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

Actual genotype:

+/+ (P = wild-type)

e mwh (Y = ebony body, mult. wing-hair)

y w sn3 (G = yellow body, white eyes, singed

bristles)

Evidence that pole plasm determines pole cells

Evidence for determination of the blastoderm - 3

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)

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

Maternal RNA and protein gradients

Gap gene - mutants Gap gene - expression

Hatched areas = deletions

Stippled replacement strcutures

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, Bx

Bithorax homeotic mutant transformation

Homeotic / segment specification genes

ANT-C and BX-C gene complexes (HOX genes)

Segments form in embryo

(germ band extension)

Segmentation and insect development

Segment positions on the blastoderm fate-map

Dorsal closure

(germ band retraction)

Adult segment pattern – embryonic pattern is retained

Conservation of homoetic gene expression in other arthropods