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10/30/2003

False Advertising in the Plant World

Since my kidney surgery nearly six weeks ago, my columns have
been primarily about things biological in nature. I’ve concluded
that not only have I become more interested in my own body’s
biology but also, in my weakened condition, find it easier to
understand and write about such simpler creatures as ants, bees
and the like. So, in that spirit, why not write about wasps?

The October 17 2003 issue of Science has articles dealing with
wasps and their manipulation by two quite different species, an
orchid and a virus. The orchid story’s detailed in a paper titled
“The Chemistry of Sexual Deception in an Orchid-Wasp
Pollination System”. There are seven authors on the paper and
their affiliations support an observation that I’ve made in earlier
columns. The authors of the article are Wittko Francke and
colleagues at the University of Hamburg in Germany and Rod
Peakall and two former colleagues at the Australian National
University in Canberra. There seems to be a strong tendency in
the biological field for workers in Europe to get hooked up with
researchers in Australia. My theory, completely unsubstantiated,
is that the Europeans want to escape the cold winters by going
Down Under, where it is summer.

The orchid in question is the Australian orchid Chiloglottis
trapeziformis. Let’s just call it C. trap. Like all flowering plants
that rely on animals or insects for pollination, it has had to figure
out some way of attracting the pollinator to do the job. We’ll see
that C. trap does indeed set a trap for its own particular insect
pollinator, the male of the thynnine wasp species Neozeleboria
cryptoides. We’ll just refer to him as the wasp.

In case you’re a bit rusty on the specifics of the pollination
process (I was), let’s review the subject briefly. As usual, I
turned to my trusty 1962 World Book Encyclopedia to enlighten
me. The flower produces pollen in its “anther”, which is like a
sac and is the equivalent of the male organ of reproduction. The
female part of the flower consists of the “stigma” and the ovary.
The stigma receives the pollen and from there the pollen travels
down through pollen tubes to the “ovules”, the egg cells residing
in the ovary. The pollen grains combine with the ovules, which
then develop into seeds to generate future plants.

Some flowers are pollinated only by pollen from other flowers
(cross-pollination) while others are pollinated by their own
pollen (self-pollination). Based on a visit to Australian National
University Web site, I conclude that C. trap belongs to the cross-
pollination group. Dr. Peakall is an associate professor in the
School of Botany and Zoology at the university and has
published extensively on orchid pollination and interactions with
wasps and other insects. He and his associates have taken
particular interest in the sexually deceptive practices of certain
orchid varieties in both Australia and Europe.

Most flowers offer their pollinators some sort of nectar or other
refreshment to repay their guests for pollinating them. However,
C. trap and other orchids have nothing to offer in that line. So,
what is a desperate orchid to do to get pollinated? One approach,
successfully employed by some sexually deceptive orchids, is to
evolve an appearance remarkably resembling a female wasp.
The male wasp falls for the trick and tries to mate with the
“wasps”, spreading the pollen around and the orchid reproduces.

Another deceptive approach is the subject of the Science paper.
Many flowers attract their insect benefactors by emitting enticing
odors. Typically, these seductive scents arise from combinations
of relatively common chemical compounds. The authors of the
paper decided to determine what kinds of compounds were
emitted by C. trap to attract the male wasps. They managed to
extract enough material from the appropriate parts of C. trap
orchids for analysis by several different modern analytical
techniques. You can imagine that the amounts of material were
quite small. They also carried out microreactions to help confirm
the chemical nature of the extracted material.

They found that, instead of a mixture of compounds, there was
just a single compound, 2-ethyl-5propylcyclohexan-1,3-dione,
that did the trick. Mercifully, they gave the compound a shorter
name, “chiloglottone” (derived from Chiloglottis trapeziformis).
To confirm that chiloglottone was indeed what was attracting the
male wasps, the researchers went into the field and treated
orchids and plastic beads with both the extracts from C. trap and
with synthetic chiloglottone. Sure enough, the male wasps were
attracted to both the treated orchids and the treated beads. Some
of the males even attempted to mate with the beads. The wasps
were not attracted to beads that were not treated with the
chiloglottone.

This unusual case in C. trap of a single chemical compound
triggering such an enthusiastic response from the male wasp
contrasts with the cases of some 80 species of a European
sexually deceptive orchid. For example, one European species
employs a mixture of 14 common chemicals to attract its own
particular insect pollinator.

But that’s not the end of the story. The researchers wanted to
know what scent the female wasp uses to attract the male and
compare it with the orchid’s deceptive scent. They extracted
material from the heads of the female wasps and analyzed it.
They then used the antennae from male wasps in some kind of
technique I’ve never heard of to check the various compounds
they found in the head extracts. They only found one compound
that elicited a response. Sure enough, the compound was
chiloglottone!

