Name ______________________________
Spring 2011 - Biol 4800 - Symbiotic Interactions - Mid-term Exam
The exam is in 5 sections; you should allot your time accordingly. Please use the backs of the exam pages
for Parts IV and V.
Part I. Define the following terms. If appropriate, use a figure to highlight key features. (Do 4 of 5). (4 pts X
4 = 16 pts.).
1. Diffuse coevolution
This is an umbrella term that describes long-term, reciprocal evolutionary interactions between at least two
groups of organisms. The response of pollinating butterflies (e.g. wing dimensions, proboscis dimensions) to
flower structure (e.g. corolla length) is an example of diffuse coevolution. Reciprocal responses in traits
associated with the immune systems of groups of hosts (e.g. circulating antibodies) to a wide range of
parasites is another example of diffuse coevolution.
2. Phenotypic matching between two coevolving partners
In some pairwise coevolutionary interactions, traits associated with protection in one species (e.g. TTX
concentrations in the skin of newts) are closely associated with exploitation countermeasures in the other
species (e.g. TTX tolerance in garter snakes). Both adaptive responses are metabolically costly, such that one
phenotype is reduced in the absence of the other. In the newt/snake example, this process leads to matching in
the expression of the phenotypes. Thus, in regions where TTX concentrations in newts are low, TTX tolerance
in snakes is low, whereas in areas where TTX concentrations are high, TTX tolerance is high.
3. Congruent host-parasite phylogeny
Host parasite phylogenies are perfectly congruent when the phylogeny of a host lineage matches one-for-one
with the phylogeny of the lineage of their parasites. When host ‘A’ diverges into host ‘B’ and ‘C’, parasite ‘a’
diverges into parasites ‘b’ and ‘c’. This type of pattern was observed among species of pocket gopher
infected with species of chewing lice. Congruent host/parasite phylogenies can be used to support the
importance of host/parasite coevolution between pairs of interacting species.
4. Tachyzoite vs. sporozoite
Tachyzoites are a rapidly multiplying (via multiple fission) stage found in some apicomplexan parasites such
as Toxoplasma. These form the characteristic cysts (bradyzoites) within a range of tissue types in a variety of
hosts that have been exposed to oocysts in cat feces, or to tissue cysts in infected tissue. Sporozoites are the
product of sporogony in Apicomplexan parasites. These tend to be relatively long and thin, facilitating their
entry in to specific host cell types (e.g. invasion of liver hepatocytes by Plasmodium spp.).
5. Evolution of nematode parasitism via phoresy
The evolution of nematode parasitism has been well documented in some lineages of nematodes. In some
cases, ancestoral types utilized ‘hosts’ for transportation between food sources (e.g. flowers). Intermediate
types use their hosts for transport, yet also feed intermittently on/in their hosts. The most derived types are
obligate parasites. The nematodes of fig wasps evolved from phoretic ancestors.
Part II. Short Answer. For these parasitic diseases of humans, describe the causative agent, and the
mechanism leading to host pathology. Do 4 of 5. (4 pts X 4 = 16 pts.).
1. Cryptosporidiosis
Merozoites of this Apicomplexan penetrate epithelial cells of a range of hosts, including humans.
Synchronized development and reproduction of intracellular merozoites can lead to massive infections of
many epithelial cells along the gut. Accommodation for developing merozoites leads to cell distortion and
death, leading to complications associated with water re-absorption, dehydration, and diarrhea.
2. Congenital toxoplasmosis
Tachzoites, and some bradyzoites, can be transferred from mother to fetus via the placenta. In cases where
mothers have been exposed previously to Toxoplasma, maternal antibodies will be transferred to the
developing fetus, leading to the arresting of Tachzoites within non-pathogenic, slowly-reproducing
bradyzoites. In cases where mothers are exposed to Toxoplasma for the first time during the initial stages of a
pregnancy, she will transfer tachzoites, but not maternal antibodies. The lack of host immunity will allow the
tachyzoites to rapidly infect large numbers of tissue cells, especially in the brain, leading to severe
developmental complications.
3. Visceral larval migrans (vlm)
When L3 larvae of canine or feline hookworm (Ancylostoma spp.) penetrate humans (a non-host), they
undergo prolonged and extensive migrations, sometimes through critical tissues such as the heart, brain,
muscles of the eye, and liver. Migration of the larvae through the viscera can lead to a wide range of severe
complications, depending on the tissue type.
4. Trichenosis
L3 larvae penetrate the gut, enter the circulation system, and ultimately encyst within nurse cells in a wide
range of striated muscle types. Encystment within a nurse cell leads to highly modified, non-functional
muscles, leading to tissue-specific complications, especially associated with the diaphragm, the heart, and
muscles associated with vision. In long-term infections (>10 years), nurse cells ultimately calcify, creating
severe problems with muscle function.
5. Elephantiasis
One outcome of infection with adult filarid nematodes of the genus Wucheria is the gross deformation of
lower limbs of infected humans. Large adult worms are found associated with various lymph systems in the
lower extremities, often leading to permanent obstruction of key blood vessels. It is likely that the worms
contribute directly to the swelling, but it also results from a host’s immunopathogenic response to the worms.
Part III. Explain the following phenomena. Do 4 of 5. (4 pts X 4 = 16 pts.).
1. Determination of Rhizobium specificity for its legume host.
2. Transmission of Plasmodium into humans by mosquitoes infected with patent (sporozoites in the salivary
glands) versus non-patent (oocysts only) infections.
3. Tolerance of newt toxin (TTX) by garter snakes.
4. Sexual reproduction in Cryptosporidium.
5. Maintenance of low-oxygen concentration in legume nodules to support nitrogen fixation by Rhizobium.
Part IV. Long Answer. Answer the following question in a single, well-constructed paragraph, using full
sentences. (1 X 8 pts = 8 pts.).
1. Schistosomiasis (e.g. S. mansoni) and falciparum malaria are diseases caused by two well-known human
parasites. In both cases, the pathogenic outcomes can be considered parasite strategies that evolved to address
particular problems that the parasite must solve to reproduce. For both parasites, describe the problem that
they are trying to solve, the strategy they use to solve it, and the linkage to host pathology.
Part V. Parasite functional morphology and life-cycle diversity.
1. In a labeled diagram, describe the functional morphology of this adult parasite. Focus on
morphologies associated with attachment, reproduction, and nutrition. This parasite was dissected
from the gut of an infected vertebrate. (5 pts)
2. In two labeled diagrams with appropriate arrows, contrast the general life-cycles of the parasites
labeled A and B. (6 pts)
3. In two labeled diagram with appropriate arrows, compare the general life cycles of these two
nematodes. Include stages that occur within their hosts. ‘A’ is from the gut of a local horse (an
Ascarid), ‘B’ is a histological section of an experimentally-infected mouse. (6 pts)
4. Describe the general morphology and life-cycle of this marine parasite. (3 pts)
5. Describe the general morphology and life-cycle of this local parasite. (3 pts)