For several decades now, amphibian species worldwide have been in decline. A significant proportion of the decline seems to be due to the spread of the chytrid fungus, Batrachochytrium dendrobatidis (Bd). Chytrid sporangia reside within the epidermal cells of infected animals, animals that consequently show areas of sloughed skin. They can also be lethargic, which is expressed through failure to hide and failure to flee. The infection cycle typically takes four to five days, at the end of which zoospores are released from sporangia into the environment. In some amphibian species, mortality rates approach 100%; other species seem able to survive the infection.
Apart from direct amphibian-to-amphibian contact, what is the most likely means by which the zoospores spread from one free-living amphibian to another?
A) by wind-blown spores
B) by flagella
C) by cilia
D) by pseudopods
E) by hyphae
Answer: B
The chytrid sporangia reside within the amphibian epidermal cells. Consequently, which term(s) apply to Bd?
1. ectosymbionts
2. parasites
3. commensals
4. pathogens
5. endosymbionts
A) 1 and 2
B) 1 and 3
C) 2 and 4
D) 2, 3, and 5
E) 2, 4, and 5
Answer: E
Which of the following are protists, the organisms thought to share the closest ancestor with the chytrids?
A) nucleariids
B) choanoflagellates
C) zygomycetes
D) algae
E) diplomonads
Answer: A
Sexual reproduction has not been observed in Bd. A Bd sporangium initially contains a single, haploid cell. Which of the following processes must be involved in generating the multiple zoospores eventually produced by each sporangium?
1. S phase
2. cytokinesis
3. mitosis
4. meiosis
A) 1 and 2
B) 1 and 3
C) 2 and 3
D) 1, 2, and 3
E) 1, 2, and 4
Answer: D
Sexual reproduction has not been observed in Bd. If its morphology and genetics did not identify it as a chytridiomycete, then to which fungal group would Bd be assigned?
A) ascomycetes
B) zygomycetes
C) glomeromycetes
D) basidiomycetes
E) deuteromycetes
Answer: E
If infection primarily involves the outermost layers of adult amphibian skin, and if the chytrids use the skin as their sole source of nutrition, then which term best applies to the chytrids?
A) anaerobic chemoautotroph
B) aerobic chemoautotroph
C) anaerobic chemoheterotroph
D) aerobic chemoheterotroph
Answer: D
If Bd cannot grow properly at temperatures above 28°C (82°F), then, assuming the amphibians can survive, in which time or place should the chytrid infection proceed most rapidly?
1. cooler months
2. warmer months
3. lower altitudes
4. higher altitudes
A) 1 or 3
B) 1 or 4
C) 2 or 3
D) 2 or 4
Answer: B
What makes it risky to rely on the presence of chitin in adult amphibian skin as the sole positive test for the presence of chytrids?
A) Other mycoses may be in progress in the same amphibian simultaneously.
B) The amphibian may harbor arthropod ectoparasites simultaneously.
C) Bacterial infections may be simultaneously underway in the amphibian.
D) Three of the responses above are correct.
E) Two of the responses above are correct.
Answer: E
The fact that infection by Bd causes lethargy in many infected amphibians can have what effect on efforts to accurately census the numbers of dead or dying amphibians at a particular time, in a particular habitat?
A) It can cause underestimation, due to infected amphibians preferring to seek out refuges relative to uninfected amphibians.
B) It can cause underestimation, due to increased predation on, and removal of, infected amphibians relative to uninfected amphibians.
C) It can cause overestimation, because infected frogs should be more readily observable to human census-takers than should uninfected amphibians.
D) All three of the above statements are plausible.
E) Two of the above statements are plausible.
Answer: E
When adult amphibian skin harbors populations of the bacterium, Janthinobacterium lividum (Jl), chytrid infection seems to be inhibited. Which of the following represents the best experimental design for conclusively determining whether this inhibition is real?
A) Inoculate uninfected amphibians with Jl, and determine whether the amphibians continue to remain uninfected by chytrids.
B) Inoculate infected amphibians with Jl, and determine whether the amphibians recover from infection by chytrids.
C) Take infected amphibians and assign them to two populations. Leave one population alone; inoculate the other with Jl. Measure the rate at which infection proceeds in both populations.
D) Take infected amphibians and assign them to two populations. Inoculate one population with a high dose of Jl; inoculate the other with a low dose of Jl. Measure the survival frequency in both populations.
Answer: C
A researcher took water in which a Jl population had been thriving, filtered the water to remove all bacterial cells, and then applied the water to the skins of adult amphibians to see if there would subsequently be a reduced infection rate by Bd when frog skins were inoculated with Bd. For which of the following hypotheses is the procedure described a potential test?
A) the hypothesis that a toxin secreted by Jl cells kills Bd cells when both are present together on frog skin
B) the hypothesis that Jl cells infect and kill Bd cells when both are present together on frog skin
C) the hypothesis that Jl outcompetes Bd when both are present together on a frog's skin
D) the hypothesis that the presence of Jl on frog skin causes a skin reaction that prevents attachment by Bd cells
Answer: A
