Speciation in arctic and alpine diploid plants
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AbstractThe main objectives of this thesis are to study patterns and processes of plant speciation in arctic and alpine diploid plants. Cryptic species are here referred to as morphologically similar individuals belonging to the same taxonomic species but that are unable to produce fertile offspring (i.e. 'sibling' species).
The arctic flora is considered as one of the most species-poor floras of the world, and the latitudinal gradient with decreasing diversity from low to high latitudes is likely the oldest recognised pattern in ecology. However, these estimates are usually based on morphological differentiation into taxonomically recognizable species and may not provide accurate numbers of biological species. Previous intraspecific crossing experiments in three diploid circumpolar species of Draba (Brassicaceae) revealed the presence of numerous cryptic biological species within each taxonomic species. The present study expands the knowledge based on these previously published results and suggests that frequent formation of cryptic biological species may be a general pattern in the arctic flora. Intraspecific crossing experiments including several distantly related circumpolar diploid plant species revealed that intrinsic postzygotic isolation has developed multiple times, even at small geographical scales. This was shown for all five selfing species investigated, whereas crosses within one outcrossing species generated fully fertile F1 hybrids. This suggests that a selfing mating system may accelerate the accumulation of hybrid incompatibilities. The barriers have in addition developed very rapidly, apparently within a few millennia, suggesting that speciation rates are unexpectedly high in the arctic flora. Cryptic biological species, although not yet recognisable morphologically, are thought to represent starting points for new evolutionary lineages that given sufficient time may develop into full-fledged new taxa. Other factors may thus account for the low diversity of the contemporary arctic flora in terms of taxonomic species. It is likely that high extinction rates rather than low speciation rates have played an important role in shaping the extent diversity in the arctic flora, possibly associated with climatic shifts during the Pleistocene glacial cycles.
The genetic mechanisms involved in the build-up of reproductive isolation are of central importance in understanding the evolution of new species. This thesis presents further insights into the mechanisms underlying reproductive isolation in Draba nivalis (Brassicaceae) – a small, circumpolar, predominantly selfing diploid herb that demonstrates numerous cryptic biological speciation events. By performing genetic linkage mapping and searching for quantitative trait loci (QTL) associated with reproductive isolation more knowledge about the mechanisms involved in the evolution of intrinsic postzygotic reproduction in this system has been gathered. The linkage map was produced by combining both codominant and dominant markers and resolved eight linkage groups that most likely correspond to the eight chromosomes of D. nivalis. Observed patterns of inheritance were consistent with the influence of both nuclear-nuclear interactions and chromosomal changes. In particular, all seed set QTLs and one pollen fertility QTL displayed underdominant effects, matching expectations of chromosomal speciation models. Theory struggles to account for the establishment of large and strongly underdominant chromosomal translocations. Draba nivalis may however be an exception as a selfing mating system, is conducive for the establishment of chromosomal rearrangements through genetic drift. Overall this study confirms that multiple genetic mechanisms are involved in the build-up of reproductive isolation in D. nivalis, suggesting the involvement of both nuclear-nuclear interactions and structural chromosomal changes.
As plants are sessile organisms, they depend largely on adapting to locally changing climatic conditions such as temperature, aridity, and day length. Natural selection acting on traits that respond to such changes has likely played an important role in the evolution of plants. Climatic cycles of the Pleistocene caused drastic changes to species’ ranges. For example, the Japanese alpine endemic plant Cardamine nipponica (Brassicaceae) probably diverged into northern and central populations during the Pleistocene climatic oscillations. The northern and central populations present highly diverged alleles of a particular photoreceptor gene phytochrome E (PHYE). Phytochromes such as PHYE monitor the surrounding light environment, and likely play an important role in the regulation of plant life cycles. The present study infers the evolutionary history of the PHYE in C. nipponica and its close relatives using maximum likelihood models. The resulting genealogical relationship suggested that standing genetic variation of PHYE, which diverged under positive selection prior to speciation, resulted in the selective differentiation between the northern and central Japanese populations of C. nipponica. This further suggests the importance of standing genetic variation in regard to quick responses to climatic changes.
List of papers. The papers are removed from the thesis due to publisher restrictions.
I. A. Lovisa S. Gustafsson, Galina Gussarova, Liv Borgen, Hajime Ikeda, Jan Suda, Loren H. Rieseberg, Christian Brochmann. High speciation rates in arctic plants. Manuscript.
II. A. Lovisa S. Gustafsson, Inger Skrede, Heather C. Rowe, Galina Gussarova, Liv Borgen, Loren H. Rieseberg, Christian Brochmann, Christian Parisod. Genetics of cryptic speciation within an arctic mustard, Draba nivalis. Submitted.
III. Hajime Ikeda, A. Lovisa S. Gustafsson, Christian Brochmann, Hiroaki Setoguchi. Pre-speciation origin of selective divergence and balancing selection in a plant photoreceptor gene, phytochrome E. Submitted.