Population genetics and morphometric variation of blue mussels in the western Baltic Sea.

Breusing, Corinna (2012) Population genetics and morphometric variation of blue mussels in the western Baltic Sea. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 89 pp.

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The investigation of hybridizing taxa can provide intriguing insights into the process of speciation and the adaptive potential of populations to environmental change. Due to its gradual nature in terms of ecology and genetics, the European blue mussel hybrid zone between North Sea Mytilus edulis and Baltic M. trossulus is an ideal study system to analyse the significance of inter-specific hybridization in evolution. Although much research has already focussed on the genetic and phenotypic structures in the Baltic Proper, less is known about the situation in the western transition area.
By means of a multi-locus genotypic assessment and multivariate morphometric analyses I have examined the genetic and morphological constitutions of several Baltic Mytilus populations. Paying special attention to the poorly resolved outer parts, I find that most blue mussels in the western Baltic Sea and Skagerrak are higher generation backcross hybrids (99 % in the Kiel Fjord; 81 % in Tjärnö). While my results suggest that gene variants of M. edulis still prevail over that of M. trossulus in these mytilids, they also demonstrate that a minority of individuals in the Kiel Fjord are introgressed by M. galloprovincialis alleles. Although the origin of these alien polymorphisms cannot be told from my data, I postulate that their movement into the Baltic gene pool is most likely human-mediated (e.g. ship traffic). Considering the increasing records of anthropogenic introductions of Mytilus species to non-native habitats and the limited power of the applied molecular markers to discriminate M. galloprovincialis, it is to expect that more cryptic invasions are detected in future, technically advanced investigations.
My analysis of M. edulis allele frequency changes from the North Sea to the Baltic Proper extends and supports an earlier work by Stuckas et al. (2009), providing evidence for discordant patterns of gene flow across the hybrid zone. These discrepancies probably arise from the concerted action of direct selection, genetic hitchhiking, stochastic evolutionary forces and shifts in the geographical position of the secondary contact area. Different to this previous study and in line with the observations by Kijewski et al. (2006), I find that introgression into the central Baltic is restricted at least for some M. edulis maternal haplotypes (D-loop), indicative of cytonuclear incompatibilities between inner and outer Baltic mussels.
While differential environmental pressures might contribute to the maintenance of semi-permeable genetic barriers between M. edulis-like and M. trossulus-like hybrid populations, they can also account for the observed dissimilarities in shell morphology and phenotypic diversity. In contrast to individuals of the Baltic Proper, which express a
Abstract | 7
population-specific, M. trossulus-like morphotype, mussels of the transition zone build a morphological continuum between parental forms. Following the reasoning by Gardner (1996) I propose that the strong environmental variability in the western Baltic selects for multiple phenotypes, while the constant and extreme conditions (e.g. low salinities, absence of predators) in the eastern Baltic favour only a single shell morphotype. It remains to be shown whether these opposing patterns are consequences of environmentally-induced discrepancies in genetic variability at fixed causal loci (level of heterozygosity, selection for multiple alleles) and/or differences in phenotypic plasticity.

Document Type: Thesis (Master thesis)
Thesis Advisor: Melzner, Frank and Reusch, Thorsten B.H.
Keywords: HOSST
Subjects: Course of study: MSc Biological Oceanography
Research affiliation: OceanRep > GEOMAR > FB3 Marine Ecology > FB3-EOE-B Experimental Ecology - Benthic Ecology > FB3-EOE-B-JRG Melzner
OceanRep > GEOMAR > FB3 Marine Ecology > FB3-EV Marine Evolutionary Ecology
Open Access Journal?: Yes
Projects: HOSST
Date Deposited: 19 Dec 2012 10:50
Last Modified: 23 Jul 2015 06:52
URI: http://oceanrep.geomar.de/id/eprint/19887

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