Large and small scale evolutionary processes in the floodplain plant Cnidium dubium: From biogeography to population dynamics in fragmented river landscapes
Final Report Abstract
Populations of Cnidium dubium, a characteristic species of floodplain meadows are strongly declining due to river regulation and floodplain fragmentation by dykes. To maintain and restore C. dubium populations and its genetic resources, knowledge is needed about clonal and genetic diversity of Cnidium stands and differentiation among the relic populations. In 2012, leaf material was sampled for genetic analyses in 50 patches along a 400 km stretch of the Elbe River taking into account locations of the recent and older floodplain using a regular grid design of 3x3m. The study area ranged from Saxony to Lower Saxony near Hamburg (middle and lower Elbe River). Additionally, further samples were collected in eight large patches where a 10x10m grid design was possible. Genotyping was conducted using six nuclear microsatellite markers, and in addition, we determined chloroplast haplotypes using three chloroplastDNA markers from which only one marker is already analysed to date. To address small scale genetic patterns we related the occurrence of clone pairs as well as genetic distances of non-clonal pairs in relation to geographic distances within patches taking into account the patch size (small/large) and the floodplain compartment (recent/older). Furthermore, clonal diversity was analysed for each patch. On the large scale, genetic diversity and differentiation was compared between patches of the middle and lower Elbe River as well as between patches of the recent and older floodplain. From altogether 933 analysed samples only 147 different genotypes could be detected. From the 50 patches sampled in a small grid 24 exhibited only one single genotype forming a large clone group and 1-2 haplotypes per patch occurred. Interestingly, in 20 patches genotypes occur which had only tiny allele differences to large clone groups suggesting somatic mutation processes. The probability of occurrence of ramet pairs not belonging to the same clone and the genetic distance of non clonal ramet pairs increased along the geographic distance and was higher in patches of larger size, in the older floodplain and in patches with two haplotypes present. Within patch-clonal diversity was significantly higher in larger patches in which two haplotypes occurred, but it was not related to floodplain compartment and location along the Elbe River. Regarding the genetic diversity and differentiation parameters no differences between recent and older floodplain and middle and lower stretch of the Elbe River could be detected and no genetic structure on the large scale was observed. The lack of genetic pattern on a large scale suggests, at first glance, that the gene flow acts efficiently across large distances along the Elbe River and across the different floodplain compartments. However, the findings regarding the genetic patterns at the within-patch scale draw a different picture concerning the intensity of gene flow since clonal diversity was low. Considering, in addition, the number of genotypes where only tiny allele differences to large clone groups occurred suggesting somatic mutation processes, we can assume that sexual reproduction processes are absent in the majority of patches and that gene flow across and within Cnidium patches is extremely low. Therefore, these findings could indicate that the large scale genetic pattern of Cnidium dubium is not the result of recent intensive gene flow by pollination and seed dispersal, but the result of gene flow processes of former times in which Cnidium dubium was more abundant maybe due to environmental conditions more suitable for this plant. Taken together, the species already severely threatened by the destruction of suitable habitats through river regulation and floodplain degradation is predestined to further decline due to the low number of individuals, the lack of sexual reproduction processes and isolation of small stands.
