MALE MOTHS HIJACK BATS' CALLS TO CATCH A MATE

Full text

Page 1

O Ou ut ts si id de e J JE EB B
Outside JEB
iv
Keeping track of the literature
isn’t easy, so Outside JEB is a
monthly feature that reports the
most exciting developments in
experimental biology. Short
articles that have been selected
and written by a team of active
research scientists highlight the
papers that JEB readers can’t
afford to miss.
ALLIGATORS, LIKE BIRDS,
BREATHE ONE WAY ONLY
It is well established that bird lungs move
air in a one-way loop through their gas-
exchanging tubes during inhalation and
exhalation. In contrast, mammals and
presumably all other vertebrates breathe
tidally, bringing air into the dead-end gas-
exchange structures and back out through
the same pathway. One-way airflow is a
highly efficient method of breathing,
allowing more oxygen to be extracted per
breath compared with tidal breathing.
Unidirectional airflow is thought to be
unique to birds, which have evolved to
meet the high oxygen demands of flight. As
it turns out, alligators also have a one-way
path of breathing, similar to birds.
Conventional wisdom suggests that one-
way air flow can only occur with the help
of air sacs, bellow-like structures that are
found only in birds. C. G. Farmer and Kent
Sanders, from the University of Utah, USA,
noticed that key features of the alligator
lung looked remarkably similar to the bird
lung, though alligators lack air sacs. To test
the hypothesis that airflow in alligators is
unidirectional, the authors performed three
separate experiments. First, they artificially
ventilated excised lungs from four
American alligators and monitored airflow
using flowmeters, termed thermistors. Next,
they surgically implanted thermistors on six
anaesthetized alligators and monitored
airflow during normal breathing. Finally,
they filled an excised alligator lung with
saline fluid containing fluorescent beads
and visualized the water flow during
artificial ventilation.
All three experiments showed the same
remarkable result, that flow occurs in one
direction through the lungs of alligators.
Notably, air flowed unidirectionally even in
excised lungs, suggesting that one-way
airflow in alligators is caused by the
structural arrangement of the gas
exchanging tubes themselves. These results
VENTILATION
demonstrate that unidirectional airflow can
occur in the absence of air sacs, and
alternative forms of breathing, such as the
hepatic piston method used by alligators, do
not preclude unidirectional airflow.
This extraordinary finding has important
implications for how scientists think about
the evolution of breathing. Farmer and
Sanders suggest that unidirectional airflow
evolved far earlier than previously thought.
They propose that unidirectional airflow
was present in basal archosaurs, the
common ancestor of crocodilians, birds and
dinosaurs. Mammal-like reptiles, termed
synapsids, were the dominant land animals
during the Permian. However, following the
Permian–Triassic extinction, archosaurs
emerged as the champions and dominated
earth until the Cretaceous–Tertiary
extinction, after which synapsids regained
supremacy. Farmer and Sanders suggest
that unidirectional airflow gave archosaurs
a competitive advantage during the low
oxygen conditions of the early Triassic by
increasing their capacity for exercise. Thus,
the alligator’s one-way path for breathing
could help explain where dinosaurs got the
breath to climb to dominance of the earth.
10.1242/jeb.036558
Farmer, C. G. and Sanders, K. (2010). Unidirectional
airflow in the lungs of alligators. Science 327, 338-340.
Erika J. Eliason
University of British Columbia
eeliason@interchange.ubc.ca
THE JOURNAL OF EXPERIMENTAL BIOLOGY

