Deck 23: Microbial Symbioses

Question 1

Which of the following is a common feature of primary insect symbionts?

A) accelerated mutation rates only
B) extreme genome reduction only
C) high A+T content only
D) accelerated mutation rates, extreme genome reduction, and high A+T content

Answer 1

D

Question 2

Myc factors are

A) a type of strigolactone hormone.
B) lipochitin oligosaccharide signaling proteins produced by arbuscular mycorrhizae.
C) examples of chemicals that mycorrhizae obtain from the soil.
D) Carbohydrate structures that form connections between mycorrhizae and plants.

Answer 2

B

Question 3

Horizontal gene transfer between insects and their symbionts

A) happens frequently because of coevolution.
B) causes genome reduction in the symbiont and genome expansion in the host.
C) causes genome reduction in both the host and the symbiont.
D) has been observed, but is considered a rare event.

Answer 3

D

Question 4

The mutualism of Azolla-Anabaena is useful for

A) aquaculturalists.
B) corn farmers.
C) rice farmers.
D) tropical forest community succession.

Question 5

What is the primary difference between a bacteroid a bacterial cell?

A) There are genetic differences that can be detected using 16S rRNA gene sequencing.
B) They are biochemically very different from bacterial cells.
C) Their cell morphology is different from bacterial morphology.
D) They have genetic differences that can be detected using DNA sequencing.

Question 6

The specificity of certain rhizobia to infect only particular plants is in part due to the

A) abundance of nutrients present in the soil.
B) nifH genes they possess.
C) rhizobial lipids that act as signaling molecules.
D) season (time of year).

Question 7

What is the role of the vir genes on a Ti plasmid?

A) to cause crow gall disease (virulence)
B) to confer resistance to viral infection
C) to initiate replication of the plasmid
D) to allow T-DNA transfer

Question 8

How do arbuscular mycorrhizae (AM) help plants obtain more nutrients from the soil?

A) By-products of AM metabolism serve as primary nutrient sources for the plants.
B) The AM increase the total surface area to absorb more nutrients.
C) The hyphae develop as specialized nodules that directly produce nutrients for the plant.
D) Signaling molecules in the plant are passed to the AM to initiate electron flow, which is in turn used to create ATP.

Question 9

Frankia are

A) a type of arbuscular mycorrhizae.
B) glomerulomycetes that live in root nodules.
C) a species of tree that has root nodules.
D) actinomycetes that fix nitrogen.

Question 10

The fungal division containing arbuscular mycorrhizae is the

A) Betaproteobacteria.
B) Basidiomycota.
C) Glomerulomycota.
D) Rhizobia.

Question 11

Most nitrogen-fixing bacteria symbiotically associated with plants are called

A) bacteroids.
B) mycorrhizae.
C) rhizobia.
D) symbiodinia.

Question 12

The function of leghemoglobin in root nodules is to

A) bind oxygen.
B) chelate iron.
C) produce iron.
D) produce nitrogen.

Question 13

Which compounds serve as major sources of energy for ruminant animals?

A) acetic acid and glucose
B) acetic, butyric, and propionic acids
C) dimethylsulfoniopropionate, gluconate, and protocatechuate
D) glucose and sucrose

Question 14

Plant root nodules are

A) formed from gall-rotting bacteria that decompose plant roots.
B) harmful to plants, because the bacteria outcompete the plants for nutrients.
C) sites where nitrogen fixation occurs.
D) structures created by fungi and are found in all agricultural crops.

Question 15

In insects, primary symbionts are ________, while secondary symbionts are ________.

A) heritable / not heritable
B) able to replicate outside of the host / obligate symbionts
C) required for host reproduction / not required for host reproduction
D) obligate symbionts / sometimes free-living

Question 16

The rumen is an ________ habitat that depends on ________ to digest cellulose for ruminant animals.

A) aerobic / cellulolytic fungi
B) aerobic / cellulolytic and fermentative bacteria
C) anaerobic / methanogens
D) anaerobic / cellulolytic and fermentative bacteria

Question 17

Which bacterium contains a large Ti plasmid and causes crown gall disease in plants?

A) Agrobacterium rhizogenes
B) Agrobacterium tumefaciens
C) Aliivibrio fischeri
D) Butyrivibrio fibrisolvens

Question 18

Where are ectomycorrhizae most highly developed?

A) boreal and temperate forests
B) tundra
C) tropical dry forests
D) tropical rain forests

Question 19

Two organisms that both benefit from each other are in a symbiotic relationship called

A) ammensalism.
B) commensalism.
C) mutualism.
D) parasitism.

