Patterns of hybrid seed production in adjacent seed orchards of Acacia auriculiformis and A. mangium in Vietnam

Hybrid seed production in adjacent seed orchards of Acacia auriculiformis and Acacia mangium was influenced by (i) flowering times of the two species and (ii) the distances of parent trees from the inter-orchard boundary. Approximately 80% of hybridisation events were found within 60 m from the boundary.

Context Understanding pollen dispersal has important implications for breeding and seed production of pure species and hybrid Acacia.
Aims We examined patterns of hybrid production in adjacent clonal seed orchards of A. auriculiformis and A. mangium in Vietnam.
Methods We assessed the frequency of hybrid offspring using four species-diagnostic SSR markers in seed collected from a total of 72 trees (75 seedlings per tree) at distances ranging from 4 to 144 m from the inter-orchard boundary. The number of hybrid was determined from SSR allele peak sizes in pooled sample (10 seedlings per pool, 540 pools). Calibrations were developed from pools with known proportions of pure-species and hybrid material.
Results Hybrid frequency differed significantly among individual clones (P < 0.001) but not between species (A. mangium = 3.4%, A. auriculiformis = 2.8%). Two late-flowering clones of A. auriculiformis yielded no hybrids. The level of interspecific hybridisation declined significantly (P < 0.001) with increasing distance, and no hybrid seed was produced by trees located more than 116 m from the inter-orchard boundary.
Conclusion Pooling of tissue samples for analysis of species-specific DNA polymorphisms was an efficient, low-cost strategy for detecting hybrid genotypes among the offspring of pure-species parents, and a low rate of associated error was demonstrated. The inferred decline in inter-species pollen flow with increasing distance from the boundary between the two orchards provides guidance for the design of hybridising orchards and isolation requirements to prevent contamination of seed orchards by external pollen sources.

Clonal seed orchard, Contamination, Hybrid seed, Pollen dispersal, Seed orchard design and management, Seed production

Le, S., Harwood, C.E., Nghiem, C.Q. et al. Annals of Forest Science (2019) 76: 46.

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Data availability
Raw measurement relating to this study are available in the University of Tasmania Open Access Repository (Le et al. 2019). Le S, Harwood CE, Nghiem CQ, Griffin AR, Vaillancourt RE (2019) Number of hybrid per clone and ramet, Son Le et al. 2019. Version 05 March 2019. University of Tasmania. [Dataset].

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