Download Selection On Gall Size And Placement

Selection on Gall Size 1
Selection On Gall Size And Placement
Natural selection is a major force that drives evolutionary change. Although selection
can be examined experimentally in manipulative laboratory experiments, observing the
force of selection in the wild is far more challenging. If directional selection is the rule,
then we should see changes in the phenotypic distribution of species over several
generations. Examples are the changing beak size of some finches in the Galapagos
islands (as rainfall patterns changed), the change in color of many species of moths in
the US and England as the color of trees changed during the industrial revolution and
later efforts to clean up pollution, and the change in tolerance to lead by plants near
roadsides exposed to lead pollution. There are many other such examples, but why are
there not lots and lots more? That is, why don't we see changing phenotypes for most
species?
The answer must be that selection is often stabilizing; this means some intermediate
phenotype has highest fitness. Stabilizing selection will PREVENT evolutionary change.
Stabilizing selection has been observe in nature when two or more competing
challenges must be met by organisms. This is the idea of "trade-offs" that may balance
out and result in some compromise phenotype.
This lab exercise will examine selection on a species of insect that causes galls in the
local goldenrods. During earlier lab periods you have inspected the local population of
goldenrods and must have noticed the presence of swellings on the stems. These are
galls that form when a fly, Eurosta salidaginis, lays an egg on the tip of the growing
plant. The plant produces the gall to contain the fly larva. The fly larva then feeds on the
plant tissue within the gall, and eventually pupates and emerges as an adult the next
season. What does the plant get out of this? After all, it makes the gall, yet the fly
continues to survive and eat the plant's hard-earned tissue. Many species of plants
produce galls in response to the attack of a wide variety of insect pests. Thus, galls
have evolved independently many times in the angiosperms. Perhaps galls reduce the
harm the insect might do if it were to wander randomly through the plant, consuming
more valuable tissues. In contrast, perhaps galls are strictly under the control of the
insect -- the plant doesn't have a "choice" about producing a gall. Most likely, gall
development is under some influence of both the insect and plant.
We will examine two kinds of phenotypes of the galls, their size (diameter) and
placement (how high on the stem they occur). The size is determined both by the insect
and plant (as suggested above). The placement depends on when the adult fly lays its
eggs....how high the gall occurs depends on the size of the plant when it was attacked.
What selective forces could influence these two phenotypic characters?
The fly Eurosta salidaginis is subject to predation while living within its gall. The most
important predator is a parasitoid wasp. The female wasp locates a gall, somehow
detects the presence of the fly larva within, and inserts its ovipositor through the gall
and into the fly larva. The wasp lays a single egg in the larva. The egg hatches and
consumes the fly larva, then exits the fly and finishes its development by feeding on gall
tissue. Another source of predation are birds that locate the gall and peck it open to
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feed on any insect within. Smaller woodpeckers, such as the downy woodpecker, and
chickadees find insect prey by punching a hole in the galls. Bird predation is obvious
because the gall has a hole punched in it. The woodpeckers and chickadees feed on
gall insects mostly in the winter months when other insect prey are difficult to find.
How should selection work on gall size? The wasp's ovipositor length limits the size of
gall it can use to lay its eggs; the fly larva can "hide" better within a large gall because
the wasp cannot reach the fly with its ovipositor. Thus, selection should work on the fly
to alter the plant to produce a large gall. However, the birds prefer large galls (they need
something large to grip while pecking). The birds therefore exert a selective force on the
fly to cause the plant to produce smaller galls. This would be a case of stabilizing
selection at work....two conflicting selective forces at work to produce some compromise
gall size. Let's test this hypothesis with the goldenrod galls we find at Centennial woods.
Selection on gall placement
Vermont has very harsh winters, with some days windy, others still; some days very
cold, others can be balmy; some winters can be very snowy, others dry. What effect
would all of this have on the galls and insect within? Should the fly "want" its gall to be
high on the plant where it might remain out of the snow (but be subject to attack by
birds), or would it be better off under the snow where climatic changes are buffered,
wind cannot reach, and it is protected from the birds? How could an adult female fly
alter the height of the gall that its young will inhabit? As the eggs are laid at the top of
the growing plant, the timing of egg laying must have an effect on the final placement of
the gall. Thus, selection could work on the egg laying schedule of the fly (of course lots
of other factors influence the optimal timing of egg laying).
To get some idea of answers to these questions, we need to determine how much
variation exists in the height of the galls on plants. If selection strongly favors one
height, then we should find a narrow range of gall placement on the plants. If selection
is weak, or if there are multiple selective forces, we might find a broad range of gall
heights (or might find gall height be bimodal!).
Procedure
We have waited until the end of semester to do this project because we want to be sure
that the birds have been pecking at the galls (it may still be too early to find any birdpecked balls if the weather has been mild).
The lab will travel to Centennial Woods to inspect the galls there. Let's do some basic
natural history of the goldenrod galls. Your TA will tell you which of these to address.
1. Any bird damage to be seen on the galls?
2. Check galls within the patch vs. those at the edge. Is bird damage more
common at one site or the other?
3. Are there more galls on the plants in dense goldenrod patches vs. more
sparse patches. How about in different parts of the Woods?
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4. Do any plants have more than one gall? If so, are they larger or smaller than
galls that are solitary?
Each student will sample five galls from plants that have only one gall, and one gall from
plants with multiple galls. The labs at the end of the week may have to wander around
the Woods to find enough galls! First, pick some galls randomly. You might throw a twig
over your shoulder into the goldenrod patch, then pick your way to its location (without
damaging the other plants! other students are coming later!) and examining the five
galls closest to the twig.
Measure the full height of the plant, then the height of the gall --is it one of the solitary
galls, or one of the galls that share a plant? Cut the gall off and place it into a baggie
with your data for that gall. Was the gall opened by a bird? Return to lab.
Now, measure the diameter of the gall and cut it open with a razor blade (careful not to
injure yourself or the insect within). Determine the fate of each gall. There are three
possible outcomes: The gall may contain a fly larva, a parasitoid wasp, or it may have
been opened by a bird. How to tell a fly larva from the wasp? The fly larva is a fat
creature that is often as wide as it is long and you can see the black mouthparts. The
wasp is thin and smaller and is often bent into a U shape. (Most parasitoids complete
their development on their host; our wasp species however eats the fly larva, then goes
on to eat the gall. It is therefore, both a parasitoid and herbivore!)
Add your data to the class pooled data set. The instructor will now lead you in deciding
how to analyze the data to determine if the two hypotheses (stabilizing selection for gall
size and some complex selection for placement) are supported by the data.
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