Remarkably, the orchid has evolved to make an exact copy of
this sex pheromone used by the female wasp. This may be the
first time that any plant has been found to evolve and copy
exactly the sex pheromone of its pollinator. How sneaky can you
get? Obviously there’s a wealth of research remaining to be
done. For example, how did the orchid learn of the chiloglottone
without some contact with the female wasp? Could it be that the
female wasp actually copied the orchid and not vice versa? Old
Bortrum’s getting skeptical in his old age! Another question I
have is, does the male wasp live long enough that he finally
tumbles to the fact that he’s been attempting to mate with a
flower? Obviously, if the orchid requires cross-pollination, the
wasp has to fall for the deception at least a couple of times to
pollinate another orchid.

This isn’t the only case of a wasp being manipulated. The same
issue of Science contains a news article by Elizabeth Pennisi
discussing the Australian work and that of a French researcher,
Julien Viraldi, whose work appears in the online edition of
Science. Viraldi studied the egg laying behavior of a certain type
of parasitic wasp. This female wasp has the inimitable habit of
laying its eggs by inserting them into young fruit flies. Usually,
the female limits her insertions to one egg per fly. As the fruit
fly matures and the egg hatches the wasp larvae dines on the fly,
eventually doing in the unsuspecting host.

Wasps from Portugal were found to be quite finicky about
limiting their parasitic behavior to one egg per fly. However,
when French wasps were housed with a limited number of flies,
the French would stick in four or more eggs in one larva. As we
know, the French don’t tend to conform. The strange thing was
that, when both Portuguese and French wasps colonized the same
fly larvae, most of the resulting Portuguese wasp offspring no
longer stuck to one egg per fly. Some three-quarters of the
Portuguese offspring, and even their own offspring, followed the
French example.

This would seem to be a remarkable genetic change in just one
generation. However, Viraldi and his colleagues concluded that
it’s not a genetic mutation, but a virus that causes the change in
egg laying behavior. I’m assuming that the French wasps passed
along the virus. One postulate is that this virus somehow
destroys the ability of the wasps to detect that an egg has already
been laid. Hence the sudden change in habit. To test the
proposal, the researchers injected the virus into finicky females
and found that the female then became less finicky, inserting
more than one egg. This manipulation of the wasp behavior by
the virus benefits the virus since it appears to spread only among
wasp eggs sharing a fly larva.

It’s a strange and deceptive world out there. We all have to be
on guard! After all, wasps aren’t the only creatures being
manipulated.

Allen F. Bortrum



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-10/30/2003-      
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Dr. Bortrum

10/30/2003

False Advertising in the Plant World

Since my kidney surgery nearly six weeks ago, my columns have
been primarily about things biological in nature. I’ve concluded
that not only have I become more interested in my own body’s
biology but also, in my weakened condition, find it easier to
understand and write about such simpler creatures as ants, bees
and the like. So, in that spirit, why not write about wasps?

The October 17 2003 issue of Science has articles dealing with
wasps and their manipulation by two quite different species, an
orchid and a virus. The orchid story’s detailed in a paper titled
“The Chemistry of Sexual Deception in an Orchid-Wasp
Pollination System”. There are seven authors on the paper and
their affiliations support an observation that I’ve made in earlier
columns. The authors of the article are Wittko Francke and
colleagues at the University of Hamburg in Germany and Rod
Peakall and two former colleagues at the Australian National
University in Canberra. There seems to be a strong tendency in
the biological field for workers in Europe to get hooked up with
researchers in Australia. My theory, completely unsubstantiated,
is that the Europeans want to escape the cold winters by going
Down Under, where it is summer.

The orchid in question is the Australian orchid Chiloglottis
trapeziformis. Let’s just call it C. trap. Like all flowering plants
that rely on animals or insects for pollination, it has had to figure
out some way of attracting the pollinator to do the job. We’ll see
that C. trap does indeed set a trap for its own particular insect
pollinator, the male of the thynnine wasp species Neozeleboria
cryptoides. We’ll just refer to him as the wasp.

In case you’re a bit rusty on the specifics of the pollination
process (I was), let’s review the subject briefly. As usual, I
turned to my trusty 1962 World Book Encyclopedia to enlighten
me. The flower produces pollen in its “anther”, which is like a
sac and is the equivalent of the male organ of reproduction. The
female part of the flower consists of the “stigma” and the ovary.
The stigma receives the pollen and from there the pollen travels
down through pollen tubes to the “ovules”, the egg cells residing
in the ovary. The pollen grains combine with the ovules, which
then develop into seeds to generate future plants.

Some flowers are pollinated only by pollen from other flowers
(cross-pollination) while others are pollinated by their own
pollen (self-pollination). Based on a visit to Australian National
University Web site, I conclude that C. trap belongs to the cross-
pollination group. Dr. Peakall is an associate professor in the
School of Botany and Zoology at the university and has
published extensively on orchid pollination and interactions with
wasps and other insects. He and his associates have taken
particular interest in the sexually deceptive practices of certain
orchid varieties in both Australia and Europe.