Question 20

The insect symbiont Wolbachia can be used to suppress disease transmission by mosquitoes because

A) Wolbachia supplies essential nutrients to the mosquito Culex quinquefasciatus.
B) only Wolbachia-infected mosquitoes can transmit viral diseases such as dengue fever.
C) Wolbachia infections can spread rapidly through a mosquito population, killing all the mosquitoes.
D) Wolbachia-infected male mosquitoes sterilize uninfected female mosquitoes.

Question 21

Why do obligate symbionts often contain lower G+C content when compared to free-living organisms?

A) Ancestral symbionts had low GC content genomes by chance, and these low GC content genomes are passed onto their progeny.
B) Organisms with a low G+C content organisms are at a selective disadvantage to those with a high G+C content when free-living.
C) It is more difficult to replicate high G+C content genomes than to replicate low G+C content genomes.
D) Two common spontaneous mutations change GC pairs into AT pairs and symbionts usually have fewer DNA repair mechanisms to fix these compared with free-living organisms.

Question 22

What is the primary benefit to phototrophic microorganisms associated with corals?

A) Calcium and carbonate ions are released, which help buffer the pH and prevent dramatic pH shifts.
B) The coral skeleton is used as a source of bicarbonate for the autotrophic growth of the phototrophs.
C) The coral skeleton is an extremely efficient light-gathering structure that greatly enhances light harvesting for photosynthesis.
D) The water temperature in and around the coral skeleton is much warmer than elsewhere.

Question 23

Ruminants digest bacterial cells as a primary source of

A) carbohydrates.
B) vitamins and proteins.
C) proteins.
D) vitamins.

Question 24

How are root nodule and arbuscular mycorrhizal symbioses similar?

A) They both use lipochitin oligosaccharide signaling factors to initiate root colonization.
B) They both supply nitrogen to the host plant through nitrogen fixation.
C) They both increase absorption of nutrients from soil.
D) They are both required for the growth and reproduction of their host plants.

Question 25

What is one reason that acetogenesis is favored over methanogenesis in the termite gut?

A) The environment is anoxic.
B) Methanogens can use a wide range of substrates, but acetogens specialize in those produced in the termite gut.
C) Methanogens are not found in the termite gut.
D) Acetogens are better at colonizing the center of the gut, which is rich in H₂.

Question 26

In mycorrhizal mutualisms between plants roots and fungi

A) the plant supplies carbohydrates to the fungus and the fungus supplies phosphorus and nitrogen to the plant.
B) the plant supplies water to the fungus and the fungus supplies essential amino acids to the plant.
C) the plant protects the fungus from predation and the fungus supplies carbohydrates to the plant.
D) the fungus infects the plant roots, stimulating plant growth through myc factors that act as growth hormones in the plant.

Question 27

Secondary symbionts in insects often live

A) inside cells only.
B) in hemolymph only.
C) inside bacteriocytes only.
D) inside cells, in hemolymph, or inside bacteriocytes.

Question 28

During the Jurassic Period, several different mammalian lineages independently evolved an herbivorous lifestyle for obtaining energy. This has resulted in

A) different digestive patterns in herbivorous animals, some that depend on gut microbiota to digest plant material and some that do not.
B) different digestive patterns in herbivorous animals that all depend on gut microbiota to digest plant material.
C) the evolution of foregut fermentation, as seen in ruminants, as the only digestive pattern that depends on fermentative gut microbiota.
D) horizontal gene transfer of genes for glycoside hydrolases and polysaccharide lyases from bacteria to mammals.

Question 29

When squids that contain luminescent bacterial symbionts hatch, they do NOT contain a bacterial symbiont. Symbiont transmission in this symbiosis is

A) vertical from parent to offspring.
B) horizontal and involves specific selection of the symbiont from the environment.
C) random and results in various different species being selected as the symbiont.
D) mixed and results in multiple symbiont species colonizing the squid at once.

Question 30

What percentage of terrestrial plants are colonized by arbuscular mycorrhizae?

A) 10-15%
B) 25-30%
C) 50-60%
D) 70-90%

Question 31

The signaling factors of arbuscular mycorrhizae most likely gave rise to the signaling factors used in rhizobial nodule formation, yet less is known about arbuscular mycorrhizae because

A) they cannot be maintained in pure culture.
B) they are not as important for plant health.
C) no genetic systems have been developed for fungi.
D) they are not as important for plant health and no genetic systems have been developed for fungi.

