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MHC and genomic diversity in Atlantic salmon (Salmo salar L.)

Lohm, Jakob LU (2002)
Abstract (Swedish)
Popular Abstract in Swedish

Graden av genetisk variation inom och mellan olika populationer bestäms av balansen mellan tre faktorer: naturligt urval, slumpmässig genetisk drift och genflöde. Genom studier av genetisk variation är det möjligt att dra slutsatser om t ex selektionstryck på olika alleler (genvarianter), arters spridningsvägar till nya platser, tid sedan etablering och graden av släktskap mellan olika populationer mm. Genom val av analysmetod kan undersökningar av genetisk variation fokuseras på enskilda gener eller samla in information från flera olika genetiska regioner utspridda i hela genomet. Enskilda gener, särskilt om dessa kodar för viktiga proteiner, är normalt mer utsatta för naturlig urval medan... (More)
Popular Abstract in Swedish

Graden av genetisk variation inom och mellan olika populationer bestäms av balansen mellan tre faktorer: naturligt urval, slumpmässig genetisk drift och genflöde. Genom studier av genetisk variation är det möjligt att dra slutsatser om t ex selektionstryck på olika alleler (genvarianter), arters spridningsvägar till nya platser, tid sedan etablering och graden av släktskap mellan olika populationer mm. Genom val av analysmetod kan undersökningar av genetisk variation fokuseras på enskilda gener eller samla in information från flera olika genetiska regioner utspridda i hela genomet. Enskilda gener, särskilt om dessa kodar för viktiga proteiner, är normalt mer utsatta för naturlig urval medan utspridda genetiska markörer oftast är mer påverkade av genetisk drift och genflöde mellan angränsande populationer. I denna avhandling har jag, med hjälp av olika molekylärgenetiska metoder, studerat genomisk variation hos lax, öring och röding samt specifik genetisk variation på en immunförsvarsgen hos lax. Öring visade sig vara den mest genetiskt variabla arten följd av röding och lax. Detta är vad man kan förvänta sig om man tittar på de olika arternas spridningsförmåga, koloniseringsmöjligheter efter istiden och inte minst mänsklig påverkan av deras lekområden. I en studie av genomisk variation hos olika populationer av lax inom en älv, Kalixälven som är en av få orörda älvar i Sverige, visade det sig att graden av genetiskt likhet mellan populationer minskar med ökande geografiskt avstånd mellan lekområden. Detta resultat visar att olika lokala bestånd är genetiskt åtskilda och att olika populationer inom en älv kan vara genetiskt anpassade till olika miljöer. Resultatet pekar också på Kalixälvens unika värde som relativt opåverkad laxälv. Å andra sidan så fanns det inget samband mellan geografiskt avstånd och genetisk variation för en MHC-gen (Major Histocompatibility Complex är ett genkomplex med central roll i immunförsvaret och därför antas vara påverkad av naturligt urval) i samma älv. Detta antyder att det naturliga urvalet bevarar MHC variation över stora områden vilket inte var fallet med den neutrala genetiska variationen. Den genetiska heterogeniteten, i utspridda genetiska markörer, inom Kalixälven var jämförbar med de skillnader som fanns mellan olika älvar i norra Sverige vilket visar att ortstroheten till lekområdena är mycket stark hos dessa havsvandrande laxar. I denna avhandling visar jag också hur skillnader i specifika immunförsvarsgener kan påverka en individs livsduglighet. Olika alleler inom MHC orsakade signifikanta skillnader i överlevnad mellan olika genotyper vid experimentella infektioner av lax. Genetiska skillnader så små som ett baspar mellan olika MHC alleler hade avgörande effekter på överlevnad. Dessa experiment visade också att specifika MHC alleler påverkar individers överlevnad mer än graden av genetisk variation i sig, (heterozygoti) på dessa gener, vilket ofta har antagits tidigare. (Less)
Abstract
In this thesis I present studies concerning genomic diversity and genetic heterogeneity as well as the evolution and maintenance of genetic variation at a specific immune defence gene in Atlantic salmon (Salmo salar, L.). I here present data on nucleotide diversity showing that human activities as well as different post-glacial colonisation events have had considerable effects on the genomic diversity in different salmonid populations. Also, significant within-river heterogeneity in an unmanaged naturally reproducing population of Atlantic salmon is described. In this natural river, the subpopulation genetic structure was significantly correlated to the geographical distance between subpopulations suggesting that gene flow between... (More)
