DEVELOPMENTAL BIOLOGY Drosophila - Deveopment, Axis specification
Gilbert- editions 6, 7, or 8 – chap 9, ed. 9 or 10 - chap 6
I. Drosophila is small, easy to grow, and has a rapid (10 day) life cycle A. Drosophila is sequenced, contains ~13000 genes
- yet a fairly complex organism (many more organ systems, stages, and regulative
developmental processes than nematodes)
a. > 30% of the genes are characterized by some phenotype/biochem...
b. Highly homologous genes responsible for parallel processes in higher phyla
B. a number of special tools exist which enhance the use of Drosophila
1. transgenic animals can be constructed to assess the effect of an introduced gene
on the whole organism, and to verify gene assignments through gene rescue
-this is acheived through a well characterized P-transposon element system
2. genetic manipulations, epistasis studies are well developed & sophisticated
3. techniques for detecting molecular determinants are sophisticated
II. The life cycle of Drosophila (four stages, all times listed at 25°C)
A. embryogenesis consists of 24 hours in an egg case
1. cleavage takes place as a syncitial blastoderm with the yolk in the center of the egg
-cellular blastoderm forms at div. 14 after nuclei have migrated to the periphery
2. gastrulation proceeds first with the formation of the ventral tube (to be meso) then
ingresion/delamination (neurectoderm), & ant. and post. endodermal invaginations
a. it continues with the ventral sheet of cells, the germ band, curling around
the posterior end of the embryo and extending toward the anterior end
on the dorsal side - this is called germ band extension
b. the germ band reaches the head on the dorsal side, then germ band
retraction begins. Then dorsal closure makes ectoderm cover all
c. germ band will form the segmented body of the embryo (meso and ecto)
d. endoderm forms from two ends as separate invaginations
3. segments can seen at extended germband, but causative pattern formation genes are
regionally, and segmentally, expressed even in syncitial blastoderm
B. the embryo hatches as a feeding larva
1. larvae proceeds through 3 moults, increasing its size to that of the adult fly (3 days)
2. the larve consists of:
a. polytenized larval tissues - (destined to be destroyed)
b. tissues for use in both the larva and adult
c. imaginal discs with diploid cells - give rise to most adult structures
C. the 3rd instar larva (3rd stage) immobilizes and undergoes metamorphosis as a pupa
the imaginal discs divide, differentiate, become adult structures. Larval tissue is digested
D. after 4 to 5 days of pupation, the adult emerges from the pupal case
III. Embryonic pattern specification is established by two gradients defining two axes of the animal
A. the dorsal-ventral axis is defined by a large number of maternal and zygotic genes
1. A maternal cascade patterns the egg in the D-V axis (Toll/Dorsal-Cactus)
2. A zygotic cascade then furthers the execution of D-V specification (twist, snail)
B. termini (unsegmented A-P): established by the the terminal gene group, (note-torso)
C. the anterior-posterior axis of the segmented body is established by 2 gradients of
maternal genes' factors- these are then interpreted by zygotic genes
anterior maternal morphogen-bicoid
posterior maternal morphogen- nanos
zygotic (and maternal) master gap gene influences the rest- hunchback
D. the anterior-posterior axis is then subdivided stepwise by coarse to finer - BELOW
The Anterior - Posterior pattern (A-P) - different genetic/signaling systems
A. the anterior-posterior axis of the body is established by gradients of maternal and
zygotic genes which encode morphogens -- set up overall gradient from "head to tail"
1. terminal structures (telson and acron) are established by the action of the
terminal gene group, - chief morphogen is the gene product of the gene torso
2. segmental structures (a majority of the body) - directed by genes beginning w/
anterior maternal morphogen-bicoid
posterior maternal morphogen- nanos
zygotic (and maternal) master gap gene which controls the rest- hunchback
B. the anterior-posterior segmented portion is then subdivided stepwise by coarse to finer determinants
1. three groups of zygotic genes divide the embryo into finer divisions
gap genes- 5 regions, gaps of overlapping activity and expresion
pair rule genes - into 7 stripes of expression
segment polarity genes - into the 14 parasegments
2.then homeotic genes interpret and define the identity of each segment
The specification of the A-P axis (further detail):
A. By the stepwise (temporally and spatially) and hierarchical action of 5 sets of genes, the
embryo is subdivided from a zygote to a larva (and later adult) with 14 segments
B. Each set of genes operates on the subsequent set by determining the spatial and temporal
expression of the next set of A-P genes- first- control of transcription /
then, at the segment polarity gene stage cell-cell interactions
C. The action of a specific gene is demonstrated by:
1. the portion of the embryo missing in it's mutant alleles
