Gibson - Host/parasite Coevolution Flashcards Preview

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Flashcards in Gibson - Host/parasite Coevolution Deck (46):

what are 4 possible mechanisms of host parasite coevolution?

1. co-speciation - parasites and hosts diverge at the same time
2. Parasite divergence - parasite speciates in the same host
3. Host switch - parasite moves to a new host
4. Parasite extinction - when host goes extinct, so does parasite.


what are 3 ways of investigating co evolution?

Compare parasite fauna on different hosts.
Compare host and parasite phylogenies.
Conclusions are stronger where parasites are host specific.


what are the basic steps involved in building phylogenetic trees

choose one of the many genomes to be the reference
sequence the same part of all the genomes, and find differences between sequences.
build a matrix of the sequence differences, pairs with more differences are further apart.


4 examples of parasites which co-evolved with humans

Malaria parasite, Plasmodium spp.
Bed bug, Cimex lectularias.
Tapeworms, Taenia spp.
Lice, Pediculus humanus and Pthirus pubis.


approx how many people are affected by malaria?

~3.2 billion people at risk, almost half the global population.
212 million cases, 429,000 deaths in 2015 (WHO)


what causes malaria?

Protozoan parasites; 5 species in humans: Plasmodium falciparum, P.vivax, P.ovale, P. malariae, P. knowlesi (zoonosis)


whwre would be a good place to start looking for the origin of human malaria parasite?

The human lineage and the chimp lineage diverged 6 mya.
closest relative is chimpanzee - Pan troglodytes, so start looking at their parasites.


what are 2 possible mechanism for origin of human malaria parasite?

host switch events from other primate relatives of humans. however, host switch events v difficult to date so could have happened at any time.


what was generally accepted (before lui et al) phylogenetic relationships among plasmodium species?

Suggests divergence of human and chimp Plasmodium lineages during host divergence (co=speciation)
Human plasmodium falciparum more similar to chimp P.reichnowi than other human species.
This supports the cospeciation theory


Describe the method used to conclude P. facliparum evolved from gorilla plasmodium due to host switch

Liu et al 2010.
Collected faecal samples from wild apes = non-invasive sampling. (but prevented in protected areas).
Amplified Plasmodium spp. DNA by PCR and sequenced mitochondrial, apicoplast and nuclear genes.
Extensive analysis: ~3000 faecal samples; 100’s of DNA sequences.
- results: Plasmodium spp. highly prevalent in chimps and western lowland gorilla, but not seen in bonobo or eastern lowland gorilla.
Commonly mixed infections.
Used Single Genome Amplification (SGA) to ensure non-chimeric sequences.
gorilla plasmodium spp have very different branch lengths, suggesting it is a v heterogeneous group.


what is the apicoplast?

apicomplexans retain a chlopoplast with a complete genome


why use mit dna for analysis?

to get the female lineage. however it evolves very fast so should be used in conjunction with other genomic data.


what did Liu et al conclude?

P falciparum aquired from gorilla, not chimp as previously thought, so was a host switch not co-speciation.
likely to be single host switch event, estimated to be 100,000-1m years ago, by Baron et al 2011.
further support from limited variation in human species whereas much variation in gorilla species.


What are 2 opposing conclusions to Liu et al's conclusions about origin of P falciparum?

Valkiunas 2011 - argues that the parasite might not undergo a whole lifecycle in these hosts, as no evidence that sexual forms or gametocytes, so could have been parasites of different animals.
Silva et al 2015
Genome data dates divergence of p. falciparum and p. reichnowi at 3-5.5mya, and co-speciation event.


where is plasmodium vivax found?

Prevalent in Asia and Latin America, but absent from West and Central Africa, because humans are genetically resistant – DARC negative


what is the plasmodium vivax presumed origin?

S. E. Asian monkeys
Then why did it spread across Africa?


who investigated the origin of plasmodium vivax and how?

Liu et al (2014) analysed Plasmodium vivax-related parasites by PCR and sequencing using DNA extracted from faecal samples of wild chimps and gorillas in central Africa
used 3 genetic markers (mit, nuclear and apicoplast genomes) to form a phylogenetic tree.


what 3 points did liu et al conclude about evolution of p vivax?

1. evolved from related parasites that infect primates in Africa – this is contrary to present-day distribution.
2. One human P. vivax lineage gave rise to all existing P. vivax parasites – perhaps a bottleneck event coinciding with human migration from Africa.
3. emergence of genetic resistance in the African human population led to the extinction of P. vivax in Africa.


from where is p knowlesi aquired?

SE asia, macaques
huge outbreak in borneo


define zoonosis

disease transmitted between humans and other vertebrates.


what is a potential reason why P knowlesi has only recently been discovered?

Confused with P. malariae in diagnosis from stained blood smears.


what is the origin of human P. knowlesi?

evolved in primates and recent host switch to humans. - Silva et al 2015.
divergence of knowlesi from vivax 18-34mya.


which stages of cestode lifecycle are found in each host?

The definitive host (predator) harbours the adult worm(s), while the intermediate host (prey) harbours the larval stage.
Larval stages in muscle or other tissues are ingested by the definitive host.


describe the beef tapeworm lifecycle

Taenia saginata
human - def host
- adult tapeworm expels eggs in faeces which contaminate pasture
- ox - intermediate host, ingests eggs and larvae hatch - form cysticerci in muscles.
- cysticerci ingested by human in meat.


how many taenia species infect humans?

taenia saginata - beef
taenia solium - pork
taenia asiatica - pork


What is the classically accepted origin of human tapeworms?

