Does variability matter? [Elektronische Ressource] : major histocompatibility complex (MHC) variation and its associations to parasitism in natural small mammal populations / von Yvonne Meyer-Lucht

Meyer-Lucht, Giordano Yvonne

Sujets

• Biologie

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• Publié par: universitat_potsdam
• Publié le: 01 janvier 2009
• Nombre de lectures: 33
• Langue: English
• Poids de l'ouvrage: 8 Mo

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Does variability matter? Major histocompatibility complex (MHC)
variation and its associations to parasitism
in natural small mammal populations


















Ph.D. Thesis
Yvonne Meyer-Lucht Leibniz Institut für Zoo- und Wildtierforschung
Forschungsgruppe Evolutionsgenetik



Does variability matter? Major histocompatibility complex (MHC)
variation and its associations to parasitism
in natural small mammal populations




Dissertation
zur Erlangung des akademischen Grades
"doctor rerum naturalium"
(Dr. rer. nat.)
in der Wissenschaftsdisziplin "Evolutionsbiologie"





eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät
der Universität Potsdam



von
Yvonne Meyer-Lucht


Potsdam, Mai 2009














































Published online at the
Institutional Repository of the University of Potsdam:
URL http://opus.kobv.de/ubp/volltexte/2009/3641/
URN urn:nbn:de:kobv:517-opus-36419
[http://nbn-resolving.org/urn:nbn:de:kobv:517-opus-36419]










Meiner Familie

Table of contents


Preface 6
Summary 7
Zusammenfassung 9
Introduction 11
Study Framework and Study Species 17
Article Summaries 20
Article 1 20
Selection, diversity and evolutionary patterns of the MHC class II
DAB in free-ranging Neotropical marsupials
Article 2 22
Variety matters: adaptive genetic diversity and parasite load in
two mouse opossums from the Brazilian Atlantic forest
Article 3 24
Number of MHC alleles is related to parasite loads in natural
populations of yellow necked mice (Apodemus flavicollis)

Discussion 26
References 34
Appendix 41
Article 141 rticle 271
Article 3101
Acknowledgements 121
6 PREFACE

Preface

In my Ph.D. thesis I have studied adaptive genetic variation at the major histocompatibility
complex (MHC) in different small mammal populations and assessed its relevance in
pathogen resistance in the natural environment. My thesis comprises three article
manuscripts, which can be read independently. The first article is published, while the
remaining two are submitted to international peer-reviewed scientific journals. Because all
articles include co-authors, they are written in the first person plural. Each article summary
contains a declaration of the authors’ individual inputs to highlight my own independent
contributions.

SUMMARY 7
Summary

The adaptive evolutionary potential of a species or population to cope with
omnipresent environmental challenges is based on its genetic variation. Variability at
immune genes, such as the major histocompatibility complex (MHC) genes, is assumed to
be a very powerful and effective tool to keep pace with diverse and rapidly evolving
pathogens. In my thesis, I studied natural levels of variation at the MHC genes, which have
a key role in immune defence, and parasite burden in different small mammal species. I
assessed the importance of MHC variation for parasite burden in small mammal
populations in their natural environment. To understand the processes shaping different
patterns of MHC variation I focused on evidence of selection through pathogens upon the
host. Further, I addressed the issue of low MHC diversity in populations or species, which
could potentially arise as a result from habitat fragmentation and isolation.
Despite their key role in the mammalian evolution the marsupial MHC has been rarely
investigated. Studies on primarily captive or laboratory bred individuals indicated very little
or even no polymorphism at the marsupial MHC class II genes. However, natural levels of
marsupial MHC diversity and selection are unknown to date as studies on wild populations
are virtually absent. I investigated MHC II variation in two Neotropical marsupial species
endemic to the threatened Brazilian Atlantic Forest (Gracilinanus microtarsus, Marmosops
incanus) to test whether the predicted low marsupial MHC class II polymorphism proves to
be true under natural conditions. For the first time in marsupials I confirmed characteristics
of MHC selection that were so far only known from eutherian mammals, birds, and fish:
Positive selection on specific codon sites, recombination, and trans-species polymorphism.
Beyond that, the two marsupial species revealed considerable differences in their MHC
class II diversity. Diversity was rather low in M. incanus but tenfold higher in G. microtarsus,
disproving the predicted general low marsupial MHC class II variation.
As pathogens are believed to be very powerful drivers of MHC diversity, I studied
parasite burden in both host species to understand the reasons for the remarkable
differences in MHC diversity. In both marsupial species specific MHC class II variants were
associated to either high or low parasite load highlighting the importance of the marsupial
MHC class II in pathogen defence. I developed two alternative scenarios with regard to
MHC variation, parasite load, and parasite diversity. In the ‘evolutionary equilibrium’
scenario I assumed the species with low MHC diversity, M. incanus, to be under relaxed
pathogenic selection and expected low parasite diversity. Alternatively, low MHC diversity 8 SUMMARY
could be the result of a recent loss of genetic variation by means of a genetic bottleneck
event. Under this ‘unbalanced situation’ scenario, I assumed a high parasite burden in
M. incanus due to a lack of resistance alleles. Parasitological results clearly reject the first
scenario and point to the second scenario, as M. incanus is distinctly higher parasitised but
parasite diversity is relatively equal compared to G. microtarsus. Hence, I suggest that the
parasite load in M. incanus is rather the consequence than the cause for its low MHC
diversity.
MHC variation and its associations to parasite burden have been typically studied
within single populations but MHC variation between populations was rarely taken into
account. To gain scientific insight on this issue, I chose a common European rodent
species. In the yellow necked mouse (Apodemus flavicollis), I investigated the effects of
genetic diversity on parasite load not on the individual but on the population level. I
included populations, which possess different levels of variation at the MHC as well as at
neutrally evolving genetic markers (microsatellites). I was able to show that mouse
populations with a high MHC allele diversity are better armed against high parasite burdens
highlighting the significance of adaptive genetic diversity in the field of conservation
genetics. An individual itself will not directly benefit from its population’s large MHC allele
pool in terms of parasite resistance. But confronted with the multitude of pathogens present
in the wild a population with a large MHC allele reservoir is more likely to possess
individuals with resistance alleles. These results deepen our understanding of the complex
causes and processes of evolutionary adaptations between hosts and pathogens.