Mating system and seed variation of Acacia hybrid (A. mangium x A. auriculiformis)

Limited phylogeographic and genetic connectivity in Acacia species of low stature in an arid landscape

Widespread plant species are expected to maintain genetic diversity and gene flow via pollen and seed dispersal. Stature is a key life history trait that affects seed and potentially pollen dispersal, with limited stature associated with limited dispersal and greater genetic differentiation. We sampled Hill's tabletop wattle (Acacia hilliana) and curry wattle (Acacia spondylophylla), two co-distributed, widespread, Acacia shrubs of low stature, across the arid Pilbara region of north-western Australia. Using chloroplast sequence and nuclear microsatellite data we evaluated patterns of population genetic and phylogeographic diversity and structure, demographic signals, ratios of pollen to seed dispersal, evidence for historical refugia, and association between elevation and diversity. Results showed strong phylogeographic (chloroplast, G ST = 0.831 and 0.898 for A. hilliana and A. spondylophylla, respectively) and contemporary (nuclear, F ST = 0.260 and 0.349 for A. hilliana and A. spondylophylla, respectively) genetic structure in both species. This indicates limited genetic connectivity via seed and pollen dispersal associated with Acacia species of small stature compared to taller tree and shrub acacias across the Pilbara bioregion. This effect of stature on genetic structure is superimposed on moderate levels of genetic diversity that were expected based on widespread ranges (haplotype diversity h = 25 and 12; nuclear diversity He = 0.60 and 0.47 for A. hilliana and A. spondylophylla, respectively). Contemporary genetic structure was congruent at the greater landscape scale, especially in terms of strong genetic differentiation among geographically disjunct populations in less elevated areas. Measures of diversity and connectivity were associated with traits of greater geographic population proximity, population density, population size, and greater individual longevity, and some evidence for range expansion in A. hilliana. Results illustrate that low stature is associated with limited dispersal and greater patterns of genetic differentiation for congenerics in a common landscape and highlight the complex influence of taxon-specific life history and ecological traits to seed and pollen dispersal.

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

BACKGROUND AND AIMS

Understanding 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.

METHODS

Progeny 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 RESULTS

Pollen 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.

CONCLUSIONS

Extensive 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.

The breeding systems of diploid and neoautotetraploid clones of Acacia mangium Willd. in a synthetic sympatric population in Vietnam

Colchicine-induced neoautotetraploid genotypes of Acacia mangium were cloned and planted in mixture with a set of diploid clones in an orchard in southern Vietnam. Following good general flowering, open-pollinated seed was collected from trees of both cytotypes and microsatellite markers were used to determine the breeding system as characterised by the proportion of outcrosses in young seedling progeny. As predicted from the literature, the progeny of diploid clones were predominantly outcrossed (t(m) = 0.97). In contrast, the progeny of the tetraploid clones were almost entirely selfs (t(m) = 0.02; 3 of 161 seedlings assayed were tetraploid outcrosses and there were no triploids). Segregation at loci heterozygous in the tetraploid mothers followed expected ratios, indicating sexual reproduction rather than apomixis. Post-zygotic factors are primarily responsible for divergence of the breeding systems. Commonly, less than 1 % of Acacia flowers mature as a pod, and after mixed pollination, diploid outcrossed seed normally develops at the expense of selfs. Selfs of the tetraploid trees appear to express less genetic load and have a higher probability of maturing. However, this does not fully explain the observed deficiency of outcross tetraploid progeny. Presumably, there are cytogenetic reasons which remain to be investigated. In nature, selfing would increase the probability of establishment of neotetraploids irrespective of cytotype frequency in the population. Breeders need to review their open-pollinated breeding and seed production strategies. It remains to be seen whether this is an ephemeral problem, with strong fertility selection restoring potential for outcrossing over generations.