Relationships between population density, fine-scale genetic structure, mating system and pollen dispersal in a timber tree from African rainforests.

J Loo,O J Hardy,K Daïnou,D K Kaviriri,J Duminil,P Gillet,J-L Doucet

doi:10.1038/hdy.2015.101

Abstract

Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (∼16%) and patterns of spatial genetic structure (Sp ∼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.

Highlights

The population density of reproductive individuals (‘population density’ for short) is central in the context of sustainability of tree resources under selective logging practices
The population in Gabon presented differences in genetic diversity among cohorts, but this can be attributed to a sampling effect
Mating system parameters are based on the genotypes of seeds (Cameroon and Democratic Republic of Congo (DRC)) or seedlings (Gabon), whereas fine-scale spatial genetic structure estimates are based on the genotypes of adults. aF1: mean kinship coefficient between individuals at the first distance class. *Significant F1 values (Po0.001). bOutcrossing rate t as calculated through the relation t = 1 − 2F(null). ct as estimated through a paternal analysis in CERVUS, numbers in brackets refer to the range of t observed in the population using progeny array with a minimum size of six offspring. dtm: multilocus population outcrossing rate (MSF). etm: multilocus population outcrossing rate (MLTR). ftm − ts: indirect estimation of the presence of biparental inbreeding (MLTR)

Summary

Introduction

The population density of reproductive individuals (‘population density’ for short) is central in the context of sustainability of tree resources under selective logging practices. A reduction of population density may limit the amount of pollen available for outcrossing, which could reduce seed production (pollen limitation) in obligate outcrossing species and/or increase selfing rate in self-compatible hermaphrodite species (Naito et al, 2008). In a number of species, a reduction of population density has been shown to be counterbalanced by a change in pollinator behaviour that allows higher pollen dispersal distance (Hardy et al, 2006; Carneiro et al, 2011) Another potential long-term impact of a demographic reduction is a higher rate of loss of rare alleles, reducing the population adaptive potential (Beardmore et al, 2014). Again, this impact might be counterbalanced if effective pollen dispersal distances increase at lower density. That pollen dispersal distances are smaller below some density threshold where a disruption of pollen flow between individuals would occur because of the unavailability of mates (Forsyth, 2003; de Waal et al, 2015)

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Discussion

Conclusion