Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/6897
Parasites and parallel divergence of individual MHC allelic richness in Icelandic threespine stickleback (Gasterosteus aculeatus L.): Contrasting habitats and population divergence
When individuals of a single species encounter different environments, local adaptation and thus phenotypic diversification among populations is expected. This intraspecific diversity, which is common among northern freshwater fish, can be observed even between populations that live in close proximity (sympatry). Common ecological selection pressures often lead to parallel evolution as similar habitats favour the same phenotypic optima resulting in independent evolution of the same traits towards the same direction. Local adaptation is reflected in the external morphology of the fish as many morphological traits are associated with performance and fitness i.e. locomotion activity, foraging efficiency and protection against predators. However, in environments characterized by great niche availability utilization of vacant niches can result in increased intrapopulation phenotypic variation. Differential exploitation and specialization to new alternative resources can also occur between sexes of the same population. This intersexual competition for resources can lead to sexual dimorphism and its extent has been found to increase with niche availability as sexes have the potential to explore a wider range of alternative niches.
An important ecological factor that can contribute to divergence among populations is the interaction between hosts and parasites. According to theory, differential parasitism between host populations has the potential to promote adaptation to local environments and thus rapid divergence due to host-parasite co-evolutionary interactions. Parasite mediated selection has been found to operate on the highly polymorphic genes of the major histocompatibility complex (MHC) which is part of the adaptive immune system of the jawed vertebrates.
In the first part of the thesis (manuscript I) threespine sticklebacks (Gasterosteus aculeatus) sampled from four lakes located in South-Western Iceland, two of which are inhabited by two distinct stickleback morphs, apparently adapted to different benthic habitats, -lava and mud-, and two single morph lakes, were screened for parasites and genotyped for MHC class IIB diversity. The level of parasitic infection was found to differ consistently between the sympatric morphs but also between populations found in similar foraging habitats with fish from the lava/rocky habitats being more heavily infected. A parallel pattern was also found in individual MHC allelic variation with lava stickleback morphs exhibiting lower levels of variation compared to the mud morphs. The parallel divergence in MHC allele number between the sympatric morphs may be caused by different selection pressures imposed by varying abundance of parasite species causing different optima to be favoured in contrasting habitats. Consequently, parasite mediated selection could play a part in the divergence of the benthic lava-mud morphs in Icelandic lakes.
The second part of the thesis (manuscript II) investigates sexual dimorphism and morphological variation in relation to substrate type and lake size among populations and the sexes in threespine stickleback (Gasterosteus aculeatus). Threespine stickleback were sampled from 34 sites in 22 lakes in Iceland inhabiting three main substrate types: mud, vegetation and rock. Lake size, generally representing increased niche availability, was positively correlated with phenotypic variance within populations and sexual dimorphism, most likely representing increased ecological opportunity. Finally, our result do not support that increased sexual dimorphism restricts phenotypic variation within each sex at the population level.