Questions
1-6 are based on the PLoSOne article
entitled “How Ebola Impacts Genetics of Western Lowland Gorilla Populations” by
Le Gouar et. al (2009).
- In
an evolutionary sense, why is it informative to study human influenza and
its implications in gorillas?
Describe and define the mechanisms that have increased the risk of
interspecific disease transmission between humans and non-human apes.
a.
Great apes and
humans share around 98% of their genetic material. This remarkable similarity means
that gorillas are susceptible to many human viruses and vice versa. With this
insight, scientists have been able to gain a better understanding of the
mechanisms that allow the transmission of disease between humans and non-human
apes. Transmission of disease between these two groups typically occurs through
various forms of interaction. Tourist, conservation scientist, and poachers are
just a few examples of ways in which humans and apes come in contact with one
another and open a door for interspecific disease transmission. Censored
humanly interaction is important in the future in order to prevent both humans
and apes from further infection.
- Generally
speaking, what are the evolutionary consequences of bottlenecks caused by infectious
disease outbreaks?
a.
Generally
speaking, population bottlenecks arise when a population’s size decreases over
the duration of at least one generation. Genetic drift acts at a faster rate to
reduce genetic variation in small populations. Consequently, undergoing a
bottleneck could potentially reduce a population’s genetic variation immensely,
even if the bottleneck does not exist for many generations. Reduced genetic
variation results in the population being unable to adapt to new selection
pressures, like climatic change or a shift in available resources, because the
genetic variation that selection would
act on may have already drifted out of the population.
If there were to be an infectious disease outbreak in a species, this
would immensely affect a population. The population would be extremely small
due to the bottleneck, therefore the genetics of the population would be very similar.
Thus, making an infectious disease very detrimental to a population that has
undergone the bottleneck phenomena.
- What measures of variation did the authors
use to assess variation in the gorilla populations pre- and
post-outbreak? What did they find?
a.
In order to assess the variation in the
gorilla populations pre- and post-outbreak, the authors measured: marker
polymorphism, impact of mortality events on genetic diversity, changes in
allele frequencies, immigration, and selection pressure. By the end of the study, the authors were
able to conclude that although there was an absence of genetic diversity loss
after the Ebola epidemic, there were some temporal changes of allele frequencies
in the gorilla populations.
The authors attribute
social organization, migration of both sexes, and low genetic structure between
populations to lack of genetic diversity loss after the demographic crash. Although the state of genetic diversity
remains stable for now, the authors believe that these results could change in
the near future. This positive result
could have also resulted from the fact that there was a high enough effective
population size during the short amount of time surveyed after the
decline. With more time and data, the
authors would have been able to draw a more precise conclusion on the effect
the Ebola outbreak had on the genetic diversity in the gorilla populations.
In regards to the
temporal changes of allele frequencies in the gorilla populations, it is
believed that the cost of sociality induced by Ebola and the sex-biased
dispersal of the species could have been the cause. Through social organization and migration of
both sexes, the authors hope that the gorillas will be able to rebuild their population
without extreme effects from the Ebola epidemic.
- Why is genetic variation important to a
population (hint: revisit the Crotaphytus
paper)?
a.
The Western
lowland gorilla, a critically endangered species, shows the consequences of the
lack of variance in a population. The gorilla population lost its genetic
diversity because of a disease around 300 years ago, this caused a bottleneck
effect. This bottleneck effect caused the loss of genetic diversity through an
increase of inbreeding and the fixation of alleles. If the Gorillas had more
variance, a disease like Ebola wouldn’t wipe out the entire population because
the population wouldn’t be identical. With more genetic variation the gorillas
would also be more able to adapt to their environments. If there is no gene
flow in the population, then variance can’t occur, which again would lead to
the fixation of alleles.
- Did
evolution occur between pre- and post-epidemic gorilla populations? How do you know? Which evolutionary mechanism(s) is/are thought
to be operating in these populations?
a.
In the Lokoué population both pairwise FST analyses and tests of
temporal changes in allele frequencies agreed that no changes occurred and
therefore there was no evolution that occurred in the Lokoué population. In
contrast, the Lossi population experienced significant temporal changes in
allele frequencies were revealed at for four loci (D1s550 n = 12, D4s243 n = 8,
D16s2624 n = 13, and vWF n = 12). We know evolution occurred because for these
loci Waples' temporal tests were performed to investigate whether sampling
error and genetic drift alone could explain the heterogeneity in allele
frequencies over time. A significant test implies that sampling error and
genetic drift were not sufficient to explain the differences in allele
frequencies. The tests were significant for D16s2624 and vWF for all the value
of Ne tested and therefore genetic drift and sampling error. The tests for
D1s550, and D4s243 loci in Lossi were not significant. Those results indicate
that for the Lossi population and the loci D16s2624 and vWF other factors than
sampling error and genetic drift were responsible for the observed allele
frequency changes between the pre- and post-epidemic samples.
There are a few explanations for the
absence of genetic diversity loss. In the Lokoué population, the absence of
loss of genetic diversity is hypotheisezed to be a result of immigration in post-epidemic
population since genetic diversity levels were in the same range than those
previously published on Western lowland gorilla. In the Lossi population, In
Lossi, no immigration from the peripgery had been detected. Another likely
explanation for the absence of allelic diversity loss is a high enough remnant
effective population size and the short time elapsed after the decline.
In contrast, temporal changes of allele
frequencies were observed in the Lossi population. The temporal heterogeneity
is small for some locus (D1s550 and D4s243) and can be explained by drift or
sampling error. However, other factors than drift and sampling errors were
responsible for the observed allele frequency changes between pre and
post-epidemic samples for two loci (D16s2624 and vWF) of the Lossi population.
One hypothesis for the change in allele frequency at the two speciiced locus is
the Ebola disease induced a non random mortality through selection in the Lossi
population. A second possibility for the changes in allele frequencies between
pre and post-epidemic samples in the Lossi population could result from
evolutionary survival bias. This survival bias could have led to a bias
composition of sampled individuals in the post-epidemic sample.
- Why
is it important to study protein folding/misfolding in Ebola?
a.
It is important to study the protein
folding/misfolding in Ebola because the protein folding/misfolding determines
the structure of the protein. Since the structure of a protein determines its
function, once the structure is determined drugs can be developed and used to
disrupt the function of the Ebola protein. On the topic of Ebola protein
structure Zygmunt Derewenda, a biophysicist at the University of Virginia in
Charlottesville, says, “When you have a protein with a novel amino-acid
sequence, you never know if the structure is going to fall into one of the
known families or if it’s going to be something new.” The initial, basic
research on Ebola’s structure is important because it will tell us how Ebola
operates once it infects a host, and also more importantly how to combat the
virus.
