Browsing by Subject "speciation"

Adaptive radiation is an important mechanism of evolution, which can lead to emergence of sympatric species or morphotypes. Among other biological interactions, parasitic pressure can have significant evolutionary implications for host populations by reducing the fitness of the host individuals. Parasite community structure of fishes is typically strongly dependent on both host ecology (e.g. habitat and feeding behaviour) and environmental factors (e.g. water quality and temperature). However, the relative importance of these factors for parasite-mediated speciation is not known. Also, host gender-specific differences in parasite communities can have an effect on the differentiation of host morphs. In this Master's thesis, I focused on differences in parasite communities of sympatric morphs of three-spine stickleback (Gasterosteus aculeatus) in two large Icelandic lakes, Thingvallavatn and Mývatn. In these lakes, the habitats of sympatric mud and lava morphs are the same (soft/hard bottom), but the habitat water temperatures are opposite between the lakes. In this unique system, it is thus possible to compare between the effects of host ecology and water temperature on parasite community structure and strength of parasite-mediated selection. Additionally, I studied the effect of host sex on the parasitic pressure. There is also a third stickleback morph, Nitella morph, inhabiting the cold limnetic habitats in Lake Thingvallavatn. I sampled the stickleback morphs from both lakes and identified their parasite species. I discovered a total of five parasite genera: trematodes Apatemon and Diplostomum, and cestodes Diphyllobothrium, Proteocephalus and Schistocephalus. Most of the observed parasites have negative effects on health and fitness of the host. I found more parasites in sticklebacks living in higher temperature in both lakes regardless of the morph. When comparing the cold water morphs in Thingvallavatn, parasite abundance was higher in the limnetic Nitella morph than in the shallow water lava morph. Fish gender had an effect on parasitism only in Thingvallavatn as males of both lava and Nitella morphs were more heavily infected with cestodes. Similarities in parasite communities with water temperature indicate that water temperature mainly determines parasite infections in this system instead of host ecology. As similar fish morphotypes exist in different lakes under opposite parasitic pressure, parasitism has unlikely initiated host differentiation, but differences in infection probably have emerged secondary to the ecological specialization of the morphs to different habitats. These results are among the first to tackle the key question in parasite-mediated divergent selection: at which point of the speciation process parasite communities become differentiated and thus can have an effect on speciation. However, the comparison between the cold water morphs (lava and Nitella) indicates that although water temperature seems to be the main factor controlling infections in this system, its effect may still be over ridden by host ecology. Sex-depended differences in parasitic pressure, on the other hand, are likely to reflect specific characteristics of each fish population and lake. These results suggest complex interactions between host ecology and abiotic environment, such as water temperature, in determining the parasite community structure. Hence both factors have to take into consideration when studying the role of parasites in speciation processes. In future, it is necessary to pinpoint the stage of the host speciation process when parasite infections become differentiated in replicated systems to gain comprehensive understanding of the role of parasites in adaptive radiations.

Climatic cycles lead to changes in habitat suitability, which in turn can lead to allopatry, i.e. isolation, between populations. Lack of gene flow between allopatric populations causes them to diverge through accumulation of genetic differences that can create incompatibilities between lineages upon secondary contact in the form of lowered survivability or reproduction rate in hybrids. Incompatible genes act as reproductive barriers and keep lineages isolated by selection against hybrids, while gene flow and recombination work as a counterforce to selection promoting admixture. Reproductive barriers like these are most often found inside hybrid zones and are well demonstrated in nature, but the isolating effect of individual genetic incompatibilities on genome-wide gene flow is still an open question. Here we test if selection counteracting gene flow maintains a narrow hybrid zone between two subspecies of the meadow grasshopper Pseudochorthippus parallelus. We targeted 0,01 % of the 13 GB genome, recovering a 29,1 mean coverage per locus per individual in targeted regions, when mapping against a transcriptome. We find that, for the nuclear markers, the hybrid zone is narrower than expected under a neutral scenario of no selection, suggesting that it is maintained by selection against hybrids. We also find significant isolation by distance, suggesting gene flow across the hybrid zone despite selection against hybrids. Different parts of the genome show significant excess or deficit of heterozygotes, suggesting that selection and gene flow are heterogeneous throughout the genome. Combined, our results show that reproductive isolation between recently diverged lineages can evolve quickly despite gene flow in neutral and positively selected sites.