Page 2

HOT BUGS SHOW EFFECTS OF
CLIMATE CHANGE
Predicting how climate change will affect
the survival and development of organisms
will be a key challenge for 21st century
biology. A recent study of the southern
green stink bug, Nezara viridula, gives an
illustration of how we might go about
investigating the effects of global warming.
The stink bug can grow up to 2cm long
and is found in tropical and sub-tropical
regions around the world. It is a major
agricultural pest on a wide variety of crops,
and has been expanding its range towards
more temperate zones as a result of global
warming.
To discover how N. viridula responds to
increased temperatures, a group of scientists
in Kyoto, led by Dmitry Musolin, kept a
population of the bugs outside; half of them
were reared in transparent incubators that
were 2.5°C warmer than the ambient
temperature. The advantage of having
glass-walled incubators was that the
experimental insects were subject to the
same daylight conditions as a group of
insects that were kept alongside them in
transparent plastic containers with no
additional heating.
Every 2weeks in summer 2006, new
clutches of N. viridula eggs were added to
the experiment, until the insects began to
over-winter from November to March. In
spring 2007, two further egg clutches were
added and allowed to grow. Carried out
over 15 months, the study examined how
many eggs hatched, and the rate at which
the young insects reached maturity and
mated, and also measured various physical
characters such as size and colouration.
Higher temperatures affected every aspect
of the bugs’ biology. The insects in the
incubators became active earlier in the year,
grew more rapidly and laid more eggs.
However, in the hot summer months the
experimental group showed several
responses that suggested that they could not
cope with the higher temperatures: the bugs
found difficulty shedding their skin as they
grew, and tended to die earlier and show
physical abnormalities.
The authors point out that their study was
somewhat artificial – the temperature
increase was constant throughout the year,
no predators could get at the bugs, and the
experiment was carried out on only one
experimental group. Nevertheless, this is
one of the first attempts to understand how
climate change could affect the future of
life. In particular, it suggests that species’
responses to increased temperatures may be
complex. At some points in the year the
warmed bugs prospered, but they did not
produce an extra generation because of the
damaging effects of the increased summer
temperature. The exact reason for these
effects is unknown, but it is possible that
the bacteria that live in the gut of the bugs
were unable to cope with the high
temperatures.
In the real world, increased temperatures
would affect not only the bugs and their
bacteria but also their host plants and their
predators. Each of these components of the
bugs’ ecosystem would probably respond
differently to climate change, making the
overall outcome hard to predict. However
complex this might seem on paper, the
reality will soon become apparent to
everyone on the planet. This article gives us
a glimpse of what the future might hold.
10.1242/jeb.036541
Musolin, D., Tougou, D. and Fujisaki, K. (2010). Too
hot to handle? Phenological and life-history responses
to simulated climate change of the southern green
stink bug Nezara viridula (Heteroptera: Pentatomidae).
Glob. Chang. Biol. 16, 73-87.
Matthew Cobb
University of Manchester
cobb@manchester.ac.uk
THE FLY SAYS LESS IS MORE!
Throughout animal evolution, several
innovations allowed organisms to generate
genetic ‘novelty’. In vertebrates, repeated
genetic duplication events fostered the
expansion of several gene families. For
example, a family that has three or 4
distinct genes in mammals may only have
one equivalent gene in insects. So a
recurrent question is: how do insects or
other animals that didn’t experience these
large duplication events generate functional
diversity? Nigel Price and colleagues
examined this question by looking at the
metabolic enzyme carnitine
palmitoyltransferase 1 (CPT1) in
Drosophila, as most insects only possess
one copy of this gene. This enzyme is
essential to import long-chain fatty acids
into the mitochondria to break them down
for energy production. In comparison to
flies, mammals express three distinct
isoforms in a tissue specific manner, each
isoform differing in its affinity for its
substrate (carnitine), and its response to its
inhibitor (malonyl-CoA). Price and his team
specifically hypothesized that Drosophila
(and other insects) generated a similar
kinetic diversity of this enzyme through
alternative splice variants of their single
gene. That is, this single ‘insect CPT1
gene’ could give rise to distinct CPT1
proteins through alternative expression of
different parts of the gene.
Initially, the team searched genomic
databases to identify possible alternative
versions of the CPT1 gene in insects. They
discovered that most insects possessed a
region of their CPT1 gene that could
generate two versions of the protein via
usage of two interchangeable expressed
sequences (exons).
To confirm these predictions, the team
undertook gene expression analysis to see
whether and how these two putative CPT1
versions were expressed in Drosophila
tissues. First, through searches in available
Outside JEB
v
CLIMATE CHANGE
FUNCTIONAL DIVERSITY
THE JOURNAL OF EXPERIMENTAL BIOLOGY

