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Vorburger lab: Possible topics for MSc & BSc Projects (updated May 2016)
⇒ We suggest to first have a look at http://homepages.eawag.ch/~vorburch/research.html
to have the context of these project suggestions.
Costs of harbouring a defensive endosymbiont: Regiella insecticola strain 5.15
Aphids benefit from increased resistance to parasitoids when harbouring the defensive endosymbiont
Hamiltonella defensa. This protection does not come for free. In the absence of parasitoids, infected aphids
suffer from a reduced longevity and lifetime reproduction. In our lab, we have recently discovered a strain
of another endosymbiont, Regiella insecticola isolate 5.15, that also increases resistance to parasitoids,
albeit via a different molecular mechanism (details not important here). We do not know yet if harbouring
this symbiont is also associated with a longevity cost. This could be tested experimentally. We already
produced aphid lines that are genetically identical (same clone) but do or do not have R. insecticola 5.15.
We even have such lines from three different aphid species. These are available for the test.
Suitable for a 3-month project.
Comparing costs of harbouring Hamiltonella defensa between natural and experimental infections
An experiment introducing multiple isolates of H. defensa into previously uninfected clones of Aphis fabae
revealed substantial fitness costs of harbouring the symbiont. In the absence of parasitoids, aphids
harbouring H. defensa suffered from a reduced lifespan, resulting in lower lifetime reproduction (Vorburger
& Gouskov 2011, JEB). However, it is possible that this experiment overestimated the costs, because we
generated host-symbiont combinations that were untested by natural selection. Naturally occurring
infections may be less costly because only successful host-symbiont combinations prevail, which may be
those in which the symbiont is least harmful to the host. This is testable by comparing the costs of
harbouring the same strains of H. defensa beween the clones in which they were found naturally and
clones into which they were introduced experimentally. We already have the experimental infections in
two different clones, and we still possess the naturally infected clones that served as donors for the
experimental infections. Curing those of H. defensa with antibiotics would enable such a comparison.
Only possible for a 6-month MSc when the curing is done a couple of generations before the start of the
MSc.
Local adaptation to defensive symbionts in aphid parasitoids?
The aphid parasitoid Lysiphlebus fabarum exhibits host-associated genetic differentiation (HAD). Wasps
collected from different aphid hosts are typically less related than wasps collected from the same aphid
host, which is suggestive of local adaptation. Most of their host species from the genus Aphis are protected
by heritable bacterial endosymbiont called Hamiltonella defensa. It is possible that different species of
Aphis harbour different strains of Hamiltonella, and that parasitoids exploiting these species are locally
adapted to the Hamiltonella found in 'their' host. This can be tested by genotyping (multi-locus sequence
typing, MLST) Hamiltonella from different aphid species to verify their distinctness, introducing them into a
common genetic background (a single clone from the species Aphis fabae), and comparing the infectivity of
wasps collected from the different hosts on aphids containing the different symbiont strains.
This project is better suited for a 12-month MSc.
The specificity of chemical mimicry in aphid parasitoids of the genus Lysiphlebus
Aphid parasitoids of the genus Lysiphlebus are specialized on ant-tended aphid species. These aphids
engage in a symbiosis with ants, which provide protection against predators and parasitoids in exchange for
honeydew from the aphids. To exploit such aphids despite their protection by ants, Lysiphlebus parasitoids
have evolved chemical mimicry. The chemical profile of the hydrocarbons coating their cuticula closely
matches that of the aphids, 'fooling' ants into 'believing' they are aphids and leaving them unmolested
(Liepert & Dettner, 1996, J Chem Ecol). However, it is still unknown how precise this mimicry needs to be to
be functional. As a first attempt to answer this question, the student will exchange Lysiphlebus parasitoids
between different ant-tended aphid species, to test whether parasitoids remain undetected by ants even
against a 'background' consisting of different aphid species. Field experiment suitable for a 3-month
project.