Question 32

A mutant of Rhizobium leguminosarum is able to survive and reproduce in the laboratory outside of plant roots, but can no longer initiate root nodule formation. What type of genes are most likely mutated in this mutant?

A) rhz genes
B) myc genes
C) nif genes
D) nod genes

Question 33

Which of the following are important adaptations specifically to herbivores?

A) having a large, anoxic chamber where fermentation can occur and having an extended retention time for food
B) having a large, oxic chamber where fermentation can occur and having an extended retention time for food
C) having a large, anoxic chamber where fermentation can occur and having relatively rapid food passage through the digestive tract
D) having a large, oxic chamber where fermentation can occur and having relatively rapid food passage through the digestive tract

Question 34

Which of the following do dinoflagellates most commonly associate with in a mutualistic relationship?

A) clams
B) corals
C) flatworms
D) snails

Question 35

Bacterial symbionts of termite hindguts play a critical role in the termiteʹs

A) reproduction.
B) nitrogen metabolism.
C) resistance to fungal infection.
D) reproduction and nitrogen metabolism.

Question 36

Two main symbionts of termites are ________ and ________.

A) archaea / bacteria
B) bacteria / fungi
C) bacteria / protists
D) fungi / protists

Question 37

Which of the following is a common benefit of a microbe-plant symbiosis?

A) increased nutrient availability only
B) decreased pathogen colonization only
C) increased affinity for carbon dioxide
D) increased nutrient availability and decreased pathogen colonization

Question 38

A species of insect that lives solely on pine sap was found to contain a bacterial symbiont. Genome sequencing of both the host and the symbiont revealed that the symbiont lacked many genes required for energy production, and the host lacked genes for biosynthesis of several essential amino acids. What is/are the most likely mechanism(s) that caused the loss of these genes?

A) coevolution
B) horizontal gene transfer
C) increased mutation rate
D) horizontal gene transfer and increased mutation rate

Question 39

The termite hindgut is

A) entirely oxic.
B) entirely anoxic.
C) variable in oxygen concentration, with distinct microbial niches maintained by patterns of microbial oxygen consumption.
D) entirely oxic or entirely anoxic depending upon the diet of the termite.

Question 40

Symbionts such as Rickettsia in whiteflies and Buchnera in aphids are transmitted ________ to ensure the safe transfer of the symbiont to the next host generation.

A) vertically to offspring
B) horizontally through infected water
C) horizontally through infected secondary hosts
D) vertically through the air

Question 41

Several rhizobial species within a cross-inoculation group should be able to grow with particular legume species such as alfalfa. However, the same cross-inoculation group would be unable to associate with another legume species such as peas.

Question 42

Fungi and protists serve an important role in the anaerobic digestion of cellulose in ruminants.

Question 43

Leguminous plants such as alfalfa, beans, clover, peas, and soybeans all benefit from symbiotic bacteria that leach vitamins into their roots.

Question 44

Lichens are a mutualistic association of a fungus and either an alga or a cyanobacterium in which

A) the fungus protects the photosynthetic partner from erosion.
B) the fungus slowly engulfs the photosynthetic partner.
C) the fungus helps collect sunlight for the photosynthetic partner.
D) the phototroph provides phosphorus to the fungus.

Question 45

Archaea are commonly present in rumens.

Question 46

Which of the following is TRUE of Riftia tube worm symbionts?

A) they have never been maintained in laboratory culture
B) they are obligate symbionts
C) they have a free-living stage
D) they have an unusually small genome, even for a symbiont

Question 47

An individual tree can maintain a symbiotic relationship with many different species of mycorrhizae simultaneously.

Question 48

The structural and trophic foundation of coral reefs is a mutualistic relationship between

A) cyanobacteria and sponges (Porifera).
B) chemolithotrophic bacteria and stony corals (Cnidaria).
C) dinoflagellates and stony corals (Cnidaria).
D) cyanobacteria and stony corals (Cnidaria).

Question 49

Consortia of a phototrophic green sulfur bacteria and motile heterotrophs are found worldwide in freshwater stratified sulfidic lakes.

Question 50

Bacterial symbionts most often provide marine invertebrates in hydrothermal vents with

A) fixed nitrogen in the form of ammonia.
B) sulfate and other sulfur compounds.
C) cellulolytic enzymes to help digest plant material.
D) fixed carbon dioxide in the form of organic compounds.

Question 51

Bacteria associated with plants fix very small amounts of nitrogen, therefore they do NOT contribute significantly to the global N cycle.

Question 52

While rhizobia usually associate with plant roots, nodules can also be formed along the stems of leguminous plants.

