Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis

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.

Limiting inbreeding in disjunct and isolated populations of a woody shrub

“…1999; Hoebee et al. 2007; Duminil et al. 2009), but in H. oldfieldii outcrossing rates did not differ significantly among populations of different size, shape, or plant density.…”

“…The nearest population was over 1 km away and therefore this provides evidence for some pollen dispersal between populations across different habitat at a level similar to those found in other woody species with disjunct populations (Robledo‐Arnuncio and Gill 2005; Hoebee et al. 2007; Sampson et al. 2015).…”

“…Several studies suggest that even low levels of gene flow may be sufficient to maintain diversity within populations (Hoebee et al. 2007; Hsieh et al. 2013; Millar et al.…”

“…1999; Hoebee et al. 2007), and also by individual plant characteristics such as plant size, floral display, flowering phenology, pollination, and self‐incompatibility (SI) systems (Barrett and Harder 1996; Richards 1997; Coates et al. 2007; Hoebee et al.…”

Contemporary pollen flow, characterization of the maternal ecological neighbourhood and mating patterns in wild cherry (Prunus avium L.)

“…Bacles et al (2004) suggested that low pollen dispersal arose from localized foraging of pollinators and infrequent travel between remnant populations at their windy site. The effect of isolation on pollen immigration was further highlighted in a study of two Sorbus torminalis woods, one of which was part of a continuous large population and the other a small more-isolated population (Hoebee et al, 2007). In the continuous population, immigrant pollination accounted for 36-39% of paternity compared with only 8% in the isolated population.…”