Resistance-conferring strains of Spiroplasma in pea aphids?
For a collaborative project on Spiroplasma-conferred resistance in insects (funded by a Sinergia grant from
the Swiss National Science Foundation), we have collected pea aphid clones infected with different strains
of the bacterial endosymbiont Spiroplasma. By sequencing we could establish that we have a dozen
different strains (multi-locus sequence types – MLSTs). To test their effects on resistance to parasitoids,
these strains will have to be transferred into a common genetic background (same aphid clone) by
microinjection, followed by exposing these newly formed lines to the parasitoid Aphidius ervi.
Suitable (if ambitious) project for a 6-months MSc.
The genomic basis of parasitoid infectivity on symbiont-protected hosts (QTL mapping)
Parasitoids can adapt to the presence of defensive endosymbionts in their aphid hosts. We have evolved
parasitoids (Lysiphlebus fabarum) on aphids with and without the defensive symbiont Hamiltonella. These
evolved lines are now available for QTL mapping of genomic loci involved in this adaptation. The work will
involve experimental crosses with parasitoids wasps, followed by RAD-tag sequencing of a mapping
population (Method already established at Uni Lausanne).
AND SOMETHING NON-INSECT:
Genetic population structure at the limit of a species' geographic range: the endangered stone crayfish
(Austropotamobius torrentium) in central Switzerland
Central Switzerland is the natural range limit of the stone crayfish, Austropotamobius pallipes, one of three
native freshwater crayfish species in Switzerland. It is red-listed as 'endangered' due to habitat degradation,
competition by introduced crayfish species, and a deadly disease (crayfish plague) vectored by these
introduced species. To inform and support the conservation of the remaining stone crayfish populations in
central Switzerland, the MSc candidate will analyze their genetic population structure using molecular
markers. The goal is to quantifiy the connectivity of the still exisiting populations, to potentially detect
distinct gene pools that should be treated as separate conservation units, and to identify suitable source
populations for possible re-introductions, e.g. populations still possessing a high genetic diversity.
Possible projects under the supervision of Dr. Hugo Mathé-Hubert
([email protected])
Do defensive symbionts promote the acquisition of additional symbionts?
Background: Aphids arbour a large diversity of facultative bacterial symbionts that are vertically
transmitted from mother to offspring. These facultative symbionts provide diverse advantages to
their aphid host, which in some environments can improve their offspring production. This in turn is
beneficial to the symbionts that are transmitted to these offspring.
The research question involves two phenomena linked to these symbionts. Firstly, some
symbionts protect aphids against a parasitoid wasp that lays its eggs within the aphids. In the
absence of protective symbionts, the wasp’s egg develops within the aphid, and kills it while ending
its development. Secondly, Gehrer and Vorburger (2012) have shown that the ovipositor of the
wasp works as a “dirty needle”: when a wasp successively stings different aphids, it happens that
some symbionts stick to the ovipositor and get released in a new aphid (horizontal transmission).
The idea: When some “newcomer symbionts” are released in the host by an ovipositing wasp, a
three-way competition can occur between the “newcomer symbionts”, the “resident symbionts”,
and the wasp’s egg. More precisely, if “resident symbionts” are protecting against the wasp, they
kill the egg, and then they face the “newcomer symbionts”, and possibly compete with them for the
host resources and for the vertical transmission to offspring. But, if on the opposite, the “resident
symbionts” of the new aphid are not protecting against the wasp, then the aphid, the “resident
symbionts” and the “newcomer symbionts” will all die during the development of the wasp’s egg.
According to this scenario, the rate of successful horizontal transmission would be much higher
if the new host is protected against the wasp by its resident symbionts. This in turn would lead to an
asymmetry, where protective symbionts would be more strongly selected to be able to outcompete a
newcomer than a non-protective symbiont that tends to die anyway after the parasitism, whatever its
capacity to outcompete a newcomer symbiont.
