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 segmentincluding the body part that will develop atmetamorphosis.

Means this genes are master regulatory genes whichspecify the types of appendages and other structuresthat 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 ofparticular segments, transforming them intocopies of other segments.

The principal pioneer of genetic studies ofhomeotic mutations is Edward Lewis.

More recent molecular analysis has been done inmany laboratories, including those of ThomasKaufman, Walter Gehring, William McGinnis,Matthew Scott, and Welcome Bender.

Lewis’s pioneering studies were on a cluster ofhomeotic genes called the bithorax complex (BX-C).

BX-C determines the posterior identity of the fly,namely, thoracic segment T3 and abdominalsegments 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.

The fly shown is a homozygous for three separate

mutations in the Ubx gene, Abd-A, Abd-B.

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.

Another well-studied group of mutations definesanother large cluster of homeotic genes called theAntennapedia complex (ANT-C).

ANT-C determines the anterior identity of the fly,namely, the head and thoracic segments T1 andT2.

ANT-C contains five genes, Labial (lab),proboscipedia (Pb), Deformed (Dfd), Sex combsreduced (Scr), and Antennapedia (Antp).

Most ANT-C mutations are lethal.

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.

Antennapedia.

(a) Scanning electron micrograph of the antennal

area of a wild-type fly.

Antennapedia.

(b) Scanning electron micrograph of the antennalarea of the homeotic mutant of Drosophila,Antennapedia, in which the antenna istransformed into a leg.

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.

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.

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.

Other genes of EmbryoSeveral 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.

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.

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.

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.

In female embryos, there is an excess ofnumerator proteins that form transcriptionfactors that activate a master regulatory gene forsex determination, Sex-lethal (Sxl).

In male embryos, numerator proteins are boundto denominator proteins, so no activation of Sxloccurs.

This key transcription regulatory event sets inmotion a cascade of regulated alternative RNAsplicing events that ultimately leads todifferentiation into female-specific or malespecific cells.

REFERENCE:iGenetics –A Molecular Approach

By, Peter J. Russell.

Developmental Biology.

By, Gilbert.

Genetics.

By, P. K. Gupta.

Genetics - Edition VIII.

By, Lewin.

THANK YOU...