GENECTIC REGULATION OF DEVELOPMENT IN DROSOPHILA. BY, MAYURA D. JOSHI K. J. SOMAIYA COLLEGE OF SCIENCE AND COMMERCE. MSC--PART II--BOTANY PAPER--VI HIERARCHY OF GENE ACTIVATION. MATERNAL GENES. SEGMENTATION GENES OF EMBRYO. HOMEOTIC GENES OF EMBRYO. HOMEOTIC SELECTOR GENES. MUTATIONS IN HOMEOTIC GENES. OTHER GENES OF THE EMBRYO. SEX DETERMINATION IN DROSOPHILA. THE HOMEOTIC SELECTOR GENES OF EMBRYO. Once the segmentation pattern has been determined, a major class of genes called the homeotic (structure-determining) selector genes are activated. This genes specify the identity of each segment including the body part that will develop at metamorphosis. Means this genes are master regulatory genes which specify the types of appendages and other structures that each segment will form. Mutations in homeotic genes produce flies with structures in incorrect places. Homeotic genes control the expression of genes responsible for specific anatomical structures. E.g. Where antenna should form, where legs should appear etc. Mutations in Homeotic genes. The homeotic genes have been defined by mutations that affect the development of the fly. That is, homeotic mutations alter the identify of particular segments, transforming them into copies of other segments. The principal pioneer of genetic studies of homeotic mutations is Edward Lewis. More recent molecular analysis has been done in many laboratories, including those of Thomas Kaufman, Walter Gehring, William McGinnis, Matthew Scott, and Welcome Bender. Lewis’s pioneering studies were on a cluster of homeotic genes called the bithorax complex (BX-C). BX-C determines the posterior identity of the fly, namely, thoracic segment T3 and abdominal segments AI-A8. BX-C contains three genes called Ultrabithorax (Ubx), Abdominal-A (Abd-A), and Abdominal-B (Abd-B). Each of these genes constitutes of one protein-coding transcription unit. Mutations in these homeotic genes often are lethal and the fly typically does not survive. Some nonlethal mutant alleles have been characterized. These nonlethal mutant alleles allow an adult fly to develop. The figure given here, shows the abnormal adult structures that can result from bithorax mutations. A diagram showing the segments of a normal adult fly in the Figure (a); note that the wings are located on segment Thorax 2 (T2). The pair of halteres (rudimentary wings used as balancers in flight) are on segment T3. A photograph of a normal adult fly clearly showing the wings and halteres is presented in Figure (b). Figure (c); shows one type of developmental abnormality that can result from nonlethal homeotic mutations in BX-C.

1. GENECTIC REGULATION OF
DEVELOPMENT
IN DROSOPHILA.
BY,
MAYURA D. JOSHI
K. J. SOMAIYA COLLEGE OF
SCIENCE AND COMMERCE.
MSC--PART II--BOTANY
PAPER--VI

2. HIERARCHY OF GENE
ACTIVATION.
MATERNAL GENES.
SEGMENTATION GENES OF EMBRYO.
HOMEOTIC GENES OF EMBRYO.
HOMEOTIC SELECTOR GENES.
MUTATIONS IN HOMEOTIC GENES.
OTHER GENES OF THE EMBRYO.
SEX DETERMINATION IN DROSOPHILA.

3. THE HOMEOTIC SELECTOR
GENES OF EMBRYO.
Once the segmentation pattern has been determined,
a major class of genes called the homeotic (structure-
determining) selector genes are activated.
This genes specify the identity of each segment
including the body part that will develop at
metamorphosis.
Means this genes are master regulatory genes which
specify the types of appendages and other structures
that each segment will form.

4. Mutations in homeotic genes produce flies with
structures in incorrect places.
Homeotic genes control the expression of genes
responsible for specific anatomical structures.
E.g. Where antenna should form, where legs
should appear etc.

5. Mutations in Homeotic genes.
The homeotic genes have been defined by
mutations that affect the development of the fly.
That is, homeotic mutations alter the identify of
particular segments, transforming them into
copies of other segments.
The principal pioneer of genetic studies of
homeotic mutations is Edward Lewis.
More recent molecular analysis has been done in
many laboratories, including those of Thomas
Kaufman, Walter Gehring, William McGinnis,
Matthew Scott, and Welcome Bender.

6. Lewis’s pioneering studies were on a cluster of
homeotic genes called the bithorax complex (BX-
C).
BX-C determines the posterior identity of the fly,
namely, thoracic segment T3 and abdominal
segments AI-A8.
BX-C contains three genes called Ultrabithorax
(Ubx), Abdominal-A (Abd-A), and Abdominal-B
(Abd-B).
Each of these genes constitutes of one protein-
coding transcription unit.