Page 3

Outside JEB
vi
collections of expressed genes, they
identified two separate CPT1 gene
transcripts in Drosophila. These two
transcripts only differed by one small
portion of the gene where only one or the
other functional alternative exon was
expressed, the remainder of the gene
transcript being identical. The team
confirmed the presence of these splice
variants in most Drosophila tissues and
quantified their relative levels in flight
muscle and fat body. Their results
established that these two variants were
differentially expressed in these tissues
indicating possible regulatory and
functional differences between the
alternative Drosophila CPT1s.
Finally, the team characterized the
enzymatic properties of the two splice
variants in yeast. They determined that the
isoform predominantly expressed in flight
muscle (a tissue with high levels of fat
oxidation) had overall greater enzymatic
activity than the other CPT1 variant,
suggesting fine-tuning of the CPT1 enzyme
profile according to the metabolic demands
of the tissue.
The results of the current study clearly
show that flies can still create functional
diversity in their metabolic processes
despite having comparatively fewer genes
than some other animals. In addition, the
unique kinetic characterization of a non-
mammalian CPT1 enzyme generates
interesting hypotheses on the functional
diversity in CPT1 across animals.
Furthermore, this work begs the question of
how functionally different the various CPT1
would be in an animal that experienced
repeated duplication events and as a result
possesses many additional CPT1 copies.
10.1242/jeb.036533
Price, N. T, Jackson, V. N., Muller, J., Moffat, K.,
Borthwick, K. L., Orton, T. and Zammit, V. A. (2010).
Alternative exon usage in the single carnitine
palmitoyltransferase 1 (CPT 1) gene of Drosophila
generates functional diversity in the kinetic properties
of the enzyme. Differential expression of alternatively
spliced variants in Drosophila tissues. J. Biol. Chem.
doi: 10.1074/jbc.M109.072892.
Chris Le Moine
McMaster University
clemoin@univmail.cis.mcmaster.ca
FROG MUSCLES START
STRETCHED
The limb muscles of a frog produce high
levels of power to propel the animal into
the air during a jump. To do this, they
generate large forces quickly, and contract
over relatively long distances (up to 30% of
their resting length). However, the notion of
muscles generating high forces while
contracting over a large distance presents a
bit of a conundrum because muscles have a
relatively narrow range of lengths over
which they generate their highest forces. It
seems unlikely that muscles that shorten
over great distances, like those in frog legs,
will spend much time at lengths where
forces can be maximized. Manny Azizi and
Tom Roberts of Brown University recently
addressed this conundrum and, in the
process, revealed some surprising results.
Using bullfrogs, Azizi and Roberts chose to
measure in vivo activation patterns and
length changes in the major ankle extending
plantaris muscle during jumping. Following
these jumping trials, they used an in vitro
preparation to determine a force–length
curve for each muscle they measured in
vivo. Force–length curves describe how a
muscle’s length affects its capacity for
generating force. They consist of three
parts: (1) an ‘ascending limb’ where force
capacity increases as muscles go from short
to intermediate lengths, (2) a plateau at
intermediate lengths where muscles can
maximize force production and (3) a
‘descending limb’, where further stretching
leads to decreased forces. Azizi and Roberts
wanted to know where the plantaris was
operating in relation to its force–length
properties as it contracted during a jump.
They predicted that the muscle would start
near its optimal length and move down its
ascending limb as it shortened during
contraction. But they were wrong.
Prior to a jump, it turns out the plantaris is
resting at a relatively long length,
positioned on the descending limb of its
force–length curve, well beyond its
optimum length for generating force.
During the jump, the muscle contracts
enough to put it onto the plateau of its
force–length curve – in short, the plantaris
ascends its descending limb. The authors
were also able to show that this particular
shortening pattern, where the muscle starts
stretched and ends up at intermediate,
optimal lengths, generates higher forces
than the alternative strategy of starting at
optimal lengths and contracting from there.
These are surprising results! Operating at
such long lengths is, or I should say was,
considered an anathema for muscles. But
this view may be mammal-centric. In
mammals, passive tension in stretched
muscles increases very rapidly, and it is
almost absurd to think of mammalian
muscle starting to contract at the sorts of
lengths these frog muscles are using
regularly (1.33 resting length). However, as
Azizi and Roberts show, passive tension in
frog muscles stretched to this degree is
quite low, allowing them to operate safely
at such long lengths. The next time you
find a frog, look closely at how the thigh,
lower leg and foot are folded on top of one
another, and remember that this seemingly
awkward limb positioning is likely critical
for stretching the underlying muscles so
that they can contract over large distances
without losing too much force.
10.1242/jeb.036566
Azizi, E. and Roberts, T. (2010). Muscle performance
during frog jumping: influence of elasticity on muscle
operating lengths. Proc. R. Soc. B. 277, 1523-1530.
Gary B. Gillis
Mount Holyoke College
ggillis@mtholyoke.edu
BIOMECHANICS
THE JOURNAL OF EXPERIMENTAL BIOLOGY