All 3 species are ingested with meat from our livestock.
Hypothesis: humans acquired these worms from our livestock when these animals were domesticated.
Domestication of livestock dates to ~10,000 years ago.


what did Hoberg et al 2001 study?

Made a Phylogenetic analysis of human taenid tapeworms and African carnivores using 28 morphological and life history characters.
calculated the dates of divergence from gene sequence data.
T. saginata and T. asiatica = 0.78 – 1.71 MYA
supported by other studies.
however, this indicates that human tapeworms diverged long before domestication of livestock.

human taenids don't form a single clade, and represent 2 host switch events. instead, the origin of human taenids is associated with adaption to carnivory.


what is a more recent view of human tapeworm origin, based on hoberg's results?

human taenids don't form a single clade, and represent 2 host switch events. instead, the origin of human taenids is associated with adaption to carnivory.
domestic livestock secondarily aquired infection from humans.


which phylogenies are not in agreement with Hoberg's phylogeniy?

those based on molecular data.
1. Knapp et al (2011) Taenia phylogeny based on 3 protein-coding genes.
2Terefe et al (2014) analysed Taenia from roadkill hyenas in Ethiopia – including T. crocutae.

These confirmed the close relationship of saginata and asiatica, but no close relationship btw t. solium and jackal or hyena. jackal anf hyena tapeworm both more closely related to saginata and asiatica.
closest relative of solium is from brown bear - suggesting divergence in eurasia rather than Africa.


when were bed bugs common?

Europe and n america before widespread use of insecticides in 20th C.
currently global resurgence, maybe due to resistance to insecticides, movement of people and goods and secondhand shops.


describe feeding and dispersal beh of bedbug

Cimex lectularius
wingless, bloodsucking insect.
pest of human habitation
hides in cracks in walls in the day and merges to feed at night. also parasite of bats.
locally within a building, possible active dispersal. often passive dispersal in suitcases, clothes and furniture.


what is the hypothesis regarding origin of human bedbugs?

batsare the original host. humans aquired them through sharing cave dwelling with bats, and took them into houses.


describe study of bedbug evolution from bats

Booth et al 2015 - sampled bedbugs from around europe, in human dwellings and bat roosts.
microsatellite analysis. 2 genetically distinct groups.


what did balvn et al find?

balvin 2012 et al - bedbugs associated with bats and humans differed in morphological traits.
human bedbugs have longer legs, maybe better adapted for dispersal within dwellings?
bat associated - short strong legs, for clinging onto fur for dispersal?


what are the general conclusions about human bedbug origin?

bedbug races are biologically isolated although sometimes occur in the same dwelling. bats have not contributed o the current resurgence of bedbugs.
host switch onto humans in early human evolution and than 2 host associated pops diverged.


what are lice?

highly specialized, blood-sucking ectoparasites that live on a single host species.
Can't fly or hop like other bloodsucking insects, but get from host to host by close contact.


which species of lice do humans host?

3 types
1. Head louse - Pediculus humanus capitis. harmless and common in kids.
2. body louse - Pediculus humanus corporis. adults and nymphs in unwashed clothing. vector of bacterial diseases: typhus, relapsing fever and trench fever.
3. pubic louse Pthirus pubis . same genus as gorilla louse.


how many lice do primates have?

each of humans closest relatives: chimo, gorilla, OW monkeys have 1 louse species each.


what is a hominin

any species more closely related to humans than a chimpanzee


what is predicted about the evolution of human body lice?

human body lice arose from head lice when humans started to wear clothing.
Clothes creates a new ecological niche, no direct evidence for when Homo started to wear clothes. found sewing needles and thread from about 22,000-34000 ya.


evidence of evolution of body lice?

Kittler et al 2003 - molecular evolution of pediculus humanus.compared DNA sequences of head and body lice samples globally, using 2 mit and 2 nuclear genes.
found human head and body lice are closely related and molecular clock analysis indicates that human body lice originated 107,000ya - far before we started wearing clothes.


alternative theory in light of evidence of human body lice origin

human body lice arose from human head lice on modern humans, perhaps correlated with global expansion of humans from Africa.


what theories has the new date of origin of clothing lice brought forward?

Toups et al 2011
indicates early clothing use by anatomically modern humans in Africa.


what did Veracx & Raoult 2012 argue?

human head and body lice are ecotype variants of one species and there is continuous exchange between niches, especially in high population densities.


what is interesting about the dates which Kittler et al used on their phylogenetic tree of head and body lice evolution in 2003?

used mitochondrial COX1 gene.
emergence of clade B predates the evo of homo sapiens 0.77-1.2mya.
suggests contact between homo erectus and homo antecessor, as lice require a human host at all points in life.
future papers are more cautious and analysis of nuclear genes rather than mit have given different results.


what is one hypothesis for the origin of human pubic lice? why might it be wrong?

human pubic lice are from same genus as gorilla lice.
human pubis and head lineages diverged because hominins lost most body hair and retained 2 separate hair niches - head and genital.
phylogeny suggests common ancestor 13mya.
doesnt agree with the divergence of humans and gorillas., 7mya.
Reed et al 2007 concluded that hominins aquired a second species of louse 3-4 mya, via host switch.similar phylogeny derived from lice endosymbiotic bacteria.