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09/22/2004
One Egg - Many Siblings
On September 11, my wife and I drove to the Brandywine region
of Pennsylvania to visit my brother and his wife, who have
moved to a retirement community close to Longwood Gardens.
Earlier that day, I had watched some of the 9/11 anniversary
ceremonies in New York in which parents and grandparents were
reciting the names of the victims of the World Trade Center
attack. For those of us in the New York area, the wound of 9/11
is still raw and there was criticism that in at least one section of
New York there was a festive street fair in progress that day.
Therefore, it felt strange that evening to find ourselves sitting on
a lawn in Pennsylvania watching fireworks, definitely not the
way New Yorkers would mark the occasion. The fireworks, in
Longwood Gardens, were the most impressive and unusual
fireworks I’ve ever seen, with overlapping rings, multicolored
rings and rings with puffs of light above and below them. Not
being in the Gardens, we weren’t privy to any patriotic words or
music that accompanied the fireworks, but assume that they were
in good taste.
During our visit, my brother and I reminisced about our youth
and I was amazed to find that we shared one particular memory.
In 1941, our mother learned to drive so she could assist our
father on the long drive from our home in Mechanicsburg,
Pennsylvania to visit our grandparents in Denver, Colorado.
There were no interstate highways in those days. My mother
was passing a truck on a road outside Red Oak, Iowa and found
herself partly on the left-hand shoulder. She seemed not to have
the car under complete control and Dad grabbed the wheel. We
swung across the road and slammed into a guardrail that
prevented us from going down an embankment. Happily, the
truck driver had slowed down and didn’t hit us.
Our hood flew up and the horn was blowing. My brother, 10 at
the time, said to our mother, “I knew it. I knew you’d do it!”
We now agree that our father shared responsibility for the
accident. We ended up in a garage in Red Oak, left the car there
to be fixed and caught a train to Denver. I can still see Grandma
and Grandpa greeting us in the Denver station. It’s no surprise
that both my brother and I remember the accident quite clearly.
What surprised me was that, 63 years later, my brother said that
the water in that Red Oak garage was the best water he’s ever
tasted. I too can still taste that water. Were we both in shock
and revived by the water or was it really outstanding water?
We’ll never know.
During our visit, I also found there was sibling rivalry that I,
being the older brother, had not recognized. I had preceded my
brother at Dickinson College by several years. When he arrived
there, he suffered the inevitable comparisons with his brother and
resolved to follow a different path. I went to Pitt for my graduate
work; he went to Penn State, Pitt’s archrival at the time. I
worked at Bell Labs; he was a professor at the University of
Delaware. Neither of us realized, however, that our paths were
remarkably similar. We both initially had visions of becoming
organic chemists. Strangely, the professor at Dickinson who
turned me on to organic chemistry turned my brother off the
subject. I was turned off by a course in organic chemistry at Pitt
taught by Professor Dull. He was a very nice man but the course
was indeed dull! My brother and I both ended up being physical
chemists.
One day, after returning from our visit, I found an angry wasp
trapped between a window and the screen and spent about a half
hour getting it into a position where I could dispatch him (or
her). I was musing about the wasp and my discovery of the
sibling rivalry when I ran across an article by Andy Gardner and
Stuart West in the September 3 issue of Science. The article is
titled “Spite Among Siblings” and deals with sibling interactions
in, appropriately, wasps! The wasp in question is Copidosoma
sosares, a “polyembryonic” parasitic wasp. Polyembryony was
new to me and even the explanatory statement “A single wasp
egg proliferates asexually (clonally) to produce multiple
larvae…” left me unsatisfied.
Visits to the Web sites of the Natural Environment Research
Council in the UK and the honorary fraternity Sigma Xi
enlightened me. In a polyembryonic parasitic wasp, the female
wasp selects either a caterpillar or a caterpillar egg and deposits a
single, maybe a couple of eggs, in the host. The polyembryony
is the fact that a single egg develops into anywhere from several
hundred to several thousand larvae! Polyembryony isn’t
confined to wasps. The pregnant female nine-banded armadillo
invariably produces a set of four identical siblings of the same
sex. Apparently, her uterus can only accommodate one
implanted egg. However, that egg then splits into four
genetically identical eggs. The polyembryonic wasp egg beats
the armadillo egg by a factor of hundreds.
I have trouble coming to grips with what happens to that single
wasp egg. The egg gives rise to these hundreds or thousands of
genetically identical clones. When I think of clones, whatever
the cloned creature, I think that the clones pretty much resemble
each other. Not so with this wasp. There are two distinct types
of larvae. One is the “soldier” larva, a snake-like little critter
with mandibles that can chomp down on other larvae, as well as
on the caterpillar. The other, “normal” wasp larva is more like a
fat blob that goes on to develop into a full-grown wasp capable
of reproduction.
The soldier larva is sterile and its primary function is to defend
the normal larvae. When the latter pupate and get ready to
emerge into the world as full-fledged wasps, the soldiers die.
Research groups led by Ian Hardy of the University of
Nottingham and by Mike Hardy of the University of Georgia
have looked closely into the sibling interactions of the soldiers
with other larvae. They’ve shown that the difference between
the soldier and the normal larvae is that the normal larvae inherit
a germ cell while the soldiers do not. Without the germ cell they
can’t reproduce.
These researchers have studied the dining habits of the soldiers
by labeling certain larvae with fluorescent tracers that light up
the stomachs of the soldiers when they become items on the
soldiers’ menus. The workers have also done experiments in
which they introduce alien larvae and larvae of full brothers and
sisters but are not genetically identical into a caterpillar that
already contains a brood of developing larvae.
When the soldiers hatch, they launch into murdering other larvae.
The investigators have found that, as any good sibling would do,
they tend not to eat their closest relatives but concentrate on the
aliens and the brothers or sisters that aren’t as closely related.
How do they recognize their relatives? There’s something in the
membrane that surrounds each larva that clues the marauding
soldier about the kinship of his target. The researchers have
performed microsurgery to remove membranes and transplant
membranes and have demonstrated that it’s the membrane, not
the larva, that determines whether the soldier eats the critter.
The soldier’s altruistic behavior in eating only his more distant
siblings allows the normal larva who are his closest relatives to
emerge as wasps and carry on the lineage. A surprising finding
was that even when the food supply in the caterpillar was
running short the soldiers still avoided eating their closest
relatives. This reluctance to eat one’s close sibling, even in the
face of starvation, remains a puzzle.
I’d like to thank my brother, and his wife, not only for a most
pleasant visit and but also for providing material for a segue into
the wasp sibling story. And, lest I leave the wrong impression,
there is no puzzle should my brother and I be together and faced
with starvation – I would not even consider eating him!
Allen F. Bortrum
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