The project: This project will involve raising and laboratory experiments, and it can be extended
to more theoretical questions with simulations. The student will conduct one or several experiments
to test if the rate of successful transmission by the wasp is higher when the “resident symbionts” of
the recipient host are protecting against the wasp. Two kinds of experiments are possible. The first
one is very similar to (Gehrer and Vorburger 2012), and the second one would involve population
cages. Then, whatever the outcome of experiments, some thought experiments and simulations
(Skaf and Imbert 2013) could be used to investigate the evolutionary consequences of this
asymmetry. These simulations would be conducted either with the SimuPop python module, or with
raw Python or R programming. The simulation part of the project is optional, and the relative
importance of these two parts will depend on the student’s interest and capabilities. Although one of
the experiments could be completed within 3 months, the project is better suited for six months.
Gehrer, L., & Vorburger, C. (2012). Parasitoids as vectors of facultative bacterial endosymbionts in aphids. Biology
Letters, 8(4), 613–615. doi:10.1098/rsbl.2012.0144
Imbert, C. (2013). Unfolding in the empirical sciences: Experiments, thought experiments and computer
simulations. Synthese, 190(16), 3451–3474. doi:10.1007/s11229-012-0203-y
Does the pea aphid symbiont Spiroplasma use plants as a route of horizontal transmission
between aphids?
Background: Aphids are sap-sucking insects that arbour a large diversity of facultative bacterial
symbionts which are vertically transmitted from mother to offspring. One of them is Spiroplasma.
The host range of this genus of bacterial symbionts include some insects, arachnids, crustacean, and
possibly some vertebrates. The effect of these symbionts on their hosts goes from detrimental
(parasitism) to beneficial (mutualism).
The idea & the project: Two clues suggest that Spiroplasma of pea aphids might use plants to get
horizontally transmitted. On one hand, phytopathogenic species of Spiroplasma exist and are
usually transmitted from plant to plant by sap-sucking insects such as leafhopper that suck them
with the plant sap (Weintraub and Beanland 2006 ; Capinera 2008). This means that some
Spiroplasma are able to live and reproduce in both plants and insects, and to shift from the insect
gut to its hemolymph and then to its salivary glands (Bové et al., 2003). On the other hand, CaspiFluger et al. (2012) have shown that plants can be a suitable route of horizontal transmission for
some Rickettsia, a bacterial symbiont of insects that is usually vertically transmitted, as the
Spiroplasma of pea aphids. The first part of the project is to experimentally test whether
Spiroplasma of pea aphids also use plants to get horizontally transmitted. This experiment would
use clip cages.
The second part of the project would be more theoretical. When phytopathogens are transmitted
by sap-sucking insects, they are strongly selected to be able to survive and reproduce within each of
their two hosts (plant and insect), and to get transmitted from one host to the other host. However, if
in one of the two host, the phytopathogen become able to be vertically transmitted from mother to
offspring, then its persistence might be less dependent on the other host. The second part of this
project would thus be to investigate by thought experiments and simulations (Skaf and Imbert 2013)
the similarities, the differences, and the eventual evolutionary transition between these two
symbiotic life style. This would also shed light on the role of horizontal transmissions events on the
maintenance of vertically transmitted symbionts. These simulations would be conducted either with
the Simupop python module, or with raw Python or R programming. The simulation part of the
project is optional, and relative importance of these two parts will depend on the student’s interest
and capabilities. Although the experiment could be completed within 3 months, the project is better
suited for six months, in particular if it involves simulations.
