2. Jens
Martensson
Norm of
reaction
The developing embryo is not isolated from its
environment. In numerous instances,
environmental cues are a fundamental part of
the organism's life cycle,
The earliest tradition of experimental
embryology involved altering the environmental
circumstances in which the embryos developed
and seeing if these worries changed the
phenotype, in many instances, it was found that
the genome did not necessarily encode a
particular phenotype.
Rather, the genotype demonstrated a norm of
reaction it encoded the information for a list of
possible phenotypes. The environment was then
the determining factor as to which of the
2
3. Jens
Martensson
Phenotypic plasticity
• This idea became known as
phenotypic plasticity.
• Phenotypic plasticity is the ability of
an organism to change in response
to stimuli or inputs from the
environment.
• Phenotypic variants that result from
environmental differences are often
called morphs.
3
4. Jens
Martensson
Environmental
sex
determination
• SEX DETERMINATION IN AN
ECHIUROID WORM: BONNEllIA
• Although we associate sex
determination in mammals with
specific chromosomes (X and Y), there
are numerous species for whom the
environment plays the critical
determining role.
• For instance, Baltzer (1914) showed
that the sex of the echiuroid worm
Bonellia viridis depended on where
the larva settled. The female Bonellia
worm is a marine, rock dwelling
animal, with a body about 10 cm long
and a proboscis that can extend over a
4
• The male Bonellia, however, is only 1-3 cm
long and resides within the uterus of the
female, fertilizing her eggs. Baltzer showed
that if a Bonellia larva settles on the sea floor,
it becomes a female. However, should a larva
land on a female's proboscis, it enters the
female's mouth, migrates into her uterus, and
5. Jens
Martensson
5
SEX DETERMINATION IN A
VERTIBRATE: ALLIGATOR
• The effects of the environment on
development can have important con-
sequences.
• Recent research has shown that the sex
of alligators, crocodiles, and many other
reptiles depends not on chromosomes,
but on temperature.
• After studying the sex determination of
the Mississippi alligator both in the
laboratory and in the field, Ferguson
and Joanen (1982) concluded that sex is
determined by the temperature of the
egg during the second and third weeks
of incubation.
• Eggs incubated at 30°C or below during
this fume period produce female
• (At 32°C, 87 percent of the hatchlings were
female.) Moreover, whereas nests built in
wet marshes (close to 30°C) produce
females, nests constructed on banks(close
to 34°C) give rise to males.
• This allows alligators to escape the 1: 1
sex ratio that would be imposed by
chromosomal sex determination, and have
a nearly 10: 1 ratio of females to males in
the population.
• These findings are obviously important to
wildlife managers and farmers who wish to
breed this species. They also raise
questions of environmental policy, since
the shade of buildings or the heat of
thermal effluents can have dramatic effects
on sex ratios among reptiles.
6. Jens
Martensson
6
Adaptation of embryos and larvae to their
environments
• Another program of environmental
developmental biology concerns how the
embryo adapts to its particular envi-
ronment.
• August Weismann (1875) pioneered the
study of larval adaptations, and research in
this area has provided some fascinating
insights into how an organism's devel-
opment is keyed to its environment.
• Weismann noted that butterflies that
hatched during different seasons were col-
ored differently, and that this season-
dependent coloration could be mimicked
by incubating larvae at different tem-
peratures.
• One example of such seasonal variation is
the European map butterfly, Araschnia
levana, which has two seasonal phenotypes
so different that Linnaeus classified them
• Two morphs of Araschnia levana, the
European map butterfly. The spring morph
is represented at the top, the summer
morph at the bottom. In this species, the
phenotypic differences are elicited by
differences in day length and temperature
7. Jens
Martensson
7
Adaptation of embryos and larvae to their
environments
• Another dramatic example of seasonal cllange in
development occurs in the moth Nemoria arizonaria. This
moth has a fairly typical insect life cyde.
• Eggs hatch in the spring, and the caterpillars feed on
young oak flowers (catkins). These larvae metamorphose
in the late spring, mate in the summer, and lay eggs on
the oak trees, producing another brood of caterpillars. The
second-brood caterpillars eat the oak leaves,
metamorphose, and mate. Their eggs overwinter to start
the cyde over again the following spring.
• What is remarkable is that the caterpillars that hatch in
the spring look nothing like their progeny that hatch in
the summer. The spring caterpillars that eat oak catkins
are yellow-brown, rugose, and beaded, resembling, to a
great extent, an oak catkin.
