Studying Segmentation
Mutants in Balanced Stocks
Drosophila Development
 Each egg is surrounded
by a chorion.
 The anterior end has
two filaments to allow
oxygen to enter the cell.
 Sperm enter through
the micropyle at the
anterior end.
Early Drosophila Development
 It takes 1 day for the embryo to develop into a larva.
 The larva hatches, feeds, and sheds its skin twice.
 After 5 days, the larva becomes immobile and forms
a pupa.
 During the pupal stage, cells in the imaginal discs
differentiate into adult structures.
Maternal Gene Activity in
Development
Materials transported into the egg
during oogenesis play a major role
in embryonic development.
Maternal-Effect Genes
 Maternal-effect genes contribute to the formation of
healthy eggs; effects of mutations in these genes
may not affect the phenotype of the female making
the eggs but may be seen in the next generation.
 A maternal-effect mutation causes a mutant
phenotype in the offspring of a female with a mutant
genotype.
The dorsal Gene:
Offspring of dl/dl Females are
Dorsalized and Inviable
Segmentation Genes
Segmentation genes are required for
segmentation along the anterior-posterior
axis.
They are classified into three groups based
on embryonic mutant phenotypes.
– Gap genes
– Pair-rule genes
– Segment-polarity genes
Gap Genes
 Gap genes define segmental regions in the embryo.
 Mutations in the gap genes cause a set of contiguous
body segments to be missing.
 Four gap genes have been well characterized:
Krüppel, giant, hunchback, and knirps.
 Gap gene expression is controlled by bicoid and
nanos.
 The gap genes encode transcription factors.
Pair-Rule Genes
 Pair-rule genes define a pattern of segments within
the embryo.
 Pair-rule genes are regulated by the gap genes and
are expressed in seven alternating bands, dividing
the embryo into 14 parasegments along the anteriorposterior axis.
 In pair-rule mutants, every other parasegment is
missing.
 The pair-rule genes encode transcription factors.
Expression of fushi tarazu (ftz) in
a Drosophila Blastoderm Embryo
Segment-Polarity Genes
 Segment-polarity genes define the anterior and
posterior compartments of individual segments.
 Mutations in segment-polarity genes cause part of
each segment to be replaced by a mirror-image copy
of an adjoining half-segment.
 Segment-polarity genes refine the segmental pattern
established by the pair-rule genes.
 These genes encode transcription factors and
signaling molecules.
Segmentation Gene Mutants
Chapter 21
The Genetic Control of Animal
Development
Sex Determination in
Drosophila and C. elegans
 The sex determination signal in both animals is the
ratio of X chromosomes to autosomes. If the ratio is
1.0 or greater, the animal is a female; if the ratio is
0.5 or less, the animal is a male.CLASSIC Definition
 But wrong
 In Drosophila, the key genes in sex determination
encode proteins that regulate RNA processing.
Sex Determination in Drosophila
Components of the sex-determination
pathway include
– A system to ascertain the X:A ratio ,
– A system to covert this ratio into a
developmental signal, and
– A system to respond to this signal by
producing either male or female structures.
Ascertaining the X:A Ratio
 The system that ascertains the X:A ratio involves
interactions between maternally synthesized proteins
in the egg cytoplasm and embryonically synthesized
proteins encoded by several X-linked genes.
 The X-linked gene products are called numerator
elements and are twice as abundant in XX embryos
as in XY embryos.
 The autosomal gene products are called
denominator elements and antagonize the products
of the numerator elements.
The Sex-lethal (Sxl) Gene
Sxl is the mater regular of the sex
determination pathway in Drosophila.
The X:A ratio is converted into a
molecular signal that controls the
expression of the X-linked Sxl gene.
Function of SXL
SXL regulates splicing of its own
transcript to maintain SXL protein
expression in XX embryos.
SXL also regulates splicing of the
transformer (tra) gene.
Differentiating in Response to
the Signal
 TRA, along with TRA2, regulate splicing of doublesex
(dsx) and fruitless (fru).
 In XX embryos, where TRA is present, dsx transcripts
are processed to encode a DSX protein that
represses the genes for male development.
 In XY embryos, where TRA is absent, dsx transcripts
are processed to encode a DSX protein that
represses the genes for female development.
Fruitless (fru)
Males homozygous
for the fru mutation
court other males.
The fru gene
encodes a zincfinger transcription
factor that regulates
the genes for male
sexual behavior.
Loss-of-Function Mutations in SexDetermination Genes in Drosophila
Mutations in Sxl prevent SXL protein from
being made in males; homozygous mutants
would develop into males but die as embryos.
Mutations in transformer and transformer2
cause both XX and XY animals to develop
into males.
Mutations in dsx cause both XX and XY
embryos to develop into intersexes.
Key Points
In Drosophila the pathway that controls
sexual differentiation involves some genes
that ascertain the X:A ratio, some that convert
this ratio into a developmental signal, and
others that respond to the signal by producing
either male or female structures.
The Sex-lethal (Sxl) gene plays a key role in
Drosophila sexual development by regulating
the splicing of its own transcript and that of
another gene (tra).