Mechanisms for partial reproductive isolation in a Bombina hybrid zone in Romania [Elektronische Ressource] / vorgelegt von Sonja Köhler

Koehler, Sonja

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• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2003
• Nombre de lectures: 9
• Langue: English

Extrait

Mechanisms for partial reproductive isolation
in a Bombina hybrid zone in Romania
Dissertation
zur Erlangung des Doktorgrades
der Fakultät für Biologie
der Ludwig-Maximilians-Universität München
vorgelegt im Juli 2003
von
Sonja Köhler
aus Bocholt1. Gutachter: PD Dr. Beate Nürnberger
2. Gutachter: Prof. Dr. Wolfgang Stephan
Dissertation eingereicht am: 31.07.2003
Tag der mündlichen Prüfung: 24.11.2003Abstract
Differences between taxa which have developed in allopatry can contribute to reproductive
isolation in the case of secondary contact. Hybrid zones are ideal study systems in which to
investigate the role of pre- and postzygotic mechanisms for the reduction or inhibition of gene
flow. This thesis describes a hybrid zone between the fire-bellied toads Bombina bombina and
B. variegata in Romania.
The spatial arrangement of populations in this hybrid zone resembles a broad mosaic, with B
bombina restricted to scattered big ponds and B. variegata-like hybrids occupying the
surrounding less permanent water bodies. This structure is in striking contrast with the steep
clinal transitions found in hybrid zones in Croatia, Poland and the Ukraine. A detailed
comparison between the transects in Romania and Croatia revealed that the underlying
distribution of habitat is the most likely factor determining the structure of a Bombina hybrid
zone. Furthermore, habitat preference is stronger in Romania than in Croatia. Despite habitat
preference, B. bombina adults occasionally migrate out of ponds and reproduce in
intermediate habitat, thus causing introgression at neutral markers in the B. variegata-like
populations there. In Vines et al. (in press), we used the genetic structure to quantify this
migration and then assessed how much selection is required to counteract the breakdown of
adaptive differences. The necessary level of selection is plausible but neutral divergence is
probably collapsing.
Breeding site preference in adults and natural selection in embryos and tadpoles may be
important forces against immigrant B. bombina alleles in B. variegata-like populations. I
found a consistent shift in breeding habitat preference towards B. variegata in intermediate
habitat. I also quantified natural selection in tadpoles as this should constitute a similarly
important but postzygotic mechanism for partial reproductive isolation. There was significant
intrinsic selection against B. bombina alleles in B. variegata-like families. This fits the
prediction that selection should be against immigrant B. bombina alleles rather than
heterozygotes. There was no direct evidence for extrinsic selection in tadpoles, although it is
strongly suggested by breeding habitat preference in adults. This issue is worth further
investigation. I also investigated tadpoles after selection at the phenotypic level. B. variegata-
like tadpoles grow and develop faster than B. bombina-like ones in intermediate habitat,
which affords them an adaptive advantage in the face of desiccation. Considering phenotypic
plasticity, B. bombina-like tadpoles show the same high level and continuous range as
B. variegata. This finding is probably related to the high rate of introgression in the Romanian
hybrid zone.
I showed that habitat preference and selection are important mechanisms for the maintenance
of reproductive isolation in this Bombina mosaic hybrid zone and may play an important role
for reproductive isolation in incipient species.Abbreviations
cm centimeter
dNTP Desoxyribonucleosidtriphosphate
EDTA Ethylenediaminetetraacetate
km kilometer
m meter
Mmolar
mM millimolar
min minutes
rpm rotations per minute
SDS Sodium Dodecyl Sulfate
TA Tris-acetate
TBE Tris/Borate/EDTA
TNES Tris/NaCl/EDTA/SDS
V voltTABLE OF CONTENTS
1 INTRODUCTION 1
1.1 Overview 1
1.2. Reproductive isolation 3
1.2.1 Modes of reproductive isolation 3
1.2.2 The evolution of reproductive isolation 4
1.3 Hybrid zones 7
1.3.1 The origin of hybrid zones 8
1.3.2 The fate of hybrid zones
1.3.3 Patterns of hybrid zones 10
1.4 Bombina hybrid zones 16
1.4.1 The genus Bombina 16
1.4.2 Biogeography
1.4.3 Differences between Bombina bombina and Bombina variegata 18
1.4.4 Previous work on Bombina hybrid zones 21
1.5 Aims of thesis and Chapter outline 24
2 THE APAHIDA HYBRID ZONE 26
2.1 Overview 26
2.2 The study site
2.3 Material and Methods 28
2.3.1 Collection and processing of the animals 28
2.3.2 Collection of ecological data 29
2.3.3 Multivariate statistics for the analysis of the ecological data 30
2.3.4 Solutions for the molecular analysis 31
2.3.5 Genotyping 31
2.3.6 Statistical techniques for the analysis of genetic data 34
2.4 Results 35
2.4.1 Habitat types 35
2.4.2 Concordance between loci 38
2.4.3 The distribution of adult genotypes 40
2.5 Summary and conclusions 43
3 HABITAT PREFERENCE, MIGRATION AND SELECTION 45
3.1 Introduction 45
3.2 Methods 45
3.3 Results 46
3.4 Discussion 49
3.5 Summary 52
4 INTRINSIC SELECTION 54
4.1 Introduction 54
4.1.1 Intrinsic selection in hybrid zones 55
4.1.2 Inelection in Bombina
4.1.3 Measuring intrinsic selection 564.2 Methods 59
4.2.1 Selecting the sites 59
4.2.2 Egg collection 59
4.2.3 Rearing scheme and measuring viability 60
4.2.4 Genotyping 61
4.2.5 Preparing the data set 61
4.2.6 Tadpole mortality 64
4.2.7 Test for intrinsic selection 65
4.3 Results 66
4.3.1 The data set 66
4.3.2 Tadpole mortality 68
4.3.3 Intrinsic selection 69
4.4 Discussion 72
4.5 Summary