Download Evidence for determination of the blastoderm

Animal Physiology and Development
Insects –3 JCS
Lecture Outcome:
By the end of this lecture student
should have:
a)
Become aware of the early
processes in insect embryogenesis
following fertilisation
b) Understood the evidence for
maternal information in the
specification of the blastoderm
c)
Appreciated the roles of this
information in producing the ant.post. axis of the embryo
d) Seen how this information leads to
the segmentation of the embryo
and the overall specification of the
insect body plan.
JCS 6/11/02
Early embryogenesis in Drosophila
Long and short germband embryos
Short
Long
Semi-long
From Wolpert ‘Principles of development
Evidence for determination of the blastoderm -1
Fate map
UV-irradiation
Evidence for determination of the blastoderm - 2
Bisecting embryos and
culture
Evidence for determination of the blastoderm - 3
Evidence that pole plasm determines pole cells
Actual genotype:
+/+ (P = wild-type)
e mwh (Y = ebony body,
mult. wing-hair)
y w sn3 (G = yellow body,
white eyes, singed
bristles)
Early effect female sterile mutations
Major developmental question:
What is put into the egg by way of information for
development ?
i.e what is the maternal contribution to the egg ?
_________________________________________
One way to look is to search for female-sterile (fs)
mutations which arrest normal development at the
earliest possible stages with major effects on
development.
Christiana Nusslein-Volhard and Eric Wieschaus –
Nobel Winners in 1995!
Cross to show a recessive female sterile mutation
(fs)
+ / fs female X + / fs male

+/+
+ / fs
fs / fs
(fs/fs are female sterile but for many different
reasons;
some – e.g. bicoid, nanos, oskar, torso - produce
eggs which develop with major aberrations of the
embryonic body plan)
Early body plan maternal effect mutants
Wild type
bicoid
Rescue of bicoid and oskar by
cytoplasmic transplantation.
(oskar is a gene which interacts with nanos,
(nos) to form the posterior determinant)
Maternal RNA and protein gradients
bicoid RNA is
localised/fixed to the ant.
pole. bicoid protein diffuses
to form a gradient.
bcd = bicoid
hb = hunchback
hunchback RNA is through
out the egg;
nanos RNA is localised to
post. pole; nanos protein
diffuses and represses
hunchback protein synth.
creating two gradients.
nos = nanos
Gap gene - mutants
Hatched areas =
deletions
Stippled replacement
strcutures
Gap gene - expression
How maternal product gradients specify gap
gene expression
T¦A shows how gradient specifies
thorax/abdomen boundary
Examples of gap, pair-rule and segment polarity
mutants
Control of hairy (h) - a pair rule gene expression by Kruppel and knirps (gap genes).
Expression patterns and/or domains of action of
the various maternal, gap and pair rule genes
md: mandibular segment; ma: maxillary segment; la:
labial segment; proct: proctodeum
Expression of Kruppel (gap) and hairy (pair-rule)
genes in long and short germ band insects
Antennapedia homeotic mutant transformation
Wild type
Antp
Bithorax homeotic mutant transformation
Bithorax, Bx
Homeotic / segment specification genes
ANT-C and BX-C gene complexes (HOX genes)
Segmentation and insect development
Segment positions
on the blastoderm
fate-map
Segments form in
embryo
(germ band extension)
Dorsal closure
(germ band
retraction)
Adult segment
pattern – embryonic
pattern is retained
Conservation of homoetic gene expression in
other arthropods