The ecology, genetics and evolution of two Saxifraga species with different fragmentation histories
The aim of this thesis was to study the ecology, genetics and evolution of two congeneric species with different fragmentation histories. Saxifraga sponhemica is a glacial relict species of long-term fragmented lowland rock and scree habitats, and has been naturally rare for thousands of years with...
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|The aim of this thesis was to study the ecology, genetics and evolution of two congeneric species with different fragmentation histories. Saxifraga sponhemica is a glacial relict species of long-term fragmented lowland rock and scree habitats, and has been naturally rare for thousands of years with a disjunct distribution in Central Europe. Saxifraga granulata is a formerly common species of species-rich semi-natural grasslands that has become recently fragmented due to the intensification of agricultural practices. Fragmentation generally leads to loss of genetic diversity due to drift and inbreeding, reduced mean fitness and increased extinction rates of populations. Formerly common species are expected to be particularly susceptible to the recent anthropogenic fragmentation of their habitats.
An analysis of the genetic diversity and the genetic structure of S. sponhemica populations based on RAPD-markers showed that in most populations considerable genetic variability has been preserved. An isolation by distance pattern of genetic differentiation suggested historical gene flow during the last glaciation when suitable habitats were much more abundant. Long-lived plant species can thus maintain historic genetic patterns despite the small size and strong isolation of populations.
We grew plants form several families per population in a common garden to study the quantitative genetic variation within and among populations. We found several lines of evidence for divergent selection. Most population trait means were significantly related to climate gradients, indicating adaptation. Quantitative genetic differentiation increased with climatic distance and with geographical distance, even when neutral molecular divergence was controlled for, and quantitative genetic differentiation (QST) exceeded molecular genetic differentiation (FST) for some traits. Traits under strong selection showed little genetic variation within populations. The evolutionary potential of a population was not related to its size, the performance of the population or its neutral genetic diversity. However, performance in the common garden was lower for plants from populations with reduced molecular genetic variation, suggesting inbreeding depression due to genetic erosion. Studies of molecular and quantitative genetic variation may thus provide complementary insights important for the conservation of rare species. S. sponhemica does not appear to be genetically threatened in the short term, but populations are threatened by habitat destruction. A conservation measure could be to create new populations in suitable habitats with seeds from the same region to avoid local maladaptation.
We also studied the RAPD molecular and the quantitative genetic structure of 19 populations of the declining grassland plant S. granulata in a geographically restricted area in Luxembourg and Germany. Differentiation for quantitative traits (QST) was slightly lower than differentiation for molecular markers (FST) suggesting homogenising selection for optimal trait values. Contrary to our expectations, the level of differentiation among fragmented S. granulata populations was low, and molecular genetic diversity was high and was not correlated with the size or the mean plant performance of populations. Gene flow by long distance dispersal or the longevity, clonality and polyploidy of S.granulata may have prevented genetic erosion due to drift. To avoid genetic erosion in the future, extant populations should be preserved and gene flow among populations should be maintained.
Habitat fragmentation has led to increased inbreeding and inbreeding depression in many species. We investigated the effects of increased inbreeding and of intra- and interpopulation crosses on the reproduction and performance of S. granulata. Between population crosses may result in increased performance (heterosis), but may also lead to the disruption of coadapted gene complexes and to decreased performance (outbreeding depression). Inbreeding depression affected all traits in the F1 generation, but was stronger for traits expressed late during development and varied among families. Multiplicative fitness of the F2 generation after serial inbreeding was extremely low, but there was heterosis after crossing inbred lines. Outbreeding depression was however not observed in the F2. We also subjected the first generation of offspring to a fertilization and stress treatments (competition and defoliation). The adaptive plasticity of offspring from selfing and from interpopulation crosses in response to nutrient addition was reduced. Outbreeding depression was also observed in response to stress. The results suggest that continuous inbreeding may drastically reduce the fitness of plants, but effects may be environment-dependent.
Overall, the results of this thesis advance knowledge on the role of time since habitat fragmentation, of historic connectivity among populations, and of life history traits such as longevity and clonality on the processes of selection and drift that shape the genetic variation within and among populations. It stresses the importance of using both molecular and quantitative genetic tools to gain complementary insight for the conservation of rare and endangered plant species. It shows how knowledge about the vulnerability to increased inbreeding and the potential risks of artificially increasing gene flow between populations of recently fragmented species contributes to their effective conservation.