Download I. Embryology: selected aspects

Entomology: May 22, 2006
I.
Embryology: selected aspects
A.
Insect eggs are oblong, with distinct regions (Fig. 5.6)
1.
Two outer layers
a)
Vitellin membrane
b)
Eggshell (=chorion)
2.
Nucleus
3.
Cytoplasm (Where concentrated?)
4.
Yolk
a)
Most of the material inside the egg
5.
Micropyle region: five “holes” at apex of egg
B.
Fertilization occurs
1.
Sperm enter the egg through a micropyle (chemotactic location?)
a)
More than one sperm may enter the egg
2.
Sperm entry triggers meiosis of the egg nucleus, forming the pronucleus
plus three polar bodies
a)
Oops, I erred in the last lecture by stating the ovariole tissue
was haploid. It is not. All tissues and cells associated with the
ovary are diploid.
b)
So, the eggs are actually diploid right up until fertilization…
3.
Pronucleii fuse to form the zygote
C.
Mosaic egg
1.
Many insects, such as Drosophila, possess a mosaic egg:
a)
What is this?

What happens if you remove particular cells early in
development?

What happens if you move particular cells early in
development?

What does the antennapedia mutant look like?

What does the ultrabithorax mutant look like?

What is another way to create these mutants besides
moving cells around?
2.
Some insects have regulative eggs. How does the regulative egg differ
from the mosaic egg?
D.
Cleavage occurs to form the blastoderm and blastocoel
1.
Most insects have yolky eggs. Like yolky vertebrate eggs, only the
cytoplasm fated to become the embryo and associated tissues divide,
whereas the yolk does not. Thus, cleavage is meroblastic.
a)
Note that cleavage occurs inside the yolk, but then cells migrate to
the outside of the egg
2.
Just after the blastoderm is formed, it differentiates into distinct regions.
a)
Germ band: becomes the embryo
b)
Pole cells: give rise to gametes
c)
Primary dorsal organ: function not known (glandular?)
3.
Polyembryony
a)
A single fertilized egg may undergo several complete mitotic
divisions before undergoing early development.
II.
III.

What is the result of this?

Name a type of insect in which this occurs.
E.
Gastrulation occurs
1.
This is when, theoretically, the ectoderm, endoderm and mesoderm are
formed
a)
Not straightforward in most insects with yolky eggs. Source of
controversy over which tissues are which amongst insect
embryologists.
Selected topics related to segmentation and organogenesis
A.
Parasegments
1.
Initial segmentation in embryo doesn’t correspond to eventual
segmentation in hatched individual
a)
Do initial segments resemble the ancestral segments? (Rhetorical
question—we don’t know…)
B.
Development of appendages and tagmata occurs roughly from anterior to
posterior
1.
Antennae and mouthparts develop first, followed by the thorax/thoracic
legs and finally the abdomen.
Patterns of growth and metamorphosis
A.
Apterygotes are “ametabolous”
1.
No wing development
B.
Exopterygotes undergo “hemimetabolous” development, or “simple”
metamorphosis
1.
Wings develop gradually, become ever more developed with each molt
a)
Some exceptions: wings develop only in the adults stage for
Homoptera, but they do not undergo a complete metamorphosis
(considered “paurometabolous”)
2.
Reflects gradually changing morphology of nymphal tissues into the adult.
C.
Endopterygotes undergo “holometabolous” development, or “complete”
metamorphosis
1.
Key question: from where do adult tissues originate?
a)
Imaginal discs, histoblasts and imaginal rings (Fig. 5.21)

Masses of undifferentiated tissue exist in the developing
larval stages.

Undergo mitosis only during larval stages

Triggered to develop into adult structures by
hormonal changes
2.
Pupae
a)
Often protected in cocoon (Lepidoptera, Hymenoptera,
Neuroptera, Trichoptera, Siphonaptera), usually composed of silk
secreted from salivary glands
b)
Endoparasites protected within body of host
c)
Social bees and wasps: Protective hive or “puparium”
d)
One beetle species constructs its cocoon from collapsed portion of
its digestive system
IV.
V.
Hormonal control of growth and metamorphosis in Manduca sexta (tobacco
hornworm): Fig. from Goodenough (Perspectives on Animal Behavior) plus Fig. 1
from Zee and Weeks paper.
A.
Hormones involved
1.
Juvenile hormone (JH) secreted from the corpora allata
2.
Brain hormone (BH), a.k.a. prothoracotropic hormone (PTTH)
a)
Secreted from brain tissue through neurosecretory cells into the
corpora allata (note: in most insects, BH is secreted into CC;
Lepidopterans appear to be an exception...)
b)
released from CA into hemolymph
3.
Ecdysteroids: secreted from the prothoracic gland in response to BH
B.
Hormone levels during larval stages (instars 1-5)
1.
JH levels consistently high during these stages
2.
Ecdysteroid fluctuations
a)
Molting triggered when ecdysteroids are high

Actual molt occurs after the peak. The peak itself
stimulates the production of the new exoskeleton.
b)
Molt to another larval stage triggered when both JH and
ecdysteroids are high
C.
Hormones levels/changes during 5th stage larva
1.
JH levels drop
2.
Ecdysteroids produces a small peak (labeled as CP or “commitment
pulse”) -----> onset of cessation of feeding/wandering behavior to search
for suitable location to form pupa
a)
This also triggers the reprogramming or “commitment” of tissues
from larval to adult
3.
Prepupal rise of both JH (small amount) and ecdysteroids (labeled as PP)
a)
This triggers pupation
D.
Hormonal roles during pupal stage
1.
JH again absent
2.
Ecdysteroids display a complex pattern of peaks. Precise levels and
timing of ecdysteroids control the events occurring within the pupa.
3.
Drop in ecdysteroids triggers adult emergence.
E.
Means of escape from cocoon (various insects)
1.
Should be able to list at least three different techniques.
2.
Role of hemocoel and hemolymph?
Diapause
A.
State of arrested development. Can occur in embryo, as well as postembryonically. (Known from every life stage)
1.
Feeding/other activity ceases
2.
RNA and DNA synthesis stop
3.
Metabolic rate lowers
4.
Energy comes from stored fats and glycogen
5.
Air intake is restricted to prevent water loss
B.
Adaptive value?
C.
Triggers to go into and out of diapause?
Study questions at a later date…