Page 4

Outside JEB
vii
MALE MOTHS HIJACK BATS’
CALLS TO CATCH A MATE
Vision is often almost useless at night so
many nocturnal species, such as gleaning
bats, resort to acoustic methods to perceive
their environment. They broadcast
ultrasonic echolocation calls to locate insect
prey. Yet many prey species have evolved
ultrasonic hearing in a bid to outsmart their
hunters, and some prey species have even
gone a stage further. They have
appropriated bat calls into their own
repertoire for sexual communication.
Borrowing from their predator, male corn
borer moths use ultrasonic songs
resembling bat calls to increase their
copulation success. Wondering how
mimicking bats helps the courting males,
Ryo Nakano and colleagues decided to find
out precisely how female moths respond to
the males’ advances and publish their
results in Physiological Entomology. It
seems likely that male calls induce female
moths to behave as if they are under attack,
rendering them motionless and suppressing
female interference during the act of
mating.
The team used an experimental approach to
listen in to the inner workings of the private
world of moth sexual relations. First they
silenced the males by surgically removing
the sound-producing scales located on their
forewings, while puncturing both of the
females’ tympanic membranes so that they
could not hear males’ calls. Next they
observed how deafened versus intact
females reacted to male advances, and how
frequently mute or intact males convinced
females to copulate with them.
Nakano and his team confirmed that when
both partners were intact the females
readily accepted the males as partners and
they copulated successfully 98% of the
time. However, when either partner was
acoustically impaired (through losing either
their hearing or the ability to call), their
success fell to roughly 60%: courtship
songs are instrumental in corn borer mating.
And when the team analyzed the female’s
responses to the male’s ultrasonic courtship
calls they found that the females reacted as
if they were under attack and froze, making
it easier for the males to mount them.
Focusing on call function, the team
recorded male songs to see whether they
increased the sound level of subsequent
mating attempts if the initial attempt failed.
Nakano and his colleagues found that the
moths did get louder during subsequent
mating attempts, suggesting that males can
pump up the volume as desperation kicks
in. Finally, the team played synthesized
calls to intact females at various sound
levels and found that sound level impacted
strongly on whether the males were
accepted as mates. They found that the
louder males were more successful, so
increasing the volume after a jilting makes
perfect sense.
In summary, females’ responses to
predators and amorous males are identical.
They are immobilized by the males’ calls,
which trigger their anti-predator freezing
behaviour. In effect, the male exploits a
female sensory bias against calls heralding
dangerous predators, causing her to act in a
way amenable to mating attempts.
However, the calls do not really suit classic
notions of chivalrous males wooing
females. Here, courtship seems instead to
carry decidedly manipulative undertones.
After all, males are pulling the wool over
their mates’ ears by profiting from an
existing anti-predator response in order to
mate. As males seem to be selected to
immobilize females effectively to ensure
successful copulation, it would be
interesting to investigate existing variation
across males in call quality or ability to
vary sound levels. Equally, it would be
exciting to see whether females can resist
manipulation by reliably recognizing
differences between bat sonar and male
calls.
10.1242/jeb.044529
Nakano, R., Takanashi, T., Skals, N., Surlykke, A.
and Ishikawa, Y. (2010). Ultrasonic courtship songs of
male Asian corn borer moths assist copulation
attempts by making the females motionless. Physiol.
Entomol. 35, 76-81.
Oliver Yves Martin
ETH Zurich
oliver.martin@env.ethz.ch
MIMICRY
© 2010. Published by The Company of Biologists Ltd
THE JOURNAL OF EXPERIMENTAL BIOLOGY