Soil diaspore bank of ephemeral plants

Genetic and/or phenotypic variation is required for populations to adapt to environmental change. Many annual, fugitive plant species maintain a persistent soil diaspore bank (buried viable diaspores), which adds an additional, temporal component to geographically structured genetic and phenotypic variation. Persistent soil diaspore banks may buffer local populations against dramatic changes in genetic composition occurring in conjunction with severe fluctuations in population size (genetic drift), they may retard response to selection (i.e., buffer local populations against selection), and they may act as a source of genetic novelty and may thus increase the evolutionary potential of populations, because aging of seeds is accompanied by chromosome breakage and genic mutation through progressive cleavage of DNA into fragments of lower molecular weight. Persistent soil diaspore banks are characteristic of species of ephemeral, semi-aquatic environments (communities of dwarf rushes). In Europe, these communities are declining. Historically important retreat areas of populations of dwarf rushes are secondary, man- made fishponds, with a European centre of distribution in the Bohemian Massif. The intensive disturbance regime (flooding, pond management) is expected to exert severe differential selection pressure on populations of dwarf rushes. There is little empirical evidence on the importance and functional significance of persistent soil diaspore banks for maintaining genetic and phenotypic variation. This project aims at illuminating the role of the persistent soil diaspore bank as an integral part of the life history of ephemeral, semi-aquatic plant species. The objectives and hypotheses are structured around two main components of variation in populations: (1) genetic variation as opposed to (2) variation in phenotypic and life history traits. The first objective is the study of the distribution of genetic variation within and among soil diaspore bank and surface population at study sites with different disturbance/management regimes in relation to total genetic diversity in the species. The second objective is the study of variation in quantitative traits associated with fitness as opposed to the supposedly neutral genetic variation. We aim at testing the hypotheses that adaptation processes in local populations lead to locally adapted phenotypes and that ephemeral, semi-aquatic plant species have adapted to natural water-level oscillations and irregular discharge of pond water through increased phenotypic plasticity by assessing the contribution of genotype vs. environment (availability of water) for observed phenotypic variation. Our species of interest is Cyperus fuscus (Cyperaceae), a character species of communities of dwarf rushes. We will take soil diaspore bank samples and compare soil diaspore bank vs. surface populations for their genetic composition (AFLPs and SSRs) and phenotypic variation and plasticity.

The brown galingale Cyperus fuscus L. (Cyperaceae) is a graminoid native to the Mediterranean Region and temperate Eurasia. This small summer annual plant is a typical pioneer of land interface zones of rivers, where it grows during periodic drying of these zones. As other species of mudbanks, Cyperus fuscus establishes a persistent soil seed bank, out of which seeds germinate under favourable conditions. The species itself is not endangered, but its habitat is a priority habitat of the European Habitats Directive (code: 3130) and includes many rare species. Cyperus fuscus is also found in secondary anthropogenic habitats such as traditionally used fishponds and fish storage ponds.We showed that Cyperus fuscus adopts a low-oxygen escape strategy characteristic of flood-tolerant species to avoid the negative effects of temporary partial submergence that might occur during summer floods by growing higher and developing longer and narrower leaves. Our results suggest that differences in growth between primary and secondary habitats (rivers, fishponds, and fish storage ponds) are both genetically fixed and caused by the growing conditions at the localities (e.g. high nutrient availability in fishponds). In a controlled environment, plants originating from different habitats differed in both trait means and plasticities, indicating that disruptive selection on flooding induced plasticity had taken place. Plants from river habitats performed better in general and responded better to high and fluctuating water levels than plants from secondary habitats. River populations are also more genetically variable than populations in anthropogenic habitats.Selection of genotypes out of the genotype reservoir in the soil (seed bank) seems to be mediated by the particular conditions (such as temperature) encountered during germination. There is no other differentiation between the soil seed bank and the yearly surface population. There is also no difference in the amount of genetic variation present in the soil seed bank or surface population. This suggests that the soil seed bank is a reservoir of genetic variation storing genotypes from earlier generations, which is enriched every year by immigration of seeds dispersed by running water, waterfowl, or fish transport.Little genetic geographic structure exists in the species throughout Europe, suggesting high ability for dispersal, most probably by waterfowl. A north-south gradient in genetic variation suggests postglacial colonisation of central and northern Europe from southern refugia. To our knowledge, this is the first ephemeral mudbank species, whose populations are isolated in time and space, which has been analysed for phenotypic and genetic variation across a larger geographic area. The results highlight the importance of preserving pristine river habitats, in addition to secondary habitats, for conservation of the entire variation and hence adaptive potential of pioneer vegetation of land interface zones of wetland habitats.