7. Mutations in these homeotic genes often are
lethal and the fly typically does not survive.
Some nonlethal mutant alleles have been
characterized.
These nonlethal mutant alleles allow an adult fly
to develop.
The figure given here, shows the abnormal adult
structures that can result from bithorax
mutations.

8. A diagram showing the segments of a normal
adult fly in the Figure (a); note that the wings are
located on segment Thorax 2 (T2).
The pair of halteres (rudimentary wings used as
balancers in flight) are on segment T3.

9. A photograph of a normal adult fly clearly
showing the wings and halteres is presented in
Figure (b).

10. Figure (c); shows one type of developmental
abnormality that can result from nonlethal
homeotic mutations in BX-C.
The fly shown is a homozygous for three separate
mutations in the Ubx gene, Abd-A, Abd-B.

11. Collectively, these mutations transform segment
T3 into an adult structure similar to T2.
The transformed segment exhibits a fully
developed set of wings.
The fly lacks halteres, however, because of no
normal T3 segments is present.

12. Another well-studied group of mutations defines
another large cluster of homeotic genes called the
Antennapedia complex (ANT-C).
ANT-C determines the anterior identity of the fly,
namely, the head and thoracic segments T1 and
T2.
ANT-C contains five genes, Labial (lab),
proboscipedia (Pb), Deformed (Dfd), Sex combs
reduced (Scr), and Antennapedia (Antp).
Most ANT-C mutations are lethal.

13. One group of nonlethal mutant alleles of Antp
results in leg parts instead of an antenna growing
out of the cells near the eye during the
development of the eye disc.
As shown in figure (a) and (b).
Note that the leg has a normal structure, but it is
obviously positioned in an abnormal location.

14. Antennapedia.
(a) Scanning electron micrograph of the antennal
area of a wild-type fly.

15. Antennapedia.
(b) Scanning electron micrograph of the antennal
area of the homeotic mutant of Drosophila,
Antennapedia, in which the antenna is
transformed into a leg.

16. A different mutation in Antp, called Aristapedia,
has a different effect, only the distal part of the
antenna, the arista, is transformed into the distal
part of a leg.
Therefore, the homeotic genes ANT-C and BX-C
encode products that are involved in controlling
the normal development of the relevant adult fly
structures.

17. The Antennapedia complex (ANT-C) and the
bithorax complex (BX-C) have been cloned.
Both complexes are very large.
In ANT-C, for example, the Antp gene is 103 kb
long, with many introns; this gene encodes a
mature mRNA of only a few kilo bases.
BX-C covers more than 300 kb of DNA; about 50
kb of that DNA corresponds to the protein-coding
regions of the Ubx, Abd A, Abd B, genes.

18. Organization of the bithorax complex BX-C.
The DNA spanned by this complex is 300 kb long. T = thoracic segment. A =
abdominal segment. The transcription units for Ubx, Abd A, Abd B, are shown
below the DNA; the exons are shown by control blocks and introns by bent
lines.

19. Other genes of Embryo
Several other RNAs are transcribed from BX-C,
but they are not translated.
These non-coding RNAs appear to be regulatory
RNAs that silence Ubx in early embryos.
That means RNAs act as transcriptional
repressors to ensure correct development timing
of Ubx expression.

20. Since the ANT-C and BX-C protein-coding genes
have similar functions, Lewis predicted that the
genes would have related sequences.
Analysis of the DNA sequences for the genes,
revealed the presence of similar sequences of
about 180 bp that have been named homeobox.
The homeobox is part of protein-coding sequence
of each gene, and the corresponding 60 amino
acid part of each protein is called as the
homeodomain.

21. Homeoboxes have been found in more than 20
Drosophila genes, many of which regulate
development.
Proteins play key roles in regulating the processes
of development and differentiation.
They are the key components of the structures
that are the out come of developmental and
differentiation.

22. Sex determination in Drosophila.
In Drosophila, the sex of the individual is
determined by the ratio of the number of X
chromosomes to number of sets of autosomes.
A ratio of 1.0 results in a female, and a ratio of
0.5 results in a male.
The ratio of chromosomes results in different
amounts of proteins encoded by numerator genes
on the X chromosomes versus a denominator
gene on an autosome.

23. In female embryos, there is an excess of
numerator proteins that form transcription
factors that activate a master regulatory gene for
sex determination, Sex-lethal (Sxl).
In male embryos, numerator proteins are bound
to denominator proteins, so no activation of Sxl
occurs.
This key transcription regulatory event sets in
motion a cascade of regulated alternative RNA
splicing events that ultimately leads to
differentiation into female-specific or male
specific cells.

24. REFERENCE:
iGenetics – A Molecular Approach
By, Peter J. Russell.
Developmental Biology.
By, Gilbert.
Genetics.
By, P. K. Gupta.
Genetics - Edition VIII.
By, Lewin.

25. THANK YOU...