Question 53

Bacterial cells themselves serve as major sources of protein and vitamins in hindgut fermenting animals such as rabbits and horses.

Question 54

Aliivibrio fischeri symbionts benefit the Hawaiian bobtail squid by

A) mimicking the light of the moon, which helps the squid avoid nocturnal predators.
B) providing essential amino acids missing from the squidʹs diet.
C) degrading the cellulosic cell walls of the phytoplankton the squid eats.
D) providing essential amino acids missing from the squidʹs diet and degrading the cellulosic cell walls of the phytoplankton the squid eats.

Question 55

The mycelia of mycorrhizae can form underground nutrient networks where several trees are connected.

Question 56

Plant diversity is generally lower in environments where mycorrhizae are associated with plants due to strong coevolution, which leads to competitive exclusion.

Question 57

________ in insects are intracellular bacteria that are usually localized to specialized organs within their host.

A) Symbiodinium
B) Arbuscules
C) Epibionts
D) Endosymbionts

Question 58

Bacteroids are rarely found within plant cells.

Question 59

Riftia tube worms contain unusual hemoglobins that bind(s)

A) H₂S and O₂ in order to transport these chemicals to their bacterial symbionts.
B) high concentrations of O₂ produced by their photosynthetic symbionts.
C) sulfate to transport this ion away from their bacterial symbionts.
D) ammonia that their bacterial symbionts produce.

Question 60

Aphids that feed on carbohydrate-rich but nutrient-poor foods obtain ________ from their endosymbiotic bacterial partners.

A) ATP
B) amino acids
C) ammonium
D) volatile fatty acids

Question 61

Genome sequencing of the bacterial symbionts of animals has allowed us to compare the genomic features of several symbionts. The following data was collected on two insect symbionts:
Symbiont A: 0.4 Mb genome, 20% G+C content, missing or incomplete peptidoglycan synthesis and nucleotide synthesis pathways.
Symbiont B: 1.8 Mb genome, 40% G+C content, peptidoglycan and most nucleotide synthesis pathways complete.
What can you deduce about these two insect symbionts based on their genomes?

Question 62

Based on 16S RNA phylogenetic data, what phyla are important in the ruminant gut? Give an example of how diet affects ruminant health.

Question 63

Both pathogenic bacteria and primary symbionts have a tendency to lose genes for catabolic pathways.

Question 64

Due to the close interactions with their hosts, obligate symbionts generally have larger genome sizes compared to other non-symbiotic bacteria.

Question 65

Microbial symbionts that are horizontally transferred show greater genome reduction than those that are heritable.

Question 66

Horizontal gene transfer from symbionts to the DNA of their host nematodes is rare.

Question 67

What is the difference between a primary and secondary insect symbiont? Propose how you might use an insect symbiont to control the spread of an insect-borne disease.

Question 68

Coral bleaching is primarily caused by antimicrobials that disrupt the mutualistic relationships formed between corals and their bacterial symbionts.

Question 69

How do Hawaiian bobtail squids prevent all bacteria EXCEPT Allivibrio fischeri from colonizing their light organ?

Question 70

Fibrobacteres comprise the greatest proportion of the bacterial species in ruminant guts.

Question 71

The following quote is taken from a university extension publication for farmers:
ʺGrain poisoning, also known as grain overload or lactic acidosis, is usually the result of stock consuming large quantities of grain or pellets to which they are unaccustomed. Pasture-fed cows or feedlot cattle not yet adapted to grain may become acutely ill or die after eating only moderate amounts of grain, whereas stock accustomed to diets high in grain content may consume large amounts of grain with little or no effect.ʺ
What is happening in the digestive system of the livestock that causes grain poisoning and why does a gradual shift in diet avoid this problem?

Question 72

Photorhabdus is a bacterial symbiont of insect nematodes.

Question 73

Describe the mutualistic relationship between chemolithotrophic bacteria and tube worms in hydrothermal vents.

Question 74

Ruminants are foregut fermenters.

Question 75

Gut symbionts are lost after each molting a termite undergoes.

Question 76

The reproductive systems of many insects can be manipulated by parasitic symbionts, which are passed down to each generation and can skew the sex ratio of their progeny.

Question 77

Both lichens and corals are symbioses involving phototrophic microorganisms. Compare and contrast lichens and corals, including the types of organisms involved, the role of each partner, the specificity of the interaction, and the method of transmission for each symbiosis.

Question 78

Most termites have intracellular symbionts.

Question 79

Hindgut fermenters use a cecum to contain fermentative organisms.

Question 80

Lichens always consist of a cyanobacterium and a fungus.