2. the spatial expression of it's RNA and protein
3. interactions between genes shown in double mutants, in vitro transcription tests,
and by the expression of a mutant gene in another mutant embryo's background
D. the action of the 5 sets of genes is revealed completely in the first 5 hours of development
as a "prepattern" of these gene's expressions, well before the segmentation of the
embryo is apparent morphologically. NOTE that a good deal of this pattern
determination takes place even before cellularization of the blastoderm
I. Many maternal genes help determine the A-P axis, but the actual morphogenetic substances are the
bicoid (bcd) gene product as the anterior morphogen, and nanos (nos) gene product as
posterior morphogen (a third morphogen signaling system, torso, specifies the termini)
A. bcd appears as a gradient of RNA and protein with high concentration at the anterior, the
opposite applies for nos -they establish coarse spatial information
- The localized activities are shown in transplantation and manipulation
experiments to be concentrated at the poles and to be mutually inhibitive
B. these activities are deposited in the egg non-homogeneously, and the non-homogeneity is
maintained by other gene products that bind or immobilize them (such as swa for bcd)
C. mutant bcd mutant embryos lack anterior structures, nos mutants lack posterior structures
D. bcd is a homeobox containing transcription factor, and turns on the gap genes
E. nos has the function of repressing hb translation (so it just re-inforces bcd gradient)
II. the gap genes - Kruppel (Kr), knirps (kni), hunchback (hb) [and others] are the first
zygotic genes involved in A-P patterning
A. hb is the cardinal gap gene, and responds to bcd and nos concentrations
B. they are turned on differentially (along the A-P axis) according to combinations of
maternal factors & hb- hb in segments 1-3, Kr in 4-6, and kni in 7-12 (1° bands)
C. they are expressed as large stripes at two hours of development, while the embryo is still a
syncytial blastoderm -so transcriptional activation takes place between cell-less nuclei
D. the pattern of expression of the gap genes, and the regions of the embryo missing in gap
gene mutants show a rough correspondence
E. the three gap gene products have inhibitory transcriptional effects on each other , which
causes the boundaries between them to be sharp
F. they are all DNA binding transcription factors, and their expression patterns
and overlaps are responsible for switching on the pair rule genes in seven stripes
III. The pair-rule genes are turned on in seven stripes by the gap genes-as the next step of further
subdividing the embryo into segments
A. they respond to the three regions and overlaps specified by the gap genes (can generate 7)
B. they are expressed just as the embryo becomes a cellular blastoderm (2.5 hours), and give
the embryo its first periodic character as segmented - 7 stripes (pairs of segments)
C. mutants appear as embryos missing every other segment
D. of the about 10 pair rule genes, three act first and are called cardinal pair rule geneshairy (h), runt (run), and even skipped (eve)
E. the pair rule genes are expressed in different 'registers', and subsequently turn on the
segment polarity genes in a more finely divided stripe pattern (pair rule genes are
virtually all transcription factors)
IV. The segment polarity genes are the final group setting up the segmentation of the embryo,
and are expressed in 14 stripes (one per segment)
A. each cell row is defined by a different combination of pair-rule genes, specifying (as a
combinatorial) the segment polarity gene turned on in that row
B. segment polarity mutants appear as embryos with half of each segment repeated in a
mirror image fashion (and the other half of each segment is missing)
C. they are largely proteins involved in cell surface signal transduction, and are expressed at
gastrulation (again, before the embryo shows morphological signs of segmentationstill prepattern) They are expressed once cells exist
D. The key central event is setting up two adjacent developmental organizers: a wingless (wg)
producing cell adjacent to an engrailed (en) expressing and hedgehog (hh) producing cell
V. As early as the extended germ band stage (4.5 hours) the homeotic genes are expressed, which
determine the identity of specific segments
A. they establish the identity of specific segments, such as "which segment is the second
thoracic segment (T2)"
B. mutants in these genes lead to repetition or loss of a specific segment, which can be seen in
the embryo and often in surviving adults with misplaced (homeotic) structures
C. an example is Ultrabithorax mutants which cause T3 to be T2. The result is a fly with four
wings (T2 is the segment giving rise to wings)
D. homeotic genes have the first identified homeoboxes in their gene products
NOTE: Segmentation genes are also well conserved, but the genes that control segmentation in
vertebrates are still not fully worked out – functions have changed in Drosophila