• Two morphs of Nemoria arizonaria. (A) Caterpillars that
hatch in the spring eat oak catkins and develop a cuticle
that resembles these flowers. (B) Caterpillars that hatch in
8. Jens
Martensson
PROTECTING
THE EGG FROM
UV RADIATION
• Survival in their environments poses
daunting challenges for embryos. Indeed,
as Darwin clearly stated, most (A) UV-
fiItered light (D) UV-fiItered light + UV-A
and UV-B eggs and embryos fail to survive.
• A sea urchin may broadcast tens of
thousands of eggs into the seawater, but
only one or two (if any) of the resulting
embryos will become adult urchins.
• Most eggs become food for other
organisms. Moreover, if the environment
changes, embryonic survival may increase
or decrease dramatically.
• For instance, Early blastula (B) (E) many
eggs and early embryos lie in direct
sunlight for long periods. If we lie in the
8
• The effect of ultraviolet (UV) radiation on
embryos of the sea urchin Strongylocenrrotus
droebachiensis. Eggs were fertilized and placed
in seawater lacking sources of mycosporine-like
amino acids. The first column {A-Q represents
embryos grown in light lacking ultraviolet
radiation. The second column (D-F) shows the
same stage embryos grown in the presence of
such filtered light, but with ultraviolet radiation
9. Jens
Martensson
• First, it seems that many eggs have evolved
natural sunscreens. The eggs of many marine
organisms contain high concentrations of
mycosporine-(F) like amino acid pigments,
which absorb u1traviolet radiation (UV-B).
• Moreover, just like our skin's melanin pigment,
these pigments can be induced by exposure to
UV-B radiation (Jokiel and York 1982; Siebeck
1988).
• The eggs of tunicates are very resistant to UV-B
radiation, and much of this resistance comes
from extracellular coats enriched with
mycosporine compounds (Mead and EpeI199S).
• Adams and Shick (1996, 2001) experimentally
manipulated the amount of mycosporine-like
larva amino acids in sea urchin eggs and found
that embryos from eggs with more of these
compounds were better protected from UV
damage than embryos with less.
• Moreover, when mycosporine--deficient eggs
9
10.
11. Jens
Martensson
• Increased UV-B exposure could be
an important factor in the decline
in amphibian populations seen
throughout the world during the
past two decades.
• Blaustein and his colleagues (1994;
Blaustein and Beldin 2003) tested
whether or not UV-B could be a
factor in lowering the hatching rate
of amphibian eggs.
• At two field sites, they divided the
eggs of each of true amphibian
species into three groups.
• The first group developed without
any sun filter. The second group
developed under a filter that
allowed UV-B to pass through. The
third group developed under a
filter that blocked UV-B from
reaching the eggs.
11
Hyla regilla
12. Jens
Martensson
• For Rana cascadea and Bufo boreas, however,
the UV-B blocking filter raised the percentage
of eggs hatched from about 60 percent to
close to 80 percent.
• The effects of UV-B radiation in mediating
amphibian population declines appears to be
complex, involving climate change and fungal
pathogens.
• Using long-term observational data and by
manipulating the depth of pond water in
which frogs laid their eggs, Kiesecker and col-
leagues (2001) showed that climate-induced
reductions in pond water depth increase the
exposure of eggs and embryos to UV-B
racliation and, consequently, impair their
immune systems, making them more
vulnerable to fungal infection.
• The effect of UV-B on the frog immune
system could not have come at a worse time,
12
Rana cascadea
Bufo boreas
13. Jens
Martensson
• Climate warming in Central American has
shifted the temperate in the mountain regions
into the range that is optimum for the growth
of this ·fungus."
• The combination of UV-B exposure and global
warming appears to be responsible for the
devastation of many frog populations
throughout the world.
• The environmental programs of experimental
embryology were a major part of the discipline
when Entwicklungsmechanik was first
established.
• However, it soon became obvious that
experimental variables could be better
controlled in the laboratory than in the field,
and that a scientist could do many more
experiments in the laboratory.
• Thus, field experimentation in embryology
dwindled in the first decades of the twentieth
century (see Nyhart 1995). However, with our
13
Entwick-
lungsmechanik
14. Jens
Martensson
• Hatching success rates in three
amphibian species in the field.
• At each of two sites, eggs were
placed in enclosures that were
unshielded.
• Shielded with an acetate screen that
admitted UV-8 radiation or shielded
with a Mylar screen that blocked
UV-8 radiation.
• Eggs of the tree frog Hyla regilla
hatched successfully under all three
conditions.
• Eggs of the frog Rana cascadae and
the toad Bufo boreas hatched
significantly better when protected
from UV-B radiation. (After
Blaustein et al. 1994). 14
