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Evolution > Symbiosis > Flashcards

Symbiosis Flashcards

1
Q

Define symbiosis

A

an association between different species that ranges from beneficial/mutualistic to parasitic

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2
Q

Why is symbiosis so important? give an example

A

it’s a huge source of innovation

ex. mitochondria, what now functions in respiration amongst other important things, was derived from an ancient symbiosis = allowed eukaryotes to exist

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3
Q

Describe endosymbiosis and give an example

A

occurs when one organism, usually a single-celled microbe, lives within another larger organism or within another organism’s cells

ex. mitochondria, gut microbiota

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4
Q

T or F: all endosymbiosis events are beneficial

A

false, they can be anywhere between mutualistic and parasitic

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5
Q

What is the most important factor shaping the ecology and evolution of symbiosis?

A

the mode of transmission = how an organism is transmitted to a host

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6
Q

What are the major questions to ask when trying to understand the mode of transmission?

A

how many species is the endosymbiont infecting?

is the endosymbiont transmitted vertically, horizontally, or both?

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7
Q

What is vertical transmission?

A

descent with modification (regular, Mendelian inheritance from generation to generation)

the infectious organism is inherited by offspring from the mother

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8
Q

Explain how the insect egg and bacterial infection is an example of vertically transmitted endosymbiosis

A

the bacteria finds its host, the insects, before the insects are even born = the bacterial cells are inside the insect egg

this means the bacterial symbiont is passed on through generations of the insect from the mother to her offspring

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9
Q

How are the fitnesses of the symbiont and the host related when there’s vertical transmission? Using the insect egg and bacteria to explain

A

the fitness of the symbiont is directly dependent on the number of offspring the host produces and vice versa

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10
Q

How are the evolutionary interests of the symbiont and host aligned when there’s vertical transmission?

A

very closely aligned = the fitnesses are codependent

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11
Q

What is horizontal transmission?

A

when the host is infected by the symbiont from the environment

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12
Q

How are the evolutionary interests of the symbiont and host aligned when there’s horizontal transmission?

A

usually less aligned and more conflict but sometimes there’s beneficial symbioses

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13
Q

What are some examples of beneficial symbioses that use horizontal transmission?

A

lichen
coral
bobtail squid and bioluminescent bacteria
rhizobium and legume roots

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14
Q

T or F: vertically transmitted endosymbionts are common in marine systems

A

true

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15
Q

Describe obligate inherited symbionts. Give examples

A

vertically transmitted

symbionts that are essential for host survival because they provide them with nutrients they don’t receive from their nutrient-poor diets (ex. animal blood, plant phloem or xylem)

ex. red aphids that feed on phloem and carotenoids

ex. cicadas

ex. leeches, ticks, bed bugs, lice = bugs that feed on animal blood their entire lives

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16
Q

How are the fitnesses of the symbiont and the host related for obligate inherited symbionts?

A

the host requires the symbiont for survival because their diet lacks essential nutrients and the symbiont provides those nutrients

symbionts have also lost the ability to live independently

= the fitnesses are very strongly related, neither can survive without the other

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17
Q

What are the two types of vertically transmitted endosymbionts?

A

obligate inherited symbionts
facultative inherited symbionts

18
Q

Discuss the example using aphids and their obligate nutritional symbionts, Buchnera aphidicola

A

the aphid diet lacks essential amino acids for aphid survival

B. aphidicola bacteria provide aphids with tryptophan and other AAs
they are located in bacteriocytes (specialized cells) of aphid bacteriome tissues

the B. aphidicola are vertically transmitted from mother to offspring

19
Q

What does looking at the evolutionary history of aphids and B. aphidicola bacteria tell us?

A

B. aphidicola have been vertically transmitted in aphids for ~200 million years

this is ~200 million years of coevolution between these two organisms = one cannot survive without the other and they are very well aligned

20
Q

What type of organism has the smallest genome size?

A

obligate symbionts

ex. Buchnera ~560 genes
ex. Nasuia ~140 genes (the smallest known genome)

21
Q

Is it possible that obligate nutritional/inherited symbionts a path to becoming organelles?

A

it’s possible, it’s happened before (mitochondria)

22
Q

Describe facultative inherited symbionts

A

when the host can still survive and/or reproduce without the symbiont, but the symbiont cannot survive without the host

23
Q

What are most facultative inherited symbionts classified as? What does this mean?

A

conditional mutualists: under certain conditions, the symbionts increase host fitness

24
Q

How does the fitness of a facultatively inherited symbiont relate to the fitness of its host?

