As part of a program to breed sterile triploid varieties of tropical Acacia, a series of inter-and intra-specific crosses were made among clones of neo-tetraploid A. mangium (AM-4x) and diploid A. mangium (AM-2x) and A. auriculiformis (AA-2x). The present paper reports variation in seed-crop development from anthesis to harvest, in comparison with that after open pollination of the respective parent trees. Abscission of spikes and pods within spikes commenced soon after anthesis and was more rapid in inter-cytotype crosses than in open-pollinated controls. Less than 12% of spikes were retained to maturity in either cytotype, emphasising the likely importance of resource competition during development. Inter-cytotype crosses showed higher levels of abnormal ovule development at 7 weeks after pollination and more undeveloped seeds in those pods which did develop to maturity. No inter-cytotype combination produced more than one viable seed per pod on average, and all needed to be germinated in vitro to survive. A directional effect was apparent in the inter-cytotype crosses within AM but this was not obvious when the cross was inter-specific. The study contributes new knowledge of the post-anthesis timeline for ovule, pod and spike abscission and discusses the likely genetic and environmental causes of observed differences between inter-and intra-cytotype crosses as well as the implications for breeding.
The frequency of polyploid trees in 10 populations of the predominantly diploid species Acacia dealbata subsp. dealbata Link in south-eastern Tasmania was determined using flow cytometry. At seven of the sites, all trees were diploid. At two sites, single triploid genets were found and, at a third, two tetraploids. Microsatellite markers were used to confirm that triploid trees distributed over an area of at least 930 m2 at the major study site were all ramets of a single genet. Three diploid clones were also confirmed at this site. The 16-grain polyads from the triploid genet were significantly larger than those from diploids, but only the diploid pollen showed any viability in vitro. At three months the green pods on diploids averaged 36 mm and contained four developing seeds per pod. Pods on the triploid were only 13 mm long, with no developing seeds. In spite of maturing pods, two of the diploids did not yield full seed, whereas two other diploids averaged only 0.1 seeds per pod. Seeds were not produced on triploid trees. The low reproductive output is discussed with reference to the breeding system and the impact of clonality on effective cross-pollination.
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