Bové, J. M., Renaudin, J., Saillard, C., Foissac, X., & Garnier, M. (2003). SPIROPLASMA CITRI, A PLANT
PATHOGENIC MOLLICUTE: Relationships with Its Two Hosts, the Plant and the Leafhopper Vector. Annual Review
of Phytopathology, 41(1), 483–500. doi:10.1146/annurev.phyto.41.052102.104034
Capinera, J. L. (2008). Encyclopedia of Entomology. (D. Zuzana Bernhart & S. Fabiani, Eds.) (2nd ed., Vol. 4).
doi:10.1017/CBO9781107415324.004
Caspi-Fluger, A., Inbar, M., Mozes-Daube, N., Katzir, N., Portnoy, V., Belausov, E., … Zchori-El Skaf, R., & Fein,
E. (2012). Horizontal transmission of the insect symbiont Rickettsia is plant-mediated. Proceedings. Biological
Sciences, 279(1734), 1791–1796. doi:10.1098/rspb.2011.2095
Imbert, C. (2013). Unfolding in the empirical sciences: Experiments, thought experiments and computer
simulations. Synthese, 190(16), 3451–3474. doi:10.1007/s11229-012-0203-y
Weintraub, P. G., & Beanland, L. (2006). Insect Vectors of Phytoplasmas. Annual Review of Entomology, 51(1),
91–111. doi:10.1146/annurev.ento.51.110104.151039
“Cancer evolution in symbioses”: Does the evolution of bacterial symbionts within their host,
during their host life leads to the Peto’s paradox that is meet in evolutionary cancerology?
Background: Cells can mutate during the individual's life and undergo natural selection within
the individual (Burt and Trivers 2006). In some extreme cases, this leads to cancers. Similarly,
symbionts that live within their host are likely to also undergo mutations and evolution during the
host’s life.
Evolutionary biology brought a prediction to cancerology: small and short lived species should
be less exposed to cancers because less cell divisions occur during each individual life, meaning
that there are fewer opportunities for cell evolution to occur. This prediction is not corroborated by
observations. Although at the within species level, tall and big individuals have a higher risk to
develop cancer, at the between species level there is no link between cancer risk and species size or
lifespan. This discrepancy between expectation and observation is called the "Peto's paradox"
(Caulin and Maley 2011; Roche et al., 2013).
The idea & the project: Host size and lifespan should affect the number of generations
undergone by symbionts during the host life, leading to a variation in the likelihood of within host
evolution of symbionts. These “symbiotic cancers” should entail a significant cost to the hosts. This
cost should be stronger for big and long lived hosts, leading vertically transmitted symbiosis to be
less frequent in big, long lived organisms than in small, short lived organism. The absence of such a
trend would point toward the existence of a Peto’s paradox in the symbionts. This project is to use a
meta-analysis to test if there is an equivalent of the "Peto's paradox" in symbiosis. The first and
main step will be to construct a database by extracting data from the scientific literature. Then the
database will be statistically analysed. Some other hypotheses related to the evolution of symbiosis
might emerge and be tested with the same database.
The student might also be interested in testing if mechanisms that have evolved to protect against
cancer would help the host to deal with this “symbiotic cancers”. This could be done with thought
experiments and simulations (Skaf and Imbert 2013). The project is better suited for six months.
Burt, A., & Trivers, R. (2006). Selfish Cell Lineages. In Genes in Conflict: The Biology of Selfish Genetic Elements
(pp. 420–446). Harvard University.