A

directly connected

without the host, the symbiont dies

and because these are inherited (vertically transmitted) from the mother, the fitness of the symbiont is dependent on the fitness of the host

25
Q

In the study of Drosophila flies and parasitic nematode, what type of transmission is involved? what happens to the flies?

A

the nematode is NOT a symbiont, it’s horizontally transmitted

the nematode infects fly larvae when they’re feeding on mushrooms and the initial worm will pull nutrients from the fly, replace the fly eggs with worms, and the flies will release worms onto the mushrooms when they go to lay their offspring

26
Q

What was discovered about the flies from the eastern part of the continent?

A

the eastern flies actually carried a bacterial endosymbiont that killed the parasitic nematode sterilizing the flies

27
Q

What were the results of the study on the D. flies and the parasitic nematodes when groups with the endosymbiont (eastern) were compared to those without (western)?

A

the flies with inherited endosymbiont had higher fitness (not sterile) when the worms were present compared to the flies without the endosymbiont

28
Q

How was the fitness of the parasitic nematode influenced by the presence of the inherited endosymbiont in the flies?

A

the initial, mother worms were small and very unfit

29
Q

What could explain the pattern of the inherited endosymbiont across North America (more concentrated east of the Rockies, none west)? what is the real reason?

A

the endosymbiont increases host fitness when the parasite is present = conditional mutualism - maybe there’s no nematodes in the west

might be spreading from east to west and haven’t gotten over the geographic barrier yet

maybe the western flies have coevolved with a different endosymbiont

Answer: spreading east to west = flies with the symbiont are replacing flies without

30
Q

What evidence is there to suggest that inheritance of the endosymbiont is spreading east to west?

A

the original sample sites were resampled to find there was higher concentrations of flies with the endosymbiont

sites set up to detect west of the rockies

spread is likely to persist because the parasite is common wherever the flies are

31
Q

What happens to the flies and endosymbiont when the parasite is not present?

A

inheritance of the endosymbiont decreases and may be lost - decreased fitness for the endosymbiont

this suggests there may be a cost to maintain the endosymbiont when there’s no benefit to having it

32
Q

Are defensive symbioses, as seen in the D. flies and endosymbiont against parasitic nematode, common?

A
33
Q

Does having a defensive symbiont reduce the fitness of the host? or vice versa?

A

most likely, hosting symbionts is costly because they require nutrients, regulation, and maintenance = this could cause conflict

34
Q

Is it possible for parasites to evolve to resist defensive symbionts?

A

it’s likely

35
Q

When does conflict arise between hosts and symbionts? when might this occur?

A

when the evolutionary interests of the two don’t match

this could occur because maintaining and regulating symbionts is costly

ex) if the selection for symbionts is high and there’s increased replication of symbionts, their nutritional needs will increase and may effect the host

36
Q

How are aphids an example of regulating obligate heritable symbiont to deal with conflicting evolutionary interests?

A

aphids are able to, to varying degrees, regulate the number of symbionts in their bodies

this is a mechanism for the aphids to be able to maintain their own fitness in relation to the symbiont’s presence

bigger symbiont load in body = lower fitness (fecundity decreases and developmental time increases) because it takes more energy to maintain more symbionts

37
Q

T or F: there is a negative correlation between symbiont number and aphid fitness - explain

A

true because the more symbionts there are in a host body, the more nutrients they require from the host = the host’s fitness will decrease

ex. more bacteria in aphids = lower reproductive rates of bacteria

38
Q

Why are parasites and pathogens an influential selective force?

A

because pathogens can kill hosts or pathogens can be killed = there is no middle ground, only strong selection in one direction

also because they rapidly evolve

39
Q

T or F: parasites and pathogens are strong selective forces but they evolve slowly

A

false, they are strong selective forces and they evolve rapidly

40
Q

Explain why the following statement isn’t always true “parasites and pathogens should evolve to be less harmful/virulent to their hosts”

A

the evolution of parasites and pathogens and the effects they have on their host all depends on the mode of transmission

41
Q

Why would an organism that’s fitness/survival depends on the fitness/survival of its host evolve to kill or sterilize its host?

A

it really depends on the mode of transmission

42
Q

Explain how the coevolution of myxoma virus and Australian rabbits is an example of virulence evolution

A

myxoma virus released in Australia to control invasive rabbits

myxoma and rabbits have no evolutionary history = rabbits have no resistance to myxoma = huge initial population crash of rabbits

over time, the virus evolved to be less fatal (a less fatal strain replaced the fatal one) allowing the survival of its host

over time, the rabbits also evolved resistance to the virus

this generated an arms race between rabbits as hosts and myxoma as virus

the evolution of virulence and decreased virulence of myxoma was dependent on the evolution of rabbit resistance

Evolution (11 decks)

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