Extensive long-distance pollen dispersal and highly outcrossed mating in historically small and disjunct populations of Acacia woodmaniorum (Fabaceae), a rare banded iron formation endemic.

Melissa A Millar,Margaret Byrne,David J Coates

doi:10.1093/aob/mcu167

Abstract

Background and AimsUnderstanding patterns of pollen dispersal and variation in mating systems provides insights into the evolutionary potential of plant species and how historically rare species with small disjunct populations persist over long time frames. This study aims to quantify the role of pollen dispersal and the mating system in maintaining contemporary levels of connectivity and facilitating persistence of small populations of the historically rare Acacia woodmaniorum.MethodsProgeny arrays of A. woodmaniorum were genotyped with nine polymorphic microsatellite markers. A low number of fathers contributed to seed within single pods; therefore, sampling to remove bias of correlated paternity was implemented for further analysis. Pollen immigration and mating system parameters were then assessed in eight populations of varying size and degree of isolation.Key ResultsPollen immigration into small disjunct populations was extensive (mean minimum estimate 40 % and mean maximum estimate 57 % of progeny) and dispersal occurred over large distances (≤1870m). Pollen immigration resulted in large effective population sizes and was sufficient to ensure adaptive and inbreeding connectivity in small disjunct populations. High outcrossing (mean tm = 0·975) and a lack of apparent inbreeding suggested that a self-incompatibility mechanism is operating. Population parameters, including size and degree of geographic disjunction, were not useful predictors of pollen dispersal or components of the mating system.ConclusionsExtensive long-distance pollen dispersal and a highly outcrossed mating system are likely to play a key role in maintaining genetic diversity and limiting negative genetic effects of inbreeding and drift in small disjunct populations of A. woodmaniorum. It is proposed that maintenance of genetic connectivity through habitat and pollinator conservation will be a key factor in the persistence of this and other historically rare species with similar extensive long-distance pollen dispersal and highly outcrossed mating systems.

Highlights

Patterns of pollen-mediated gene flow and variation in the mating system directly influence levels of genetic diversity, levels of genetic connectivity and genetic structure, and are key to the evolutionary potential of plant populations (Young et al, 1996; Eckert et al, 2010)
The degree of population isolation was significantly correlated with the degree of isolation from a large population (Table 2)
The nine microsatellite markers were moderately variable within A. woodmaniorum progeny cohorts, with a total of 60 alleles detected in 525 seedlings

Summary

Introduction

Patterns of pollen-mediated gene flow and variation in the mating system directly influence levels of genetic diversity, levels of genetic connectivity and genetic structure, and are key to the evolutionary potential of plant populations (Young et al, 1996; Eckert et al, 2010). The long-term impacts of restricted gene flow on rare species and others with small disjunct populations and geographically restricted ranges may be expected to include increased risk of extinction (Ellstrand and Elam, 1993). It is proposed that maintenance of genetic connectivity through habitat and pollinator conservation will be a key factor in the persistence of this and other historically rare species with similar extensive long-distance pollen dispersal and highly outcrossed mating systems

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