ABSTRACT: Habitat fragmentation often has negative consequences for genetic diversity, and thereby for the viability of populations.
However, these negative consequences might be counteracted by gene flow as the latter provides functional connectivity between
apparently isolated habitat fragments. Gene flow is itself influenced by landscape structure and composition, and it is therefore
important to understand the relationship between gene flow and landscape structure and composition. We used linear LAD regression
models to investigate the relationship between contemporary gene flow by pollen in the rare, insect-pollinated forest tree
Sorbus domestica and several landscape features. None of the landscape components—which included closed forest, deep valleys, open land and
settlements—proved to be an impermeable barrier to gene flow by pollen. We found evidence that settlements, large open areas,
and a pronounced topography increased long-distance gene flow in the landscape as compared to a random model including all
possible gene flow trajectories. These results are encouraging from a conservation view, as gene flow in species pollinated
by generalist insects seems to provide functional connectivity and may help to maintain genetic diversity in rare plant species
in fragmented landscapes.
KeywordsLandscape genetics-LAD regression-Paternity analysis-Functional connectivity-Insect pollination-Pollen dispersal-Switzerland
Landscape Ecology 04/2012; 25(6):903-911. · 3.06 Impact Factor
ABSTRACT: Precise empirical data on current gene flow by pollen, both with respect to distance and abundance, is crucial to understand whether habitat fragments are functionally connected. Based on a large-scale inventory ( approximately 100 km(2)) in which all individuals of a naturally scattered forest tree (Sorbus domestica) were mapped, we inferred current gene flow by pollen using genetic paternity analysis. We detected an extensive network of effective pollen transfer. Although short pollen flow distances were most abundant, 10% of the assigned pollen donors were more than 2 km away from their female mating partners, and 1.8% were even at a distance of 12-16 km. This latter pollen flow shows that current long-distance gene flow over a fragmented landscape clearly occurs. Pollen dispersal was well described by a fat-tailed inverse curve. Using parentage analysis of established trees, maternally inherited chloroplast markers and diameter at breast height measurements as an indicator of individual tree age, we were able to infer regular seed dispersal distances over several hundred metres up to more than 10 km. We conclude that in temperate, insect-pollinated and animal-dispersed tree species such as S. domestica, fragmented subpopulations are functionally connected by gene flow through both pollen and seed.
Heredity 09/2009; 103(6):476-82. · 4.60 Impact Factor
ABSTRACT: The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.
Heredity 08/2007; 99(1):47-55. · 4.60 Impact Factor
ABSTRACT: Distinct spatial genetic structure, as the result of various evolutionary and ecological processes, is a common feature of tree populations. The rare pioneer forest tree Sorbus torminalis occurs in scattered populations of low density and exhibits both clonal propagation and gametophytic self-incompatibility. Clonal reproduction can promote considerable spatial genetic structure and, together with a self-incompatibility system, may substantially reduce mating opportunities within S. torminalis populations, i.e. an Allee-effect owing to mate limitation. All 10 S. torminalis stands mapped in northern Switzerland and analysed with allozymes showed a considerable degree of clonal reproduction, but they were also characterised by large numbers of genotypes that occurred only once. However, spatial autocorrelation analysis revealed significant spatial genetic structure at distances between 15 and 30 m as the result of clonal reproduction. Once the effect of clonal propagation was removed from the analysis, the stands no longer exhibited significant spatial autocorrelation. This implies that seed dispersal was not locally restricted. The degree of clonal reproduction was neither correlated with population size, nor did smaller populations exhibit less genetic diversity. Because clonal patches were rather small and interspersed with other genetically unique and unrelated individuals, clonal reproduction seemed to have no negative impact on the species’ sexual reproduction. It is thus likely that the combination of an effective self-incompatibility system and high interstand gene flow helps to maintain genetic diversity in S. torminalis stands, while clonal propagation preserves the genetic diversity over time even if environmental conditions become less favourable during the course of succession.
Forest Ecology and Management.