic change in the fly having mutation in a single gene
implies that this gene encodes a product that
is very important for the design of the body plan.
Most mutation in flies leads to the loss of a structure or failure to develop it properly.
In the case of homeotic genes we see
a complete transformation of one body structure into another body structure.
Without this gene activity we see a whole T2 instead of T3.
It implies that the the product of this homeotic gene assign
T3 identity or in other words select a developmental program for T3 specification.
In the 70s, Garcia-Bellido from Madrid proposed the term of selector gene.
In his mind he thought that homeotic genes were selecting
developmental program that would be executed by downstream genes he named realizators.