In this thesis I present studies concerning genomic diversity and genetic heterogeneity as well as the evolution and maintenance of genetic variation at a specific immune defence gene in Atlantic salmon (Salmo salar, L.). I here present data on nucleotide diversity showing that human activities as well as different post-glacial colonisation events have had considerable effects on the genomic diversity in different salmonid populations. Also, significant within-river heterogeneity in an unmanaged naturally reproducing population of Atlantic salmon is described. In this natural river, the subpopulation genetic structure was significantly correlated to the geographical distance between subpopulations suggesting that gene flow between subpopulations within this natural Atlantic salmon population corresponds to an isolation-by-distance process. However, major histocompatibility complex (MHC) alleles, encoding molecules crucial for the presentation of foreign antigens to T-cells in the adaptive immune system, are homogeneously distributed along the same river. This indicates that selection maintains genetic homogeneity at this specific MHC locus over large areas, probably due to a uniformly distributed pathogen fauna within the river, which is not the situation at neutral loci where genetic drift is counterbalanced by migration. Moreover, the within-river spatial genetic heterogeneity was almost as large as between-river differentiation showing that significant local adaptation probably exists even between regions within rivers. In this thesis I also show that genetic variation at specific genes may have pronounced fitness effects. Different alleles within the MHC rendered significant differences in disease resistance during controlled experimental infections of Atlantic salmon. Nucleotide differences as small as one single base substitution between different MHC alleles had considerable effects on susceptibility. Since MHC heterozygosity did not affect disease resistance these findings suggest that frequency-dependent selection, rather than overdominant selection, is acting at on the MHC. These results are in opposition to the widely assumed hypothesis that overdominant selection is the main contributor to maintenance of MHC diversity. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Dr Miller, Kristi, Fisheries and Oceans, Pacific Biological Station, Canada
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Djurekologi, cytogenetik, Genetik, cytogenetics, Genetics, nucleotide diversity, isolation-by-distance, DGGE, AFLP, furunculosis, disease resistance, Arctic charr, brown trout, Atlantic salmon, population genetics, genetic variation, Mhc class IIB, Animal ecology
pages
76 pages
publisher
Jakob Lohm, Department of Ecology, Animal Ecology, Ekologihuset, S-223 62 Lund Sweden,
defense location
Blå Hallen, Ekologihuset, Lund
defense date
2002-10-02 10:15
external identifiers
  • Other:ISRN: SE-LUNBDS/NBZE-02/1091+76PP
ISBN
91-7105-178-3
language
English
LU publication?
yes
id
d5a7d3bb-e7df-423a-a21c-99415217e1ff (old id 464948)
date added to LUP
2007-09-05 10:34:16
date last changed
2016-09-19 08:45:15
@misc{d5a7d3bb-e7df-423a-a21c-99415217e1ff,
  abstract     = {In this thesis I present studies concerning genomic diversity and genetic heterogeneity as well as the evolution and maintenance of genetic variation at a specific immune defence gene in Atlantic salmon (Salmo salar, L.). I here present data on nucleotide diversity showing that human activities as well as different post-glacial colonisation events have had considerable effects on the genomic diversity in different salmonid populations. Also, significant within-river heterogeneity in an unmanaged naturally reproducing population of Atlantic salmon is described. In this natural river, the subpopulation genetic structure was significantly correlated to the geographical distance between subpopulations suggesting that gene flow between subpopulations within this natural Atlantic salmon population corresponds to an isolation-by-distance process. However, major histocompatibility complex (MHC) alleles, encoding molecules crucial for the presentation of foreign antigens to T-cells in the adaptive immune system, are homogeneously distributed along the same river. This indicates that selection maintains genetic homogeneity at this specific MHC locus over large areas, probably due to a uniformly distributed pathogen fauna within the river, which is not the situation at neutral loci where genetic drift is counterbalanced by migration. Moreover, the within-river spatial genetic heterogeneity was almost as large as between-river differentiation showing that significant local adaptation probably exists even between regions within rivers. In this thesis I also show that genetic variation at specific genes may have pronounced fitness effects. Different alleles within the MHC rendered significant differences in disease resistance during controlled experimental infections of Atlantic salmon. Nucleotide differences as small as one single base substitution between different MHC alleles had considerable effects on susceptibility. Since MHC heterozygosity did not affect disease resistance these findings suggest that frequency-dependent selection, rather than overdominant selection, is acting at on the MHC. These results are in opposition to the widely assumed hypothesis that overdominant selection is the main contributor to maintenance of MHC diversity.},
  author       = {Lohm, Jakob},
  isbn         = {91-7105-178-3},
  keyword      = {Djurekologi,cytogenetik,Genetik,cytogenetics,Genetics,nucleotide diversity,isolation-by-distance,DGGE,AFLP,furunculosis,disease resistance,Arctic charr,brown trout,Atlantic salmon,population genetics,genetic variation,Mhc class IIB,Animal ecology},
  language     = {eng},
  pages        = {76},
  publisher    = {ARRAY(0x94453c8)},
  title        = {MHC and genomic diversity in Atlantic salmon (Salmo salar L.)},
  year         = {2002},
}