Most flowers offer their pollinators some sort of nectar or other
refreshment to repay their guests for pollinating them. However,
C. trap and other orchids have nothing to offer in that line. So,
what is a desperate orchid to do to get pollinated? One approach,
successfully employed by some sexually deceptive orchids, is to
evolve an appearance remarkably resembling a female wasp.
The male wasp falls for the trick and tries to mate with the
“wasps”, spreading the pollen around and the orchid reproduces.

Another deceptive approach is the subject of the Science paper.
Many flowers attract their insect benefactors by emitting enticing
odors. Typically, these seductive scents arise from combinations
of relatively common chemical compounds. The authors of the
paper decided to determine what kinds of compounds were
emitted by C. trap to attract the male wasps. They managed to
extract enough material from the appropriate parts of C. trap
orchids for analysis by several different modern analytical
techniques. You can imagine that the amounts of material were
quite small. They also carried out microreactions to help confirm
the chemical nature of the extracted material.

They found that, instead of a mixture of compounds, there was
just a single compound, 2-ethyl-5propylcyclohexan-1,3-dione,
that did the trick. Mercifully, they gave the compound a shorter
name, “chiloglottone” (derived from Chiloglottis trapeziformis).
To confirm that chiloglottone was indeed what was attracting the
male wasps, the researchers went into the field and treated
orchids and plastic beads with both the extracts from C. trap and
with synthetic chiloglottone. Sure enough, the male wasps were
attracted to both the treated orchids and the treated beads. Some
of the males even attempted to mate with the beads. The wasps
were not attracted to beads that were not treated with the
chiloglottone.

This unusual case in C. trap of a single chemical compound
triggering such an enthusiastic response from the male wasp
contrasts with the cases of some 80 species of a European
sexually deceptive orchid. For example, one European species
employs a mixture of 14 common chemicals to attract its own
particular insect pollinator.

But that’s not the end of the story. The researchers wanted to
know what scent the female wasp uses to attract the male and
compare it with the orchid’s deceptive scent. They extracted
material from the heads of the female wasps and analyzed it.
They then used the antennae from male wasps in some kind of
technique I’ve never heard of to check the various compounds
they found in the head extracts. They only found one compound
that elicited a response. Sure enough, the compound was
chiloglottone!

Remarkably, the orchid has evolved to make an exact copy of
this sex pheromone used by the female wasp. This may be the
first time that any plant has been found to evolve and copy
exactly the sex pheromone of its pollinator. How sneaky can you
get? Obviously there’s a wealth of research remaining to be
done. For example, how did the orchid learn of the chiloglottone
without some contact with the female wasp? Could it be that the
female wasp actually copied the orchid and not vice versa? Old
Bortrum’s getting skeptical in his old age! Another question I
have is, does the male wasp live long enough that he finally
tumbles to the fact that he’s been attempting to mate with a
flower? Obviously, if the orchid requires cross-pollination, the
wasp has to fall for the deception at least a couple of times to
pollinate another orchid.

This isn’t the only case of a wasp being manipulated. The same
issue of Science contains a news article by Elizabeth Pennisi
discussing the Australian work and that of a French researcher,
Julien Viraldi, whose work appears in the online edition of
Science. Viraldi studied the egg laying behavior of a certain type
of parasitic wasp. This female wasp has the inimitable habit of
laying its eggs by inserting them into young fruit flies. Usually,
the female limits her insertions to one egg per fly. As the fruit
fly matures and the egg hatches the wasp larvae dines on the fly,
eventually doing in the unsuspecting host.

Wasps from Portugal were found to be quite finicky about
limiting their parasitic behavior to one egg per fly. However,
when French wasps were housed with a limited number of flies,
the French would stick in four or more eggs in one larva. As we
know, the French don’t tend to conform. The strange thing was
that, when both Portuguese and French wasps colonized the same
fly larvae, most of the resulting Portuguese wasp offspring no
longer stuck to one egg per fly. Some three-quarters of the
Portuguese offspring, and even their own offspring, followed the
French example.

This would seem to be a remarkable genetic change in just one
generation. However, Viraldi and his colleagues concluded that
it’s not a genetic mutation, but a virus that causes the change in
egg laying behavior. I’m assuming that the French wasps passed
along the virus. One postulate is that this virus somehow
destroys the ability of the wasps to detect that an egg has already
been laid. Hence the sudden change in habit. To test the
proposal, the researchers injected the virus into finicky females
and found that the female then became less finicky, inserting
more than one egg. This manipulation of the wasp behavior by
the virus benefits the virus since it appears to spread only among
wasp eggs sharing a fly larva.

It’s a strange and deceptive world out there. We all have to be
on guard! After all, wasps aren’t the only creatures being
manipulated.

Allen F. Bortrum