Caulin, A. F., & Maley, C. C. (2011). Peto’s Paradox: Evolution's prescription for cancer prevention. Trends in
Ecology and Evolution, 26(4), 175–182. doi:10.1016/j.tree.2011.01.002
Imbert, C. (2013). Unfolding in the empirical sciences: Experiments, thought experiments and computer
simulations. Synthese, 190(16), 3451–3474. doi:10.1007/s11229-012-0203-y
Roche, B., Sprouffske, K., Hbid, H., Missé, D., & Thomas, F. (2013). Peto’s paradox revisited: Theoretical
evolutionary dynamics of cancer in wild populations. Evolutionary Applications, 6(1), 109–116. doi:10.1111/eva.12025
Possible collaborative Projects with the University of Lausanne
(would require spending some time at Uni Lausanne with Dr. Casper von der Kooi, who contributed these
project suggestions, and Prof. Tanja Schwander. Here's the link to her research group:
http://www.unil.ch/dee/en/home/menuinst/research/schwander-group.html)
Inferring the genetic architecture of morphological trait variation in a parasitoid wasp
Background: In the parasitoid wasp Lysiphlebus fabarum both sexual and asexual lineages occur. Sandrock
and Vorburger (2011) found that a single recessive allele causes asexuality in L. fabarum, meaning that a
rare asexual male can be used to introgress the asexual genome into a sexual lineage. We recently
performed a crossing experiment where we introgressed the asexuality allele into sexual lineages, and
thereby established several new asexual lines. A recent study showed that asexual L. fabarum individuals
have different wing shapes than their sexual counterparts, suggesting that in this species reproductive
mode and wing shape is linked (Petrovic et al. 2015). However, the genetic architecture of wing shape
variation is unknown in this species. We recently performed a crossing experiment where we introgressed
the asexuality allele into sexual lineages, and thereby we established several new asexual lines.
Aim: We want to know if the differences in wing shape - and other morphological traits like head and body
size - in L. fabarum are caused by the same recessive allele that causes asexuality.
Methods: The student will determine the genetic architecture underlying wing shape variation in the
parasitoid wasp L. fabarum. Using individuals from our previous crossing experiment, the student will
quantify wing shape variation in individuals of different generations. Because the pedigree of the
individuals is known, the genetic architecture underlying wing shape variation can be inferred. Particularly
interesting would be to see whether wing shape variation is inherited in a similar way as the allele causing
asexual reproduction. Depending on the interest of the student, the project can be extended by also
quantifying other morphological traits (for example head, antennae and/or body size) as well as a mapping
study using a previously established RAD-sequencing panel.
Is the same locus causing asexuality in different Lysiphlebus parasitoid wasp species?
Background: The evolution and maintenance of sex is one of the major questions in evolutionary biology.
Despite the many costs associated with sexual reproduction, asexual reproduction is rare among
eukaryotes. In some taxa, however, asexual reproduction occurs frequently. To understand why asexuality
occurs frequently in some animal taxa, it is important to understand the mechanisms underlying the
transition from sexual to asexual reproduction.
In the parasitoid wasp genus Lysiphlebus several species comprise both sexual and asexual lineages.
Previous breeding experiments showed that asexuality in one species, L. fabarum, is inherited as a singlelocus recessive trait (Sandrock and Vorburger 2011) and we are in the process of mapping the locus causing
asexuality in L. fabarum using RAD-sequencing (for review, Davey et al. 2011). The high frequency of
asexual reproduction within the Lysiphlebus genus raises the question of whether the same locus causes
asexual reproduction in other species of this genus or whether different species acquired asexuality via
different mechanisms.
Aim: We will test whether the locus that causes asexuality in L. fabarum is the universal mechanism causing
asexuality in other Lysiphlebus species.
Methods: To infer the reproductive mode and test for single-locus recessive inheritance of asexuality in
different Lysiphlebus lineages and species, we will perform several breeding experiments. By using the
results from our previous mapping experiment, asexual and sexual lineages in different Lysiphlebus species
will be genotyped at the locus that causes asexuality in L. fabarum. Depending on interest, the project can
also include sampling of natural populations as well as mapping morphological traits.
References:
Sandrock and Vorburger 2011. Single-locus recessive inheritance of asexual reproduction in a parasitoid
wasp. Curr. Biol. 21:433-437
Petrovic et al. 2015. Genetic and morphological variation in sexual and asexual parasitoids of the genus
Lysiphlebus – an apparent link between wing shape and reproductive mode. BMC Evol. Biol. 15:5
Davey et al. 2011 Genome-wide genetic marker discovery and genotyping using next-generation
sequencing. Nat. Rev. Genet. 12:499-510