Ghent University Faculty of Sciences Department of Biology Academic year 2011 – 2012 Adaptive divergence in the wing-polymorphic salt marsh beetle Pogonus chalceus by Elena Dierick Supervisor: Prof. Dr. Frederik Hendrickx Tutor : Drs. Steven Van Belleghem Thesis submitted to obtain Research group: Terrestrial Ecology Unit the degree of Master in Biology “© All rights reserved. This thesis contains confidential information and confidential research results that are property to the UGent. The contents of this master thesis may under no circumstances be made public, nor complete or partial, without the explicit and preceding permission of the UGent representative, i.e. the supervisor. The thesis may under no circumstances be copied or duplicated in any form, unless permission granted in written form. Any violation of the confidential nature of this thesis may impose irreparable damage to the UGent. In case of a dispute that may arise within the context of this declaration, the Judicial Court of Gent only is competent to be notified. .” Illustration on the cover: male of Pogonus chalceus. Photographed by U. Schmidt, 2008. Acknowledgement Acknowledgement I am heartily thankful to my supervisors, Prof. Dr. Frederick Hendrickx and Drs. Steven Van Belleghem, whose encouragement, guidance and support enabled me to develop a good understanding of the subject. I would also like to thank, Charlotte, Bram and Vicky for the company and help during sample collection. Furthermore, I want to express my gratitude for the constant support of my friends, brothers and sisters. I especially want to thank Bram and Sarah for their helpful - and if not helpful then at least very funny - suggestions for my thesis. Finally, I want to thank my parents for their support and encouragement throughout my studies. 1 Abstract Abstract Previous research on the salt marsh beetle Pogonus chalceus from several European populations revealed habitat stability related divergence for both dispersal power and the pattern of allele frequencies of the isocitrate dehydrogenase 2 (IDH2) allele. Populations from temporary habitats showed near-maximum dispersal power and very high frequencies of the IDH2-L allele, whereas stable populations showed more varying degrees of dispersal power and high frequencies of the IDH2-S allele. In the present study, analysis of phylogenetic trees constructed for both the adaptive IDH2 gene and the putative neutral gene, Cytochrome Oxidase subunit one (COI), using several species of the genus Pogonus and Pogonistes from different European populations reveals a deep divergence within P. chalceus for IDH2. This deep divergence was not present within P. chalceus for COI, providing an indication that balancing selection preserves variation within the IDH2 gene. The molecular clock analysis suggests a high nucleotide substitution rate for IDH2 and a recent, single origin of the IDH2-S allele. Our results support the hypothesis that the IDH2-S allele has a recent, single origin in allopatry and entered populations throughout Europe via introgression from standing genetic variation. In the Guérande salt marshes, where the ponds and canals present a mosaic of stable and temporary habitats, this divergence is observed at a microgeographical scale. Previous studies revealed that habitat selection preserves the differentiation between the pond and canal inhabiting individuals of P. chalceus in the Guérande salt marshes, despite strong gene flow. In our study, we investigate whether reproductive barriers are present between the ecotypes of P. chalceus, which might assist habitat selection in counterbalancing the homogenizing effect of gene flow. We found no indication for the presence of temporal isolation between the ecotypes of P. chalceus in the Guérande, however, we state that the remarkable difference in willingness to mate between pond and canal males observed in the present study might prove to contribute to reproductive isolation upon further investigation. The case study of adaptive divergence in P. chalceus contributes to the growing awareness that the plurality of mechanisms and processes underlying the process of divergence often renders this process more complex than generally acknowledged. 2 1 Introduction 1 Introduction More than 150 years after Darwin published his ideas on evolution, it has become clear that they were the trigger of intense debates that are as active now as ever before. Although it is now generally accepted that the origin of species really does occur by means of natural selection, many questions still remain (Schluter 2009). The reconciliation of Darwin’s theory with important discoveries in genetics added a whole new aspect to evolutionary thinking, for it became clear where variation comes from and how it can be passed on to following generations. Within this new, genetic framework Dobzhansky and Mayr defined species and speciation based on the criterion of reproductive isolation instead of morphological and other phenotypic differences (Dobzhansky 1937, Mayr 1942). The biological species concept (BSC) defined by Ernst Mayr in 1942 stated that “species are groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups”. Hence, from that moment on, the study of speciation became equivalent to the study of the evolution of reproductive isolation (Schluter 2009). Under the BSC, speciation consists of the evolution of biological barriers to gene flow and it has become clear that there are many kinds of isolating barriers, which can be classified as either prezygotic or postzygotic barriers (Futuyama 2005). A great challenge in speciation studies is to identify the magnitude and the order of appearance of these isolating barriers and other drivers of species divergence. Sobel et al. (2009) define the most important isolating barriers as those that hamper the majority of gene flow at the time of speciation. Although speciation may indeed result from the action of only one isolating barrier, it is conceivable that, as Sobel et al. (2009) formulated so elegantly, evolutionary trajectories may reticulate in the future. This poses a huge difficulty for evolutionary biologists who try to deduce, long after the event of speciation took place, how the reproductive barriers evolved, which of them were most important and how they are currently maintained. Therefore, it is not surprising that strong disagreement remains as to how this could be accomplished. In an attempt to improve our understanding of speciation mechanisms, Schluter (2001) and Rundle and Nosil (2005) suggested a classification of speciation modes that focuses on the mechanisms that drive the evolution of reproductive isolation, which resulted in distinguishing ecological from non-ecological speciation. Ecological speciation is defined as a concept that unites speciation processes in which reproductive isolation evolves ultimately as a consequence of divergent selection on traits between environments (Schluter 2001). Non-ecological speciation includes modes of speciation in which chance events play a central role, such as genetic drift, polyploid speciation and speciation by hybridization. It may also include modes of speciation in which selection is essential, namely, speciation by sexual selection and speciation by divergence under uniform selection, in which different advantageous but incompatible mutations arise in separate populations occupying similar environments. Both natural and sexual selection may be involved in every speciation mode presented above, therefore, the presence of 3 1 Introduction either natural or sexual selection per se is not a good basis for classifying speciation events. (Rundle and Nosil 2005, Schluter 2001, 2009). Sobel et al. (2009) argue, however, that this proposed dichotomy of ecological versus non-ecological speciation is both flawed and unnecessary, having shown that ecology is involved in both uniform selection and polyploidy, and that genetic drift is unlikely to cause speciation unilaterally. They conclude that ecology is rarely, if ever, truly absent from speciation and that, therefore, ecological adaptation is the major driver of reproductive isolation. Historically, speciation modes have been classified based on the geographical arrangement of populations undergoing speciation, distinguishing allopatric, parapatric and sympatric speciation (Futuyma 2005). Allopatric speciation is defined as the evolution of genetic reproductive barriers between populations that are geographically separated. In parapatric speciation, reproductive isolation evolves between neighboring populations, whereas in sympatric speciation, reproductive isolation evolves within a single, initially randomly mating population, i.e. without geographic isolation. According to Mayr (1963), complete geographic isolation is a prerequisite for animal divergence, arguing that sympatric divergence is theoretically very unlikely. Sympatric speciation has always been a much contested topic in evolution. Many discussions can be related to the vagueness of the definition. Isolation, for example, is a matter of degree and may depend on spatial scale (Bolnick and Fitzpatrick 2007, Mallet et al. 2009). Some instances of sympatric speciation are uncontroversial, such as polyploid speciation, hybrid speciation or sexual populations that produce asexual or selfing lines via mutation. In these instances recombination does not oppose divergence. However, in most instances the key problem is that mating and recombination